[{"data":1,"prerenderedAt":3461},["ShallowReactive",2],{"wiki-page-\u002Fen-us\u002Fwiki\u002F2023-10-05-cplusplus-jiao-xue\u002F2000-1400-std-optional":3,"wiki-doc-items-\u002Fen-us\u002Fwiki\u002F2023-10-05-cplusplus-jiao-xue\u002F2000-1400-std-optional":3031,"language-switcher-data-\u002Fen-us\u002Fwiki\u002F2023-10-05-cplusplus-jiao-xue\u002F2000-1400-std-optional":3445,"wiki-i18n-paths-\u002Fen-us\u002Fwiki\u002F2023-10-05-cplusplus-jiao-xue\u002F2000-1400-std-optional":3460},{"id":4,"title":5,"body":6,"canonicalPath":3013,"chapterDepth":122,"chapterOrder":3014,"date":3015,"description":115,"docI18nKey":3016,"docKey":3017,"docRoot":3018,"docTitle":3019,"extension":3020,"i18nKey":3021,"isBlogPost":3022,"isWikiDoc":221,"isWikiIndex":3022,"layout":3023,"legacyPath":3023,"locale":3024,"localeSlug":3025,"meta":3026,"navigation":221,"path":3013,"seo":3027,"sourcePath":3028,"sourceStem":3021,"stem":3029,"wikiDepth":122,"__hash__":3030},"content\u002F_i18n\u002Fen-us\u002Fwiki\u002F2023-10-05-Cplusplus教学\u002F2000-1400-std-optional.md","std::optional",{"type":7,"value":8,"toc":2992},"minimark",[9,14,18,51,54,79,85,89,98,102,105,109,131,135,326,330,415,419,424,965,972,980,984,1468,1472,1478,1485,1488,1501,1510,1519,1529,1531,1545,1551,1563,1569,1578,1582,1959,1963,1969,1973,1976,1980,1983,1986,2089,2093,2098,2163,2167,2710,2714,2720,2729,2733,2740,2771,2774,2784,2810,2816,2825,2857,2864,2867,2884,2887,2956,2960,2988],[10,11,13],"h2",{"id":12},"what-problem-does-this-section-solve","What problem does this section solve?",[15,16,17],"p",{},"When a function returns a result, how do you represent \"no result\"?",[19,20,21,38,41,48],"ul",{},[22,23,24,25,29,30,33,34,37],"li",{},"Using a special value like ",[26,27,28],"code",{},"None"," (Python), ",[26,31,32],{},"null"," (Java\u002FJavaScript), or ",[26,35,36],{},"Optional"," (Java).",[22,39,40],{},"In some cases, returning an empty collection (like an empty list) can signify no results.",[22,42,43,44,47],{},"For numeric results, you might use a special value like ",[26,45,46],{},"NaN"," (Not a Number).",[22,49,50],{},"Throwing an exception is another approach for unexpected \"no result\" cases.",[15,52,53],{},"Common approaches have their drawbacks:",[19,55,56,70,76],{},[22,57,58,59,62,63,66,67,69],{},"Return ",[26,60,61],{},"-1"," or ",[26,64,65],{},"nullptr",": semantically unclear, and ",[26,68,61],{}," might be a valid value.",[22,71,58,72,75],{},[26,73,74],{},"bool"," + Output parameters: Code redundancy requires variables to be declared first.",[22,77,78],{},"Throw exceptions: exceptions have overhead, and \"not found\" is generally not considered an exceptional case.",[15,80,81,84],{},[26,82,83],{},"std::optional\u003CT>"," elegantly expresses the semantics of \"may or may not have a value.\"",[10,86,88],{"id":87},"what-is-this-feature","What is this feature?",[15,90,91,93,94,97],{},[26,92,83],{}," is a template class that either contains a value of type ",[26,95,96],{},"T"," or is empty (has no value). It makes the \"might not have a value\" information part of the type, and the compiler helps you check.",[10,99,101],{"id":100},"c-standard-version","C++ standard version",[15,103,104],{},"C++17",[10,106,108],{"id":107},"required-header-files","Required header files",[110,111,116],"pre",{"className":112,"code":113,"language":114,"meta":115,"style":115},"language-cpp shiki shiki-themes github-light github-dark","#include \u003Coptional>\n","cpp","",[26,117,118],{"__ignoreMap":115},[119,120,123,127],"span",{"class":121,"line":122},"line",1,[119,124,126],{"class":125},"szBVR","#include",[119,128,130],{"class":129},"sZZnC"," \u003Coptional>\n",[10,132,134],{"id":133},"basic-syntax","Basic Syntax",[110,136,138],{"className":112,"code":137,"language":114,"meta":115,"style":115},"std::optional\u003CT> opt;                    \u002F\u002F 默认空\nstd::optional\u003CT> opt = value;            \u002F\u002F 有值\nstd::optional\u003CT> opt = std::nullopt;     \u002F\u002F 显式空\n\n\u002F\u002F 检查是否有值\nif (opt.has_value()) { ... }\nif (opt) { ... }                         \u002F\u002F 隐式转 bool\n\n\u002F\u002F 获取值\nT val = opt.value();                     \u002F\u002F 若无值抛异常\nT val = *opt;                            \u002F\u002F 若无值是未定义行为\nT val = opt.value_or(default);           \u002F\u002F 若无值返回默认值\n",[26,139,140,165,190,216,223,229,244,255,260,266,286,302],{"__ignoreMap":115},[119,141,142,146,150,153,155,158,161],{"class":121,"line":122},[119,143,145],{"class":144},"sScJk","std",[119,147,149],{"class":148},"sVt8B","::optional",[119,151,152],{"class":125},"\u003C",[119,154,96],{"class":148},[119,156,157],{"class":125},">",[119,159,160],{"class":148}," opt;",[119,162,164],{"class":163},"sJ8bj","                    \u002F\u002F 默认空\n",[119,166,168,170,172,174,176,178,181,184,187],{"class":121,"line":167},2,[119,169,145],{"class":144},[119,171,149],{"class":148},[119,173,152],{"class":125},[119,175,96],{"class":148},[119,177,157],{"class":125},[119,179,180],{"class":148}," opt ",[119,182,183],{"class":125},"=",[119,185,186],{"class":148}," value;",[119,188,189],{"class":163},"            \u002F\u002F 有值\n",[119,191,193,195,197,199,201,203,205,207,210,213],{"class":121,"line":192},3,[119,194,145],{"class":144},[119,196,149],{"class":148},[119,198,152],{"class":125},[119,200,96],{"class":148},[119,202,157],{"class":125},[119,204,180],{"class":148},[119,206,183],{"class":125},[119,208,209],{"class":144}," std",[119,211,212],{"class":148},"::nullopt;",[119,214,215],{"class":163},"     \u002F\u002F 显式空\n",[119,217,219],{"class":121,"line":218},4,[119,220,222],{"emptyLinePlaceholder":221},true,"\n",[119,224,226],{"class":121,"line":225},5,[119,227,228],{"class":163},"\u002F\u002F 检查是否有值\n",[119,230,232,235,238,241],{"class":121,"line":231},6,[119,233,234],{"class":125},"if",[119,236,237],{"class":148}," (opt.",[119,239,240],{"class":144},"has_value",[119,242,243],{"class":148},"()) { ... }\n",[119,245,247,249,252],{"class":121,"line":246},7,[119,248,234],{"class":125},[119,250,251],{"class":148}," (opt) { ... }",[119,253,254],{"class":163},"                         \u002F\u002F 隐式转 bool\n",[119,256,258],{"class":121,"line":257},8,[119,259,222],{"emptyLinePlaceholder":221},[119,261,263],{"class":121,"line":262},9,[119,264,265],{"class":163},"\u002F\u002F 获取值\n",[119,267,269,272,274,277,280,283],{"class":121,"line":268},10,[119,270,271],{"class":148},"T val ",[119,273,183],{"class":125},[119,275,276],{"class":148}," opt.",[119,278,279],{"class":144},"value",[119,281,282],{"class":148},"();",[119,284,285],{"class":163},"                     \u002F\u002F 若无值抛异常\n",[119,287,289,291,293,296,299],{"class":121,"line":288},11,[119,290,271],{"class":148},[119,292,183],{"class":125},[119,294,295],{"class":125}," *",[119,297,298],{"class":148},"opt;",[119,300,301],{"class":163},"                            \u002F\u002F 若无值是未定义行为\n",[119,303,305,307,309,311,314,317,320,323],{"class":121,"line":304},12,[119,306,271],{"class":148},[119,308,183],{"class":125},[119,310,276],{"class":148},[119,312,313],{"class":144},"value_or",[119,315,316],{"class":148},"(",[119,318,319],{"class":125},"default",[119,321,322],{"class":148},");",[119,324,325],{"class":163},"           \u002F\u002F 若无值返回默认值\n",[10,327,329],{"id":328},"common-usage","Common Usage",[331,332,333,347],"table",{},[334,335,336],"thead",{},[337,338,339,344],"tr",{},[340,341,343],"th",{"align":342},"left","Operation",[340,345,346],{"align":342},"Explanation",[348,349,350,361,375,385,395,405],"tbody",{},[337,351,352,358],{},[353,354,355],"td",{"align":342},[26,356,357],{},"opt.has_value()",[353,359,360],{"align":342},"whether there is a value",[337,362,363,368],{},[353,364,365],{"align":342},[26,366,367],{},"opt.value()",[353,369,370,371,374],{"align":342},"Retrieve value (throw ",[26,372,373],{},"std::bad_optional_access"," if no value)",[337,376,377,382],{},[353,378,379],{"align":342},[26,380,381],{},"*opt",[353,383,384],{"align":342},"Retrieve the value (undefined behavior if no value exists - check first!)",[337,386,387,392],{},[353,388,389],{"align":342},[26,390,391],{},"opt.value_or(default)",[353,393,394],{"align":342},"Get value, return default value if none exists.",[337,396,397,402],{},[353,398,399],{"align":342},[26,400,401],{},"opt = std::nullopt",[353,403,404],{"align":342},"Clear",[337,406,407,412],{},[353,408,409],{"align":342},[26,410,411],{},"opt.emplace(args...)",[353,413,414],{"align":342},"in-place construct value",[10,416,418],{"id":417},"example-code","Example code",[420,421,423],"h3",{"id":422},"example-1-using-optional-to-represent-might-not-be-found","Example 1: Using optional to represent \"might not be found\"",[110,425,427],{"className":112,"code":426,"language":114,"meta":115,"style":115},"#include \u003Ciostream>\n#include \u003Coptional>\n#include \u003Cstring>\n#include \u003Cvector>\n\n\u002F\u002F 在 vector 中查找元素，返回位置（可能找不到）\nstd::optional\u003Csize_t> find_index(const std::vector\u003Cstd::string>& v,\n                                  const std::string& target)\n{\n    for (size_t i = 0; i \u003C v.size(); ++i)\n    {\n        if (v[i] == target)\n        {\n            return i;  \u002F\u002F 找到了\n        }\n    }\n    return std::nullopt;  \u002F\u002F 没找到\n}\n\nint main()\n{\n    std::vector\u003Cstd::string> names = {\"Alice\", \"Bob\", \"Charlie\"};\n\n    auto idx = find_index(names, \"Bob\");\n    if (idx)\n    {\n        std::cout \u003C\u003C \"Found Bob at index \" \u003C\u003C *idx \u003C\u003C \"\\n\";\n    }\n\n    auto idx2 = find_index(names, \"David\");\n    if (!idx2)\n    {\n        std::cout \u003C\u003C \"David not found\\n\";\n    }\n\n    \u002F\u002F 使用 value_or 提供默认值\n    std::cout \u003C\u003C \"Result: \" \u003C\u003C idx.value_or(999) \u003C\u003C \"\\n\";\n    std::cout \u003C\u003C \"Result2: \" \u003C\u003C idx2.value_or(999) \u003C\u003C \"\\n\";\n\n    return 0;\n}\n",[26,428,429,436,442,449,456,460,465,519,538,543,582,587,601,607,619,625,631,644,650,655,667,672,715,720,742,751,756,793,798,803,822,835,840,858,863,868,874,911,946,951,960],{"__ignoreMap":115},[119,430,431,433],{"class":121,"line":122},[119,432,126],{"class":125},[119,434,435],{"class":129}," \u003Ciostream>\n",[119,437,438,440],{"class":121,"line":167},[119,439,126],{"class":125},[119,441,130],{"class":129},[119,443,444,446],{"class":121,"line":192},[119,445,126],{"class":125},[119,447,448],{"class":129}," \u003Cstring>\n",[119,450,451,453],{"class":121,"line":218},[119,452,126],{"class":125},[119,454,455],{"class":129}," \u003Cvector>\n",[119,457,458],{"class":121,"line":225},[119,459,222],{"emptyLinePlaceholder":221},[119,461,462],{"class":121,"line":231},[119,463,464],{"class":163},"\u002F\u002F 在 vector 中查找元素，返回位置（可能找不到）\n",[119,466,467,469,472,475,477,480,483,486,488,491,493,495,498,500,502,504,507,509,512,516],{"class":121,"line":246},[119,468,145],{"class":144},[119,470,471],{"class":148},"::",[119,473,474],{"class":144},"optional",[119,476,152],{"class":148},[119,478,479],{"class":125},"size_t",[119,481,482],{"class":148},"> ",[119,484,485],{"class":144},"find_index",[119,487,316],{"class":148},[119,489,490],{"class":125},"const",[119,492,209],{"class":144},[119,494,471],{"class":148},[119,496,497],{"class":144},"vector",[119,499,152],{"class":148},[119,501,145],{"class":144},[119,503,471],{"class":148},[119,505,506],{"class":144},"string",[119,508,157],{"class":148},[119,510,511],{"class":125},"&",[119,513,515],{"class":514},"s4XuR"," v",[119,517,518],{"class":148},",\n",[119,520,521,524,526,528,530,532,535],{"class":121,"line":257},[119,522,523],{"class":125},"                                  const",[119,525,209],{"class":144},[119,527,471],{"class":148},[119,529,506],{"class":144},[119,531,511],{"class":125},[119,533,534],{"class":514}," target",[119,536,537],{"class":148},")\n",[119,539,540],{"class":121,"line":262},[119,541,542],{"class":148},"{\n",[119,544,545,548,551,553,556,558,562,565,567,570,573,576,579],{"class":121,"line":268},[119,546,547],{"class":125},"    for",[119,549,550],{"class":148}," (",[119,552,479],{"class":125},[119,554,555],{"class":148}," i ",[119,557,183],{"class":125},[119,559,561],{"class":560},"sj4cs"," 0",[119,563,564],{"class":148},"; i ",[119,566,152],{"class":125},[119,568,569],{"class":148}," v.",[119,571,572],{"class":144},"size",[119,574,575],{"class":148},"(); ",[119,577,578],{"class":125},"++",[119,580,581],{"class":148},"i)\n",[119,583,584],{"class":121,"line":288},[119,585,586],{"class":148},"    {\n",[119,588,589,592,595,598],{"class":121,"line":304},[119,590,591],{"class":125},"        if",[119,593,594],{"class":148}," (v[i] ",[119,596,597],{"class":125},"==",[119,599,600],{"class":148}," target)\n",[119,602,604],{"class":121,"line":603},13,[119,605,606],{"class":148},"        {\n",[119,608,610,613,616],{"class":121,"line":609},14,[119,611,612],{"class":125},"            return",[119,614,615],{"class":148}," i;",[119,617,618],{"class":163},"  \u002F\u002F 找到了\n",[119,620,622],{"class":121,"line":621},15,[119,623,624],{"class":148},"        }\n",[119,626,628],{"class":121,"line":627},16,[119,629,630],{"class":148},"    }\n",[119,632,634,637,639,641],{"class":121,"line":633},17,[119,635,636],{"class":125},"    return",[119,638,209],{"class":144},[119,640,212],{"class":148},[119,642,643],{"class":163},"  \u002F\u002F 没找到\n",[119,645,647],{"class":121,"line":646},18,[119,648,649],{"class":148},"}\n",[119,651,653],{"class":121,"line":652},19,[119,654,222],{"emptyLinePlaceholder":221},[119,656,658,661,664],{"class":121,"line":657},20,[119,659,660],{"class":125},"int",[119,662,663],{"class":144}," main",[119,665,666],{"class":148},"()\n",[119,668,670],{"class":121,"line":669},21,[119,671,542],{"class":148},[119,673,675,678,681,683,685,688,690,693,695,698,701,704,707,709,712],{"class":121,"line":674},22,[119,676,677],{"class":144},"    std",[119,679,680],{"class":148},"::vector",[119,682,152],{"class":125},[119,684,145],{"class":144},[119,686,687],{"class":148},"::string",[119,689,157],{"class":125},[119,691,692],{"class":148}," names ",[119,694,183],{"class":125},[119,696,697],{"class":148}," {",[119,699,700],{"class":129},"\"Alice\"",[119,702,703],{"class":148},", ",[119,705,706],{"class":129},"\"Bob\"",[119,708,703],{"class":148},[119,710,711],{"class":129},"\"Charlie\"",[119,713,714],{"class":148},"};\n",[119,716,718],{"class":121,"line":717},23,[119,719,222],{"emptyLinePlaceholder":221},[119,721,723,726,729,731,734,737,739],{"class":121,"line":722},24,[119,724,725],{"class":125},"    auto",[119,727,728],{"class":148}," idx ",[119,730,183],{"class":125},[119,732,733],{"class":144}," find_index",[119,735,736],{"class":148},"(names, ",[119,738,706],{"class":129},[119,740,741],{"class":148},");\n",[119,743,745,748],{"class":121,"line":744},25,[119,746,747],{"class":125},"    if",[119,749,750],{"class":148}," (idx)\n",[119,752,754],{"class":121,"line":753},26,[119,755,586],{"class":148},[119,757,759,762,765,768,771,774,776,779,781,784,787,790],{"class":121,"line":758},27,[119,760,761],{"class":144},"        std",[119,763,764],{"class":148},"::cout ",[119,766,767],{"class":125},"\u003C\u003C",[119,769,770],{"class":129}," \"Found Bob at index \"",[119,772,773],{"class":125}," \u003C\u003C",[119,775,295],{"class":125},[119,777,778],{"class":148},"idx ",[119,780,767],{"class":125},[119,782,783],{"class":129}," \"",[119,785,786],{"class":560},"\\n",[119,788,789],{"class":129},"\"",[119,791,792],{"class":148},";\n",[119,794,796],{"class":121,"line":795},28,[119,797,630],{"class":148},[119,799,801],{"class":121,"line":800},29,[119,802,222],{"emptyLinePlaceholder":221},[119,804,806,808,811,813,815,817,820],{"class":121,"line":805},30,[119,807,725],{"class":125},[119,809,810],{"class":148}," idx2 ",[119,812,183],{"class":125},[119,814,733],{"class":144},[119,816,736],{"class":148},[119,818,819],{"class":129},"\"David\"",[119,821,741],{"class":148},[119,823,825,827,829,832],{"class":121,"line":824},31,[119,826,747],{"class":125},[119,828,550],{"class":148},[119,830,831],{"class":125},"!",[119,833,834],{"class":148},"idx2)\n",[119,836,838],{"class":121,"line":837},32,[119,839,586],{"class":148},[119,841,843,845,847,849,852,854,856],{"class":121,"line":842},33,[119,844,761],{"class":144},[119,846,764],{"class":148},[119,848,767],{"class":125},[119,850,851],{"class":129}," \"David not found",[119,853,786],{"class":560},[119,855,789],{"class":129},[119,857,792],{"class":148},[119,859,861],{"class":121,"line":860},34,[119,862,630],{"class":148},[119,864,866],{"class":121,"line":865},35,[119,867,222],{"emptyLinePlaceholder":221},[119,869,871],{"class":121,"line":870},36,[119,872,873],{"class":163},"    \u002F\u002F 使用 value_or 提供默认值\n",[119,875,877,879,881,883,886,888,891,893,895,898,901,903,905,907,909],{"class":121,"line":876},37,[119,878,677],{"class":144},[119,880,764],{"class":148},[119,882,767],{"class":125},[119,884,885],{"class":129}," \"Result: \"",[119,887,773],{"class":125},[119,889,890],{"class":148}," idx.",[119,892,313],{"class":144},[119,894,316],{"class":148},[119,896,897],{"class":560},"999",[119,899,900],{"class":148},") ",[119,902,767],{"class":125},[119,904,783],{"class":129},[119,906,786],{"class":560},[119,908,789],{"class":129},[119,910,792],{"class":148},[119,912,914,916,918,920,923,925,928,930,932,934,936,938,940,942,944],{"class":121,"line":913},38,[119,915,677],{"class":144},[119,917,764],{"class":148},[119,919,767],{"class":125},[119,921,922],{"class":129}," \"Result2: \"",[119,924,773],{"class":125},[119,926,927],{"class":148}," idx2.",[119,929,313],{"class":144},[119,931,316],{"class":148},[119,933,897],{"class":560},[119,935,900],{"class":148},[119,937,767],{"class":125},[119,939,783],{"class":129},[119,941,786],{"class":560},[119,943,789],{"class":129},[119,945,792],{"class":148},[119,947,949],{"class":121,"line":948},39,[119,950,222],{"emptyLinePlaceholder":221},[119,952,954,956,958],{"class":121,"line":953},40,[119,955,636],{"class":125},[119,957,561],{"class":560},[119,959,792],{"class":148},[119,961,963],{"class":121,"line":962},41,[119,964,649],{"class":148},[15,966,967,971],{},[968,969,970],"strong",{},"Results","：",[110,973,978],{"className":974,"code":976,"language":977},[975],"language-text","Found Bob at index 1\nDavid not found\nResult: 1\nResult2: 999\n","text",[26,979,976],{"__ignoreMap":115},[420,981,983],{"id":982},"example-2-building-on-example-1-using-optional-to-represent-possibly-failed-conversion","Example 2: Building on Example 1, using optional to represent possibly failed conversion",[110,985,987],{"className":112,"code":986,"language":114,"meta":115,"style":115},"#include \u003Ciostream>\n#include \u003Coptional>\n#include \u003Cstring>\n\n\u002F\u002F 字符串转整数，失败返回 nullopt\nstd::optional\u003Cint> to_int(const std::string& s)\n{\n    try\n    {\n        size_t pos;\n        int val = std::stoi(s, &pos);\n        if (pos == s.size())  \u002F\u002F 整个字符串都被转换了\n        {\n            return val;\n        }\n    }\n    catch (...) {}\n    return std::nullopt;\n}\n\nint main()\n{\n    auto r1 = to_int(\"42\");\n    auto r2 = to_int(\"3.14\");  \u002F\u002F 带小数点\n    auto r3 = to_int(\"hello\"); \u002F\u002F 不是数字\n\n    std::cout \u003C\u003C \"\\\"42\\\" -> \" \u003C\u003C r1.value_or(-1) \u003C\u003C \"\\n\";\n    std::cout \u003C\u003C \"\\\"3.14\\\" -> \" \u003C\u003C r2.value_or(-1) \u003C\u003C \"\\n\";\n    std::cout \u003C\u003C \"\\\"hello\\\" -> \" \u003C\u003C r3.value_or(-1) \u003C\u003C \"\\n\";\n\n    \u002F\u002F 用 if 检查\n    if (auto r = to_int(\"100\"); r)\n    {\n        std::cout \u003C\u003C \"100 * 2 = \" \u003C\u003C (*r * 2) \u003C\u003C \"\\n\";\n    }\n\n    return 0;\n}\n",[26,988,989,995,1001,1007,1011,1016,1050,1054,1059,1063,1071,1096,1116,1120,1127,1131,1135,1143,1152,1156,1160,1168,1172,1191,1212,1233,1237,1285,1329,1373,1377,1382,1406,1410,1448,1452,1456,1464],{"__ignoreMap":115},[119,990,991,993],{"class":121,"line":122},[119,992,126],{"class":125},[119,994,435],{"class":129},[119,996,997,999],{"class":121,"line":167},[119,998,126],{"class":125},[119,1000,130],{"class":129},[119,1002,1003,1005],{"class":121,"line":192},[119,1004,126],{"class":125},[119,1006,448],{"class":129},[119,1008,1009],{"class":121,"line":218},[119,1010,222],{"emptyLinePlaceholder":221},[119,1012,1013],{"class":121,"line":225},[119,1014,1015],{"class":163},"\u002F\u002F 字符串转整数，失败返回 nullopt\n",[119,1017,1018,1020,1022,1024,1026,1028,1030,1033,1035,1037,1039,1041,1043,1045,1048],{"class":121,"line":231},[119,1019,145],{"class":144},[119,1021,471],{"class":148},[119,1023,474],{"class":144},[119,1025,152],{"class":148},[119,1027,660],{"class":125},[119,1029,482],{"class":148},[119,1031,1032],{"class":144},"to_int",[119,1034,316],{"class":148},[119,1036,490],{"class":125},[119,1038,209],{"class":144},[119,1040,471],{"class":148},[119,1042,506],{"class":144},[119,1044,511],{"class":125},[119,1046,1047],{"class":514}," s",[119,1049,537],{"class":148},[119,1051,1052],{"class":121,"line":246},[119,1053,542],{"class":148},[119,1055,1056],{"class":121,"line":257},[119,1057,1058],{"class":125},"    try\n",[119,1060,1061],{"class":121,"line":262},[119,1062,586],{"class":148},[119,1064,1065,1068],{"class":121,"line":268},[119,1066,1067],{"class":125},"        size_t",[119,1069,1070],{"class":148}," pos;\n",[119,1072,1073,1076,1079,1081,1083,1085,1088,1091,1093],{"class":121,"line":288},[119,1074,1075],{"class":125},"        int",[119,1077,1078],{"class":148}," val ",[119,1080,183],{"class":125},[119,1082,209],{"class":144},[119,1084,471],{"class":148},[119,1086,1087],{"class":144},"stoi",[119,1089,1090],{"class":148},"(s, ",[119,1092,511],{"class":125},[119,1094,1095],{"class":148},"pos);\n",[119,1097,1098,1100,1103,1105,1108,1110,1113],{"class":121,"line":304},[119,1099,591],{"class":125},[119,1101,1102],{"class":148}," (pos ",[119,1104,597],{"class":125},[119,1106,1107],{"class":148}," s.",[119,1109,572],{"class":144},[119,1111,1112],{"class":148},"())",[119,1114,1115],{"class":163},"  \u002F\u002F 整个字符串都被转换了\n",[119,1117,1118],{"class":121,"line":603},[119,1119,606],{"class":148},[119,1121,1122,1124],{"class":121,"line":609},[119,1123,612],{"class":125},[119,1125,1126],{"class":148}," val;\n",[119,1128,1129],{"class":121,"line":621},[119,1130,624],{"class":148},[119,1132,1133],{"class":121,"line":627},[119,1134,630],{"class":148},[119,1136,1137,1140],{"class":121,"line":633},[119,1138,1139],{"class":125},"    catch",[119,1141,1142],{"class":148}," (...) {}\n",[119,1144,1145,1147,1149],{"class":121,"line":646},[119,1146,636],{"class":125},[119,1148,209],{"class":144},[119,1150,1151],{"class":148},"::nullopt;\n",[119,1153,1154],{"class":121,"line":652},[119,1155,649],{"class":148},[119,1157,1158],{"class":121,"line":657},[119,1159,222],{"emptyLinePlaceholder":221},[119,1161,1162,1164,1166],{"class":121,"line":669},[119,1163,660],{"class":125},[119,1165,663],{"class":144},[119,1167,666],{"class":148},[119,1169,1170],{"class":121,"line":674},[119,1171,542],{"class":148},[119,1173,1174,1176,1179,1181,1184,1186,1189],{"class":121,"line":717},[119,1175,725],{"class":125},[119,1177,1178],{"class":148}," r1 ",[119,1180,183],{"class":125},[119,1182,1183],{"class":144}," to_int",[119,1185,316],{"class":148},[119,1187,1188],{"class":129},"\"42\"",[119,1190,741],{"class":148},[119,1192,1193,1195,1198,1200,1202,1204,1207,1209],{"class":121,"line":722},[119,1194,725],{"class":125},[119,1196,1197],{"class":148}," r2 ",[119,1199,183],{"class":125},[119,1201,1183],{"class":144},[119,1203,316],{"class":148},[119,1205,1206],{"class":129},"\"3.14\"",[119,1208,322],{"class":148},[119,1210,1211],{"class":163},"  \u002F\u002F 带小数点\n",[119,1213,1214,1216,1219,1221,1223,1225,1228,1230],{"class":121,"line":744},[119,1215,725],{"class":125},[119,1217,1218],{"class":148}," r3 ",[119,1220,183],{"class":125},[119,1222,1183],{"class":144},[119,1224,316],{"class":148},[119,1226,1227],{"class":129},"\"hello\"",[119,1229,322],{"class":148},[119,1231,1232],{"class":163}," \u002F\u002F 不是数字\n",[119,1234,1235],{"class":121,"line":753},[119,1236,222],{"emptyLinePlaceholder":221},[119,1238,1239,1241,1243,1245,1247,1250,1253,1255,1258,1260,1263,1265,1267,1270,1273,1275,1277,1279,1281,1283],{"class":121,"line":758},[119,1240,677],{"class":144},[119,1242,764],{"class":148},[119,1244,767],{"class":125},[119,1246,783],{"class":129},[119,1248,1249],{"class":560},"\\\"",[119,1251,1252],{"class":129},"42",[119,1254,1249],{"class":560},[119,1256,1257],{"class":129}," -> \"",[119,1259,773],{"class":125},[119,1261,1262],{"class":148}," r1.",[119,1264,313],{"class":144},[119,1266,316],{"class":148},[119,1268,1269],{"class":125},"-",[119,1271,1272],{"class":560},"1",[119,1274,900],{"class":148},[119,1276,767],{"class":125},[119,1278,783],{"class":129},[119,1280,786],{"class":560},[119,1282,789],{"class":129},[119,1284,792],{"class":148},[119,1286,1287,1289,1291,1293,1295,1297,1300,1302,1304,1306,1309,1311,1313,1315,1317,1319,1321,1323,1325,1327],{"class":121,"line":795},[119,1288,677],{"class":144},[119,1290,764],{"class":148},[119,1292,767],{"class":125},[119,1294,783],{"class":129},[119,1296,1249],{"class":560},[119,1298,1299],{"class":129},"3.14",[119,1301,1249],{"class":560},[119,1303,1257],{"class":129},[119,1305,773],{"class":125},[119,1307,1308],{"class":148}," r2.",[119,1310,313],{"class":144},[119,1312,316],{"class":148},[119,1314,1269],{"class":125},[119,1316,1272],{"class":560},[119,1318,900],{"class":148},[119,1320,767],{"class":125},[119,1322,783],{"class":129},[119,1324,786],{"class":560},[119,1326,789],{"class":129},[119,1328,792],{"class":148},[119,1330,1331,1333,1335,1337,1339,1341,1344,1346,1348,1350,1353,1355,1357,1359,1361,1363,1365,1367,1369,1371],{"class":121,"line":800},[119,1332,677],{"class":144},[119,1334,764],{"class":148},[119,1336,767],{"class":125},[119,1338,783],{"class":129},[119,1340,1249],{"class":560},[119,1342,1343],{"class":129},"hello",[119,1345,1249],{"class":560},[119,1347,1257],{"class":129},[119,1349,773],{"class":125},[119,1351,1352],{"class":148}," r3.",[119,1354,313],{"class":144},[119,1356,316],{"class":148},[119,1358,1269],{"class":125},[119,1360,1272],{"class":560},[119,1362,900],{"class":148},[119,1364,767],{"class":125},[119,1366,783],{"class":129},[119,1368,786],{"class":560},[119,1370,789],{"class":129},[119,1372,792],{"class":148},[119,1374,1375],{"class":121,"line":805},[119,1376,222],{"emptyLinePlaceholder":221},[119,1378,1379],{"class":121,"line":824},[119,1380,1381],{"class":163},"    \u002F\u002F 用 if 检查\n",[119,1383,1384,1386,1388,1391,1394,1396,1398,1400,1403],{"class":121,"line":837},[119,1385,747],{"class":125},[119,1387,550],{"class":148},[119,1389,1390],{"class":125},"auto",[119,1392,1393],{"class":148}," r ",[119,1395,183],{"class":125},[119,1397,1183],{"class":144},[119,1399,316],{"class":148},[119,1401,1402],{"class":129},"\"100\"",[119,1404,1405],{"class":148},"); r)\n",[119,1407,1408],{"class":121,"line":842},[119,1409,586],{"class":148},[119,1411,1412,1414,1416,1418,1421,1423,1425,1428,1431,1433,1436,1438,1440,1442,1444,1446],{"class":121,"line":860},[119,1413,761],{"class":144},[119,1415,764],{"class":148},[119,1417,767],{"class":125},[119,1419,1420],{"class":129}," \"100 * 2 = \"",[119,1422,773],{"class":125},[119,1424,550],{"class":148},[119,1426,1427],{"class":125},"*",[119,1429,1430],{"class":148},"r ",[119,1432,1427],{"class":125},[119,1434,1435],{"class":560}," 2",[119,1437,900],{"class":148},[119,1439,767],{"class":125},[119,1441,783],{"class":129},[119,1443,786],{"class":560},[119,1445,789],{"class":129},[119,1447,792],{"class":148},[119,1449,1450],{"class":121,"line":865},[119,1451,630],{"class":148},[119,1453,1454],{"class":121,"line":870},[119,1455,222],{"emptyLinePlaceholder":221},[119,1457,1458,1460,1462],{"class":121,"line":876},[119,1459,636],{"class":125},[119,1461,561],{"class":560},[119,1463,792],{"class":148},[119,1465,1466],{"class":121,"line":913},[119,1467,649],{"class":148},[15,1469,1470,971],{},[968,1471,970],{},[110,1473,1476],{"className":1474,"code":1475,"language":977},[975],"\"42\" -> 42\n\"3.14\" -> -1\n\"hello\" -> -1\n100 * 2 = 200\n",[26,1477,1475],{"__ignoreMap":115},[15,1479,1480,1481,1484],{},"这里的",[26,1482,1483],{},"std::stoi","函数，他的转换规则是：从字符串开头（必须数字为开头）开始，尽可能解析一个整数；只要能解析出一部分，就算 stoi 本身成功。",[15,1486,1487],{},"For example:",[19,1489,1490],{},[22,1491,1492,1493,1496,1497,1500],{},"转换",[26,1494,1495],{},"\"123\"","字符串，结果是成功的，为",[26,1498,1499],{},"123","。",[19,1502,1503],{},[22,1504,1492,1505,1496,1508,1500],{},[26,1506,1507],{},"\"123abc\"",[26,1509,1499],{},[19,1511,1512],{},[22,1513,1492,1514,1496,1516,1500],{},[26,1515,1206],{},[26,1517,1518],{},"3",[15,1520,1521,1522,1525,1526,1528],{},"而这个函数的",[26,1523,1524],{},"pos","参数表示",[26,1527,1483],{},"解析到了字符串的哪个位置，也就是第一个没有被转换的字符的位置。",[15,1530,1487],{},[19,1532,1533],{},[22,1534,1492,1535,1537,1538,1541,1542,1544],{},[26,1536,1507],{},"字符串，",[26,1539,1540],{},"val","是123,但",[26,1543,1524],{},"是3。（序号从0开始，如下图）",[110,1546,1549],{"className":1547,"code":1548,"language":977,"meta":115},[975],"123abc\n012345\n   ↑\n   pos = 3\n",[26,1550,1548],{"__ignoreMap":115},[19,1552,1553],{},[22,1554,1492,1555,1537,1557,1559,1560,1562],{},[26,1556,1206],{},[26,1558,1540],{},"是3,但",[26,1561,1524],{},"是1。（序号从0开始，如下图）",[110,1564,1567],{"className":1565,"code":1566,"language":977,"meta":115},[975],"3.14\n0123\n ↑\n pos = 1\n",[26,1568,1566],{"__ignoreMap":115},[15,1570,1480,1571,1574,1575,1500],{},[26,1572,1573],{},"pos == s.size()","是不成立的，所以返回",[26,1576,1577],{},"std::nullopt",[420,1579,1581],{"id":1580},"example-3-building-on-example-2-using-optional-as-a-struct-member-a-field-that-may-not-exist","Example 3: Building on Example 2, using optional as a struct member (a field that \"may not exist\")",[110,1583,1585],{"className":112,"code":1584,"language":114,"meta":115,"style":115},"#include \u003Ciostream>\n#include \u003Coptional>\n#include \u003Cstring>\n\nstruct Person\n{\n    std::string name;\n    int age;\n    std::optional\u003Cstd::string> nickname;  \u002F\u002F 可能没有昵称\n    std::optional\u003Cint> score;              \u002F\u002F 成绩可能尚未录入\n};\n\nvoid print_person(const Person& p)\n{\n    std::cout \u003C\u003C \"Name: \" \u003C\u003C p.name \u003C\u003C \"\\n\";\n    std::cout \u003C\u003C \"Age: \" \u003C\u003C p.age \u003C\u003C \"\\n\";\n\n    std::cout \u003C\u003C \"Nickname: \" \u003C\u003C p.nickname.value_or(\"(none)\") \u003C\u003C \"\\n\";\n    std::cout \u003C\u003C \"Score: \" \u003C\u003C p.score.value_or(-1) \u003C\u003C \"\\n\";\n\n    std::cout \u003C\u003C \"---\\n\";\n}\n\nint main()\n{\n    Person p1{\"Alice\", 20, \"Ali\", 95};\n    Person p2{\"Bob\", 22, std::nullopt, std::nullopt};  \u002F\u002F 没有昵称和成绩\n\n    print_person(p1);\n    print_person(p2);\n\n    return 0;\n}\n",[26,1586,1587,1593,1599,1605,1609,1617,1621,1628,1636,1656,1671,1675,1679,1701,1705,1731,1757,1761,1796,1832,1836,1853,1857,1861,1869,1873,1897,1924,1928,1936,1943,1947,1955],{"__ignoreMap":115},[119,1588,1589,1591],{"class":121,"line":122},[119,1590,126],{"class":125},[119,1592,435],{"class":129},[119,1594,1595,1597],{"class":121,"line":167},[119,1596,126],{"class":125},[119,1598,130],{"class":129},[119,1600,1601,1603],{"class":121,"line":192},[119,1602,126],{"class":125},[119,1604,448],{"class":129},[119,1606,1607],{"class":121,"line":218},[119,1608,222],{"emptyLinePlaceholder":221},[119,1610,1611,1614],{"class":121,"line":225},[119,1612,1613],{"class":125},"struct",[119,1615,1616],{"class":144}," Person\n",[119,1618,1619],{"class":121,"line":231},[119,1620,542],{"class":148},[119,1622,1623,1625],{"class":121,"line":246},[119,1624,677],{"class":144},[119,1626,1627],{"class":148},"::string name;\n",[119,1629,1630,1633],{"class":121,"line":257},[119,1631,1632],{"class":125},"    int",[119,1634,1635],{"class":148}," age;\n",[119,1637,1638,1640,1642,1644,1646,1648,1650,1653],{"class":121,"line":262},[119,1639,677],{"class":144},[119,1641,149],{"class":148},[119,1643,152],{"class":125},[119,1645,145],{"class":144},[119,1647,687],{"class":148},[119,1649,157],{"class":125},[119,1651,1652],{"class":148}," nickname;",[119,1654,1655],{"class":163},"  \u002F\u002F 可能没有昵称\n",[119,1657,1658,1660,1662,1665,1668],{"class":121,"line":268},[119,1659,677],{"class":144},[119,1661,149],{"class":148},[119,1663,1664],{"class":125},"\u003Cint>",[119,1666,1667],{"class":148}," score;",[119,1669,1670],{"class":163},"              \u002F\u002F 成绩可能尚未录入\n",[119,1672,1673],{"class":121,"line":288},[119,1674,714],{"class":148},[119,1676,1677],{"class":121,"line":304},[119,1678,222],{"emptyLinePlaceholder":221},[119,1680,1681,1684,1687,1689,1691,1694,1696,1699],{"class":121,"line":603},[119,1682,1683],{"class":125},"void",[119,1685,1686],{"class":144}," print_person",[119,1688,316],{"class":148},[119,1690,490],{"class":125},[119,1692,1693],{"class":144}," Person",[119,1695,511],{"class":125},[119,1697,1698],{"class":514}," p",[119,1700,537],{"class":148},[119,1702,1703],{"class":121,"line":609},[119,1704,542],{"class":148},[119,1706,1707,1709,1711,1713,1716,1718,1721,1723,1725,1727,1729],{"class":121,"line":621},[119,1708,677],{"class":144},[119,1710,764],{"class":148},[119,1712,767],{"class":125},[119,1714,1715],{"class":129}," \"Name: \"",[119,1717,773],{"class":125},[119,1719,1720],{"class":148}," p.name ",[119,1722,767],{"class":125},[119,1724,783],{"class":129},[119,1726,786],{"class":560},[119,1728,789],{"class":129},[119,1730,792],{"class":148},[119,1732,1733,1735,1737,1739,1742,1744,1747,1749,1751,1753,1755],{"class":121,"line":627},[119,1734,677],{"class":144},[119,1736,764],{"class":148},[119,1738,767],{"class":125},[119,1740,1741],{"class":129}," \"Age: \"",[119,1743,773],{"class":125},[119,1745,1746],{"class":148}," p.age ",[119,1748,767],{"class":125},[119,1750,783],{"class":129},[119,1752,786],{"class":560},[119,1754,789],{"class":129},[119,1756,792],{"class":148},[119,1758,1759],{"class":121,"line":633},[119,1760,222],{"emptyLinePlaceholder":221},[119,1762,1763,1765,1767,1769,1772,1774,1777,1779,1781,1784,1786,1788,1790,1792,1794],{"class":121,"line":646},[119,1764,677],{"class":144},[119,1766,764],{"class":148},[119,1768,767],{"class":125},[119,1770,1771],{"class":129}," \"Nickname: \"",[119,1773,773],{"class":125},[119,1775,1776],{"class":148}," p.nickname.",[119,1778,313],{"class":144},[119,1780,316],{"class":148},[119,1782,1783],{"class":129},"\"(none)\"",[119,1785,900],{"class":148},[119,1787,767],{"class":125},[119,1789,783],{"class":129},[119,1791,786],{"class":560},[119,1793,789],{"class":129},[119,1795,792],{"class":148},[119,1797,1798,1800,1802,1804,1807,1809,1812,1814,1816,1818,1820,1822,1824,1826,1828,1830],{"class":121,"line":652},[119,1799,677],{"class":144},[119,1801,764],{"class":148},[119,1803,767],{"class":125},[119,1805,1806],{"class":129}," \"Score: \"",[119,1808,773],{"class":125},[119,1810,1811],{"class":148}," p.score.",[119,1813,313],{"class":144},[119,1815,316],{"class":148},[119,1817,1269],{"class":125},[119,1819,1272],{"class":560},[119,1821,900],{"class":148},[119,1823,767],{"class":125},[119,1825,783],{"class":129},[119,1827,786],{"class":560},[119,1829,789],{"class":129},[119,1831,792],{"class":148},[119,1833,1834],{"class":121,"line":657},[119,1835,222],{"emptyLinePlaceholder":221},[119,1837,1838,1840,1842,1844,1847,1849,1851],{"class":121,"line":669},[119,1839,677],{"class":144},[119,1841,764],{"class":148},[119,1843,767],{"class":125},[119,1845,1846],{"class":129}," \"---",[119,1848,786],{"class":560},[119,1850,789],{"class":129},[119,1852,792],{"class":148},[119,1854,1855],{"class":121,"line":674},[119,1856,649],{"class":148},[119,1858,1859],{"class":121,"line":717},[119,1860,222],{"emptyLinePlaceholder":221},[119,1862,1863,1865,1867],{"class":121,"line":722},[119,1864,660],{"class":125},[119,1866,663],{"class":144},[119,1868,666],{"class":148},[119,1870,1871],{"class":121,"line":744},[119,1872,542],{"class":148},[119,1874,1875,1878,1880,1882,1885,1887,1890,1892,1895],{"class":121,"line":753},[119,1876,1877],{"class":148},"    Person p1{",[119,1879,700],{"class":129},[119,1881,703],{"class":148},[119,1883,1884],{"class":560},"20",[119,1886,703],{"class":148},[119,1888,1889],{"class":129},"\"Ali\"",[119,1891,703],{"class":148},[119,1893,1894],{"class":560},"95",[119,1896,714],{"class":148},[119,1898,1899,1902,1904,1906,1909,1911,1913,1916,1918,1921],{"class":121,"line":758},[119,1900,1901],{"class":148},"    Person p2{",[119,1903,706],{"class":129},[119,1905,703],{"class":148},[119,1907,1908],{"class":560},"22",[119,1910,703],{"class":148},[119,1912,145],{"class":144},[119,1914,1915],{"class":148},"::nullopt, ",[119,1917,145],{"class":144},[119,1919,1920],{"class":148},"::nullopt};",[119,1922,1923],{"class":163},"  \u002F\u002F 没有昵称和成绩\n",[119,1925,1926],{"class":121,"line":795},[119,1927,222],{"emptyLinePlaceholder":221},[119,1929,1930,1933],{"class":121,"line":800},[119,1931,1932],{"class":144},"    print_person",[119,1934,1935],{"class":148},"(p1);\n",[119,1937,1938,1940],{"class":121,"line":805},[119,1939,1932],{"class":144},[119,1941,1942],{"class":148},"(p2);\n",[119,1944,1945],{"class":121,"line":824},[119,1946,222],{"emptyLinePlaceholder":221},[119,1948,1949,1951,1953],{"class":121,"line":837},[119,1950,636],{"class":125},[119,1952,561],{"class":560},[119,1954,792],{"class":148},[119,1956,1957],{"class":121,"line":842},[119,1958,649],{"class":148},[15,1960,1961,971],{},[968,1962,970],{},[110,1964,1967],{"className":1965,"code":1966,"language":977},[975],"Name: Alice\nAge: 20\nNickname: Ali\nScore: 95\n---\nName: Bob\nAge: 22\nNickname: (none)\nScore: -1\n---\n",[26,1968,1966],{"__ignoreMap":115},[10,1970,1972],{"id":1971},"runtime-results","runtime results",[15,1974,1975],{},"See the \"running results\" for each example above.",[10,1977,1979],{"id":1978},"key-syntax-explanation-in-the-example","Key syntax explanation in the example",[15,1981,1982],{},"|Here is the translation of the provided Simplified Chinese Markdown fragment into natural American English, following all specified rules.",[1984,1985],"hr",{},[331,1987,1988,2009],{},[334,1989,1990],{},[337,1991,1992,1997,2000,2003,2006],{},[340,1993,1994],{"align":342},[968,1995,1996],{},"Example",[340,1998,1999],{"align":342},"Discusses what",[340,2001,2002],{"align":342},"Newly emerged syntax",[340,2004,2005],{"align":342},"Why write it this way",[340,2007,2008],{"align":342},"Precautions",[348,2010,2011,2045,2069],{},[337,2012,2013,2016,2019,2037,2040],{},[353,2014,2015],{"align":342},"Example 1",[353,2017,2018],{"align":342},"optionally return search results",[353,2020,2021,2024,2025,2028,2029,2028,2032,2028,2034],{"align":342},[26,2022,2023],{},"return i;"," \u002F ",[26,2026,2027],{},"return std::nullopt;","、",[26,2030,2031],{},"if(opt)",[26,2033,381],{},[26,2035,2036],{},"value_or()",[353,2038,2039],{"align":342},"Optional makes \"no value\" a normal return, providing clear semantics.",[353,2041,2042,2043],{"align":342},"Ensure there is a value before dereferencing ",[26,2044,381],{},[337,2046,2047,2050,2053,2059,2062],{},[353,2048,2049],{"align":342},"Example 2",[353,2051,2052],{"align":342},"optional indicates conversion failure",[353,2054,2055,2058],{"align":342},[26,2056,2057],{},"try\u002Fcatch"," packaging + optional",[353,2060,2061],{"align":342},"Converting exceptions to empty optionals makes it easier for callers to handle.",[353,2063,2064,2065,2068],{"align":342},"You can also use ",[26,2066,2067],{},"std::from_chars"," (C++17) more efficiently.",[337,2070,2071,2074,2077,2083,2086],{},[353,2072,2073],{"align":342},"Example 3",[353,2075,2076],{"align":342},"optional as a member variable",[353,2078,2079,2082],{"align":342},[26,2080,2081],{},"std::optional\u003Cstd::string>"," member",[353,2084,2085],{"align":342},"\"optional fields\" is clearer than using magic values (like -1).",[353,2087,2088],{"align":342},"optional itself occupies extra space (the size of T + bool + padding)",[10,2090,2092],{"id":2091},"optional-is-suitable-for-might-not-exist-but-not-for-error-details-are-many","\"Optional\" is suitable for \"might not exist\" but not for \"error details are many.\"",[15,2094,2095,2097],{},[26,2096,474],{}," indicates that: this value may exist or may not. It doesn’t explain “why it failed.”",[331,2099,2100,2110],{},[334,2101,2102],{},[337,2103,2104,2107],{},[340,2105,2106],{"align":342},"Scene",[340,2108,2109],{"align":342},"Recommendation",[348,2111,2112,2122,2131,2144,2155],{},[337,2113,2114,2117],{},[353,2115,2116],{"align":342},"Look up the name, it might not be found.",[353,2118,2119],{"align":342},[26,2120,2121],{},"std::optional\u003Csize_t>",[337,2123,2124,2127],{},[353,2125,2126],{"align":342},"The configuration items may not have been filled in.",[353,2128,2129],{"align":342},[26,2130,2081],{},[337,2132,2133,2139],{},[353,2134,2135,2138],{"align":342},[968,2136,2137],{},"I18N_PROTECTED_0"," Converting strings to numbers, only caring about whether it succeeds.",[353,2140,2141],{"align":342},[26,2142,2143],{},"std::optional\u003Cint>",[337,2145,2146,2149],{},[353,2147,2148],{"align":342},"Failed to open the file; this could be due to incorrect permissions, incorrect path, or format errors.",[353,2150,2151,2152],{"align":342},"Error codes, exceptions, or tools like ",[26,2153,2154],{},"expected",[337,2156,2157,2160],{},[353,2158,2159],{"align":342},"Network request failed, needs error type and error message",[353,2161,2162],{"align":342},"Error object, don't just use optional.",[420,2164,2166],{"id":2165},"example-4-the-difference-between-optional-and-error-messages","Example 4: The Difference Between Optional and Error Messages",[110,2168,2170],{"className":112,"code":2169,"language":114,"meta":115,"style":115},"#include \u003Ciostream>\n#include \u003Coptional>\n#include \u003Cstring>\n\n\u002F\u002F optional 表示“可能有值，也可能没有值”的结果。\nstd::optional\u003Cint> parse_port_simple(const std::string& text)\n{\n    try\n    {\n        int port = std::stoi(text);\n        if (port >= 0 && port \u003C= 65535)\n        {\n            return port;\n        }\n    }\n    catch (...) {}\n    return std::nullopt;\n}\n\nstruct ParseResult\n{\n    bool ok;\n    int value;\n    std::string error;\n};\n\nParseResult parse_port_with_error(const std::string& text)\n{\n    try\n    {\n        int port = std::stoi(text);\n        if (port \u003C 0 || port > 65535)\n        {\n            return {false, 0, \"port out of range\"};\n        }\n        return {true, port, \"\"};\n    }\n    catch (...)\n    {\n        return {false, 0, \"not a number\"};\n    }\n}\n\nint main()\n{\n    \u002F\u002F 程序从 main 函数开始执行，下面的语句会按顺序运行。\n    auto simple = parse_port_simple(\"70000\");\n    std::cout \u003C\u003C \"simple ok? \" \u003C\u003C (simple ? \"yes\" : \"no\") \u003C\u003C \"\\n\";\n\n    auto detailed = parse_port_with_error(\"70000\");\n    if (!detailed.ok)\n    {\n        std::cout \u003C\u003C \"error: \" \u003C\u003C detailed.error \u003C\u003C \"\\n\";\n    }\n\n    return 0;\n}\n",[26,2171,2172,2178,2184,2190,2194,2199,2233,2237,2241,2245,2263,2288,2292,2299,2303,2307,2313,2321,2325,2329,2336,2340,2348,2355,2362,2366,2370,2394,2398,2402,2406,2422,2443,2447,2468,2472,2490,2494,2501,2505,2524,2528,2533,2538,2547,2552,2558,2578,2619,2624,2642,2654,2659,2686,2691,2696,2705],{"__ignoreMap":115},[119,2173,2174,2176],{"class":121,"line":122},[119,2175,126],{"class":125},[119,2177,435],{"class":129},[119,2179,2180,2182],{"class":121,"line":167},[119,2181,126],{"class":125},[119,2183,130],{"class":129},[119,2185,2186,2188],{"class":121,"line":192},[119,2187,126],{"class":125},[119,2189,448],{"class":129},[119,2191,2192],{"class":121,"line":218},[119,2193,222],{"emptyLinePlaceholder":221},[119,2195,2196],{"class":121,"line":225},[119,2197,2198],{"class":163},"\u002F\u002F optional 表示“可能有值，也可能没有值”的结果。\n",[119,2200,2201,2203,2205,2207,2209,2211,2213,2216,2218,2220,2222,2224,2226,2228,2231],{"class":121,"line":231},[119,2202,145],{"class":144},[119,2204,471],{"class":148},[119,2206,474],{"class":144},[119,2208,152],{"class":148},[119,2210,660],{"class":125},[119,2212,482],{"class":148},[119,2214,2215],{"class":144},"parse_port_simple",[119,2217,316],{"class":148},[119,2219,490],{"class":125},[119,2221,209],{"class":144},[119,2223,471],{"class":148},[119,2225,506],{"class":144},[119,2227,511],{"class":125},[119,2229,2230],{"class":514}," text",[119,2232,537],{"class":148},[119,2234,2235],{"class":121,"line":246},[119,2236,542],{"class":148},[119,2238,2239],{"class":121,"line":257},[119,2240,1058],{"class":125},[119,2242,2243],{"class":121,"line":262},[119,2244,586],{"class":148},[119,2246,2247,2249,2252,2254,2256,2258,2260],{"class":121,"line":268},[119,2248,1075],{"class":125},[119,2250,2251],{"class":148}," port ",[119,2253,183],{"class":125},[119,2255,209],{"class":144},[119,2257,471],{"class":148},[119,2259,1087],{"class":144},[119,2261,2262],{"class":148},"(text);\n",[119,2264,2265,2267,2270,2273,2275,2278,2280,2283,2286],{"class":121,"line":288},[119,2266,591],{"class":125},[119,2268,2269],{"class":148}," (port ",[119,2271,2272],{"class":125},">=",[119,2274,561],{"class":560},[119,2276,2277],{"class":125}," &&",[119,2279,2251],{"class":148},[119,2281,2282],{"class":125},"\u003C=",[119,2284,2285],{"class":560}," 65535",[119,2287,537],{"class":148},[119,2289,2290],{"class":121,"line":304},[119,2291,606],{"class":148},[119,2293,2294,2296],{"class":121,"line":603},[119,2295,612],{"class":125},[119,2297,2298],{"class":148}," port;\n",[119,2300,2301],{"class":121,"line":609},[119,2302,624],{"class":148},[119,2304,2305],{"class":121,"line":621},[119,2306,630],{"class":148},[119,2308,2309,2311],{"class":121,"line":627},[119,2310,1139],{"class":125},[119,2312,1142],{"class":148},[119,2314,2315,2317,2319],{"class":121,"line":633},[119,2316,636],{"class":125},[119,2318,209],{"class":144},[119,2320,1151],{"class":148},[119,2322,2323],{"class":121,"line":646},[119,2324,649],{"class":148},[119,2326,2327],{"class":121,"line":652},[119,2328,222],{"emptyLinePlaceholder":221},[119,2330,2331,2333],{"class":121,"line":657},[119,2332,1613],{"class":125},[119,2334,2335],{"class":144}," ParseResult\n",[119,2337,2338],{"class":121,"line":669},[119,2339,542],{"class":148},[119,2341,2342,2345],{"class":121,"line":674},[119,2343,2344],{"class":125},"    bool",[119,2346,2347],{"class":148}," ok;\n",[119,2349,2350,2352],{"class":121,"line":717},[119,2351,1632],{"class":125},[119,2353,2354],{"class":148}," value;\n",[119,2356,2357,2359],{"class":121,"line":722},[119,2358,677],{"class":144},[119,2360,2361],{"class":148},"::string error;\n",[119,2363,2364],{"class":121,"line":744},[119,2365,714],{"class":148},[119,2367,2368],{"class":121,"line":753},[119,2369,222],{"emptyLinePlaceholder":221},[119,2371,2372,2375,2378,2380,2382,2384,2386,2388,2390,2392],{"class":121,"line":758},[119,2373,2374],{"class":144},"ParseResult",[119,2376,2377],{"class":144}," parse_port_with_error",[119,2379,316],{"class":148},[119,2381,490],{"class":125},[119,2383,209],{"class":144},[119,2385,471],{"class":148},[119,2387,506],{"class":144},[119,2389,511],{"class":125},[119,2391,2230],{"class":514},[119,2393,537],{"class":148},[119,2395,2396],{"class":121,"line":795},[119,2397,542],{"class":148},[119,2399,2400],{"class":121,"line":800},[119,2401,1058],{"class":125},[119,2403,2404],{"class":121,"line":805},[119,2405,586],{"class":148},[119,2407,2408,2410,2412,2414,2416,2418,2420],{"class":121,"line":824},[119,2409,1075],{"class":125},[119,2411,2251],{"class":148},[119,2413,183],{"class":125},[119,2415,209],{"class":144},[119,2417,471],{"class":148},[119,2419,1087],{"class":144},[119,2421,2262],{"class":148},[119,2423,2424,2426,2428,2430,2432,2435,2437,2439,2441],{"class":121,"line":837},[119,2425,591],{"class":125},[119,2427,2269],{"class":148},[119,2429,152],{"class":125},[119,2431,561],{"class":560},[119,2433,2434],{"class":125}," ||",[119,2436,2251],{"class":148},[119,2438,157],{"class":125},[119,2440,2285],{"class":560},[119,2442,537],{"class":148},[119,2444,2445],{"class":121,"line":842},[119,2446,606],{"class":148},[119,2448,2449,2451,2453,2456,2458,2461,2463,2466],{"class":121,"line":860},[119,2450,612],{"class":125},[119,2452,697],{"class":148},[119,2454,2455],{"class":560},"false",[119,2457,703],{"class":148},[119,2459,2460],{"class":560},"0",[119,2462,703],{"class":148},[119,2464,2465],{"class":129},"\"port out of range\"",[119,2467,714],{"class":148},[119,2469,2470],{"class":121,"line":865},[119,2471,624],{"class":148},[119,2473,2474,2477,2479,2482,2485,2488],{"class":121,"line":870},[119,2475,2476],{"class":125},"        return",[119,2478,697],{"class":148},[119,2480,2481],{"class":560},"true",[119,2483,2484],{"class":148},", port, ",[119,2486,2487],{"class":129},"\"\"",[119,2489,714],{"class":148},[119,2491,2492],{"class":121,"line":876},[119,2493,630],{"class":148},[119,2495,2496,2498],{"class":121,"line":913},[119,2497,1139],{"class":125},[119,2499,2500],{"class":148}," (...)\n",[119,2502,2503],{"class":121,"line":948},[119,2504,586],{"class":148},[119,2506,2507,2509,2511,2513,2515,2517,2519,2522],{"class":121,"line":953},[119,2508,2476],{"class":125},[119,2510,697],{"class":148},[119,2512,2455],{"class":560},[119,2514,703],{"class":148},[119,2516,2460],{"class":560},[119,2518,703],{"class":148},[119,2520,2521],{"class":129},"\"not a number\"",[119,2523,714],{"class":148},[119,2525,2526],{"class":121,"line":962},[119,2527,630],{"class":148},[119,2529,2531],{"class":121,"line":2530},42,[119,2532,649],{"class":148},[119,2534,2536],{"class":121,"line":2535},43,[119,2537,222],{"emptyLinePlaceholder":221},[119,2539,2541,2543,2545],{"class":121,"line":2540},44,[119,2542,660],{"class":125},[119,2544,663],{"class":144},[119,2546,666],{"class":148},[119,2548,2550],{"class":121,"line":2549},45,[119,2551,542],{"class":148},[119,2553,2555],{"class":121,"line":2554},46,[119,2556,2557],{"class":163},"    \u002F\u002F 程序从 main 函数开始执行，下面的语句会按顺序运行。\n",[119,2559,2561,2563,2566,2568,2571,2573,2576],{"class":121,"line":2560},47,[119,2562,725],{"class":125},[119,2564,2565],{"class":148}," simple ",[119,2567,183],{"class":125},[119,2569,2570],{"class":144}," parse_port_simple",[119,2572,316],{"class":148},[119,2574,2575],{"class":129},"\"70000\"",[119,2577,741],{"class":148},[119,2579,2581,2583,2585,2587,2590,2592,2595,2598,2601,2604,2607,2609,2611,2613,2615,2617],{"class":121,"line":2580},48,[119,2582,677],{"class":144},[119,2584,764],{"class":148},[119,2586,767],{"class":125},[119,2588,2589],{"class":129}," \"simple ok? \"",[119,2591,773],{"class":125},[119,2593,2594],{"class":148}," (simple ",[119,2596,2597],{"class":125},"?",[119,2599,2600],{"class":129}," \"yes\"",[119,2602,2603],{"class":125}," :",[119,2605,2606],{"class":129}," \"no\"",[119,2608,900],{"class":148},[119,2610,767],{"class":125},[119,2612,783],{"class":129},[119,2614,786],{"class":560},[119,2616,789],{"class":129},[119,2618,792],{"class":148},[119,2620,2622],{"class":121,"line":2621},49,[119,2623,222],{"emptyLinePlaceholder":221},[119,2625,2627,2629,2632,2634,2636,2638,2640],{"class":121,"line":2626},50,[119,2628,725],{"class":125},[119,2630,2631],{"class":148}," detailed ",[119,2633,183],{"class":125},[119,2635,2377],{"class":144},[119,2637,316],{"class":148},[119,2639,2575],{"class":129},[119,2641,741],{"class":148},[119,2643,2645,2647,2649,2651],{"class":121,"line":2644},51,[119,2646,747],{"class":125},[119,2648,550],{"class":148},[119,2650,831],{"class":125},[119,2652,2653],{"class":148},"detailed.ok)\n",[119,2655,2657],{"class":121,"line":2656},52,[119,2658,586],{"class":148},[119,2660,2662,2664,2666,2668,2671,2673,2676,2678,2680,2682,2684],{"class":121,"line":2661},53,[119,2663,761],{"class":144},[119,2665,764],{"class":148},[119,2667,767],{"class":125},[119,2669,2670],{"class":129}," \"error: \"",[119,2672,773],{"class":125},[119,2674,2675],{"class":148}," detailed.error ",[119,2677,767],{"class":125},[119,2679,783],{"class":129},[119,2681,786],{"class":560},[119,2683,789],{"class":129},[119,2685,792],{"class":148},[119,2687,2689],{"class":121,"line":2688},54,[119,2690,630],{"class":148},[119,2692,2694],{"class":121,"line":2693},55,[119,2695,222],{"emptyLinePlaceholder":221},[119,2697,2699,2701,2703],{"class":121,"line":2698},56,[119,2700,636],{"class":125},[119,2702,561],{"class":560},[119,2704,792],{"class":148},[119,2706,2708],{"class":121,"line":2707},57,[119,2709,649],{"class":148},[15,2711,2712,971],{},[968,2713,970],{},[110,2715,2718],{"className":2716,"code":2717,"language":977},[975],"simple ok? no\nerror: port out of range\n",[26,2719,2717],{"__ignoreMap":115},[15,2721,2722,2723,2725,2726,2728],{},"If the caller only cares about \"whether there's a value,\" ",[26,2724,474],{}," is suitable; if the caller needs to know the reason for failure, a standalone ",[26,2727,474],{}," isn't sufficient.",[10,2730,2732],{"id":2731},"common-errors","Common Errors",[15,2734,2735],{},[968,2736,2737,2738],{},"Error 1: No check performed directly ",[26,2739,381],{},[110,2741,2743],{"className":112,"code":2742,"language":114,"meta":115,"style":115},"std::optional\u003Cint> opt;\nstd::cout \u003C\u003C *opt;  \u002F\u002F ❌ 未定义行为！opt 为空\n",[26,2744,2745,2756],{"__ignoreMap":115},[119,2746,2747,2749,2751,2753],{"class":121,"line":122},[119,2748,145],{"class":144},[119,2750,149],{"class":148},[119,2752,1664],{"class":125},[119,2754,2755],{"class":148}," opt;\n",[119,2757,2758,2760,2762,2764,2766,2768],{"class":121,"line":167},[119,2759,145],{"class":144},[119,2761,764],{"class":148},[119,2763,767],{"class":125},[119,2765,295],{"class":125},[119,2767,298],{"class":148},[119,2769,2770],{"class":163},"  \u002F\u002F ❌ 未定义行为！opt 为空\n",[15,2772,2773],{},"Correct approach: First perform a microcontroller check, or use the environment setup.",[15,2775,2776],{},[968,2777,2778,2779,2781,2782],{},"Error 2: Confusing ",[26,2780,1577],{}," with ",[26,2783,65],{},[110,2785,2787],{"className":112,"code":2786,"language":114,"meta":115,"style":115},"std::optional\u003Cint> opt = nullptr;  \u002F\u002F ❌ 编译错误！应该用 std::nullopt\n",[26,2788,2789],{"__ignoreMap":115},[119,2790,2791,2793,2795,2797,2799,2801,2804,2807],{"class":121,"line":122},[119,2792,145],{"class":144},[119,2794,149],{"class":148},[119,2796,1664],{"class":125},[119,2798,180],{"class":148},[119,2800,183],{"class":125},[119,2802,2803],{"class":560}," nullptr",[119,2805,2806],{"class":148},";",[119,2808,2809],{"class":163},"  \u002F\u002F ❌ 编译错误！应该用 std::nullopt\n",[15,2811,2812,2813],{},"Correct approach: ",[26,2814,2815],{},"opt = std::nullopt;",[15,2817,2818],{},[968,2819,2820,2821,2824],{},"Error 3: Forgetting ",[26,2822,2823],{},"value()"," will throw an exception when no value is present.",[110,2826,2828],{"className":112,"code":2827,"language":114,"meta":115,"style":115},"std::optional\u003Cint> opt;\nstd::cout \u003C\u003C opt.value();  \u002F\u002F ❌ 抛出 std::bad_optional_access\n",[26,2829,2830,2840],{"__ignoreMap":115},[119,2831,2832,2834,2836,2838],{"class":121,"line":122},[119,2833,145],{"class":144},[119,2835,149],{"class":148},[119,2837,1664],{"class":125},[119,2839,2755],{"class":148},[119,2841,2842,2844,2846,2848,2850,2852,2854],{"class":121,"line":167},[119,2843,145],{"class":144},[119,2845,764],{"class":148},[119,2847,767],{"class":125},[119,2849,276],{"class":148},[119,2851,279],{"class":144},[119,2853,282],{"class":148},[119,2855,2856],{"class":163},"  \u002F\u002F ❌ 抛出 std::bad_optional_access\n",[15,2858,2859,2860,2863],{},"The correct approach is to use ",[26,2861,2862],{},"value_or(default)"," or check first.",[10,2865,2866],{"id":2866},"使用建议",[19,2868,2869,2872,2875,2878,2881],{},[22,2870,2871],{},"明确目标：在开始前确定您的具体需求，以便选择最合适的工具或教程。",[22,2873,2874],{},"充分利用资源：参考官方文档、教程和博客，这些资料能帮助您快速上手并解决问题。",[22,2876,2877],{},"实践应用：通过动手操作项目或编写代码来巩固学习成果，提升实际操作能力。",[22,2879,2880],{},"问题解决：遇到困难时，查阅参考资料或寻求社区支持，逐步培养独立解决问题的能力。",[22,2882,2883],{},"分享经验：完成项目后，可以撰写文章或博客分享心得，帮助其他学习者。",[15,2885,2886],{},"如果需要针对特定领域（如单片机、机器人或环境搭建）的进一步建议，请提供更多信息，我将为您细化内容。",[2888,2889,2890,2898,2907,2921,2933,2950],"ol",{},[22,2891,2892,2897],{},[968,2893,2894,2895],{},"Functions that may \"have no result\" should return ",[26,2896,83],{},": safer than returning -1\u002Fnullptr.",[22,2899,2900,2903,2904,2906],{},[968,2901,2902],{},"Optional fields in structs",": Using ",[26,2905,474],{}," is more semantically clear than magic numbers.",[22,2908,2909,2917,2918,2920],{},[968,2910,2911,2912,2914,2915],{},"Default to ",[26,2913,2036],{}," instead of ",[26,2916,381],{},": ",[26,2919,2036],{}," is safer.",[22,2922,2923,2928,2929,2932],{},[968,2924,2925,2927],{},[26,2926,5],{}," has extra space overhead",": equivalent to ",[26,2930,2931],{},"sizeof(T) + sizeof(bool) + padding",", minimal impact on small objects.",[22,2934,2935,2917,2940,703,2943,703,2946,2949],{},[968,2936,2937,2938],{},"C++23 introduces monadic operations for ",[26,2939,5],{},[26,2941,2942],{},".and_then()",[26,2944,2945],{},".transform()",[26,2947,2948],{},".or_else()",", making chained calls more elegant.",[22,2951,2952,2955],{},[968,2953,2954],{},"When error details are needed, don't just use optional",": optional only indicates whether a value exists, not why it doesn't.",[10,2957,2959],{"id":2958},"summary","Summary",[19,2961,2962,2970,2973,2985],{},[22,2963,2964,2966,2967,2969],{},[26,2965,83],{}," either contains a ",[26,2968,96],{}," or is empty.",[22,2971,2972],{},"Resolved the \"possibly no result\" semantic issue for the function, replacing magic values.",[22,2974,2975,2976,62,2979,2981,2982,2984],{},"Use ",[26,2977,2978],{},"has_value()",[26,2980,2031],{}," for checking, and use ",[26,2983,2862],{}," to safely get the value.",[22,2986,2987],{},"Can be used for function return values, struct members, and function parameters (though using optional for parameters is not recommended).",[2989,2990,2991],"style",{},"html pre.shiki code .szBVR, html code.shiki .szBVR{--shiki-default:#D73A49;--shiki-dark:#F97583}html pre.shiki code .sZZnC, html code.shiki .sZZnC{--shiki-default:#032F62;--shiki-dark:#9ECBFF}html .default .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: var(--shiki-default-text-decoration);}html .shiki span {color: var(--shiki-default);background: var(--shiki-default-bg);font-style: var(--shiki-default-font-style);font-weight: var(--shiki-default-font-weight);text-decoration: 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Storage","0300",{"path":3049,"stem":3050,"title":3051,"date":3015,"chapterOrder":3052,"chapterDepth":3037,"docKey":3017,"docRoot":3018,"docTitle":3019,"isWikiDoc":221,"isWikiIndex":3022},"\u002Fen-us\u002Fwiki\u002F2023-10-05-cplusplus-jiao-xue\u002F0400-shu-ru-shu-chu","_i18n\u002Fen-us\u002Fwiki\u002F2023-10-05-Cplusplus教学\u002F0400-输入输出","Input and 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