如何在 C++ 中使用 lambdas 作為值定義映射

Question

我收到以下錯誤：

braced initialization of a variable declared with a placeholder type but without `=` requires exactly one element inside the braces

  const auto temperature_units{
    {"K", [] (double x)->double {return x;}},
    {"C", [] (double x)->double {return x+273.15;}},
    {"\u2103",  [] (double x)->double {return x+273.15;}}, // degree celcius symbol
    {"\u00B0C", [] (double x)->double {return x+273.15;}}, // degree symbol plus C
    {"F",  [] (double x)->double {return (x+459.67)*5.0/9.0;}},
    {"\u2109", [] (double x)->double {return (x+459.67)*5.0/9.0;}},// degree fahrenheit symbol
    {"\u00B0F", [] (double x)->double {return (x+459.67)*5.0/9.0;}}
  };

就像是

  const std::map<std::string, auto> temperature_units{
    {"K", [] (double x)->double {return x;}},
    {"C", [] (double x)->double {return x+273.15;}},
    {"\u2103",  [] (double x)->double {return x+273.15;}}, // degree celcius symbol
    {"\u00B0C", [] (double x)->double {return x+273.15;}}, // degree symbol plus C
    {"F",  [] (double x)->double {return (x+459.67)*5.0/9.0;}},
    {"\u2109", [] (double x)->double {return (x+459.67)*5.0/9.0;}},// degree fahrenheit symbol
    {"\u00B0F", [] (double x)->double {return (x+459.67)*5.0/9.0;}}
  };

也不起作用，因為這是error: invalid use of 'auto顯然error: invalid use of 'auto 。 我不知道如何寫這個，因為顯然 lambdas 沒有定義的類型

Answer 1

using namespace std::literals;


    const std::map temperature_units {
        std::make_pair("K"s, std::function<double(double)>{[] (double x)->double {return x;}}),
        {"C"s, [] (double x)->double {return x+273.15;}},
        {"\u2103"s,  [] (double x)->double {return x+273.15;}}, // degree celcius symbol
        {"\u00B0C"s, [] (double x)->double {return x+273.15;}}, // degree symbol plus C
        {"F"s,  [] (double x)->double {return (x+459.67)*5.0/9.0;}},
        {"\u2109"s, [] (double x)->double {return (x+459.67)*5.0/9.0;}},// degree fahrenheit symbol
        {"\u00B0F"s, [] (double x)->double {return (x+459.67)*5.0/9.0;}}
    };

https://godbolt.org/z/M5er48一些變化https://godbolt.org/z/ozqG56

所以基本上問題是強制執行正確類型的初始化列表。