二维静态地图的初始化

Question

我有一个像这样的类

class MapClass {
public:
    static map<string, map<string, double>>        spreadMap;
};

既然可以这样初始化一个标准映射:

map<string,int> AnotherClass::standardMap = {
    {"A",0},
    {"B",1},
    {"C",2}
};

我试着对2D-Map做同样的事情:

map<string, map<string, double>> MapClass::spreadMap = {
    {"A", {"B", 0}},
    {"C", {"D", 1}},
    {"E", {"F", 2}}
};

但显然它不起作用。错误信息是:

error: could not convert '{{"A", {"B", 0}}, {"C", {"D", 1}}, {"E", {"F", 2}}}' from '<brace-enclosed initializer list>' to 'std::map<std::basic_string<char>, std::map<std::basic_string<char>, double> >'

有谁知道如何解决这个问题，如果可能的话，没有某种初始化函数。

非常感谢!

Answer 1

事实上，你已经在你的帖子中找到了解决方案。

您向我们展示的标准简单映射初始化是

map<string,int> AnotherClass::standardMap =
{ // opening brace for the map
    {"A",0}, // first value for the map 
    {"B",1}, // second value 
    {"C",2}  // ...
};  // closing brace for the map

但是在复杂的地图中你写(我添加了注释):

map<string, map<string, double>> MapClass::spreadMap = 
{  // opening brace for the map of map 
    {                // opening of a brace for a first element  
      "A",           // first key  
         {"B", 0}    // first value which should be a map initializer
    },               // closing of the brace for the first element 
    {"C", {"D", 1}}, // second element with second map  
    {"E", {"F", 2}}  // ...
}; // closing brace for the map of map

可以看到，当您将内部映射初始化与之前的工作代码进行比较时，存在一个不一致的地方:内部映射的初始化列表的大括号。如果内部映射需要用几个元素初始化，该如何编写呢?

解决方案是添加缺少的大括号:

map<string, map<string, double>> MapClass::spreadMap = 
{  // opening brace for the map of map 
    {                // opening of a brace for a first element  
      "A",           // first key  
         {           // <<<<= opening brace for the first value wich is a map
           {"B", 0}  // pair of key, value for the inner map
         }           // <<<<= closing brace for the inner map
    },               // closing of the brace for the first element 
    {"C", {{"D", 1}}}, // second element with second map  
    {"E", {{"F", 2},{"G",3}}}  // (the inner map could have itself several values...
}; // closing brace for the map of map

这里有一个现场演示。(顺便说一下，这里不需要=。)