C ++：2D静态地图的初始化

Question

I have a class looking like this

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

So since it is possible to initialize a standard map like this:

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

I've tried to do the same to the 2D-Map:

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

But apparently it is not working. The Error Message is:

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> >'

Does anyone know how to solve this problem, if possible without some kind of initializing-function.

Thank you very much!

Answer 1

In fact you have already the solution in your post.

The standard simple map initilisation that you showed us is

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

But in the complex map you write (I added the comments):

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

As you can see, when you compare the inner map initialization with your previous working code, there is an inconsistency : the surrounding braces for of the initializer list of the inner map. How would you write it if the inner map should be initialized with several elements ?

The solution would be to add the missing braces :

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

Here a live demo . ( By the way, you don't need the = here. )