C++：二維靜態地圖的初始化

Question

我有一個類看起來像這樣

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

那麼既然是可以初始化一個標準的地圖是這樣的：

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

我試着做同樣的2D地圖：

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

但顯然它不工作。錯誤信息是：

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> >' 

有沒有人知道如何解決這個問題，如果可能的話沒有某種初始化函數。

非常感謝！

Answer 1

事實上，你已經在你的文章解決方案。

標準的簡單地圖initilisation，你向我們展示是

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 

但在複雜的地圖，你寫的（我加了註釋）：

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 

正如你所看到的，當你比較內部映射初始化與您以前的工作代碼，有一個不一致：內部映射的初始化程序列表的周圍大括號。如果內部映射應該用幾個元素初始化，你會如何編寫它？

的解決辦法是添加缺少的括號：

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 

這裏一個live demo。 （順便說一句，你不需要=這裏。）