用结构的特定字符串元素定义unordered_map键

Question

我是unordered_map.I的新手，我想用某些结构的特定字符串元素来定义unordered_map哈希表键，定义如下：- hashtable.cpp如下所示：-

#include <tr1/unordered_map>
 #include <iostream>
 #include <stdio.h>
 using namespace std;
 using namespace std::tr1;
    struct row{
                string state;
                int population;
            };
    struct total{
                string key;
                row value;
            };
    int main ()
    {
        total data;
        unordered_map<data.key,data.value> country;
        data.key="Australia";
        data.value.state="Canberra";
        data.value.population=12000;
      return 0;
    }

我犯了一些错误：-

hashtable.cpp:17: error: ‘data’ cannot appear in a constant-expression
hashtable.cpp:17: error: ‘.’ cannot appear in a constant-expression
hashtable.cpp:17: error: ‘data’ cannot appear in a constant-expression
hashtable.cpp:17: error: ‘.’ cannot appear in a constant-expression
hashtable.cpp:17: error: template argument 1 is invalid
hashtable.cpp:17: error: template argument 2 is invalid
hashtable.cpp:17: error: template argument 3 is invalid
hashtable.cpp:17: error: template argument 4 is invalid
hashtable.cpp:17: error: template argument 5 is invalid
hashtable.cpp:17: error: invalid type in declaration before ‘;’ token

Answer 1

你做错了。也许，正确的方法是

int main ()
{
    total data{"Australia", {"Canbeera", 12000}};
    unordered_map<std::string,row> country;
    count.insert({ data.key, { data.value } });
  return 0;
}

检查此页面以供参考

Answer 2

unordered_map<data.key,data.value> country;

你想要的显然是来自c++11和上面的东西

unordered_map<decltype(data.key),decltype(data.value)> country;

因为您无论如何都使用tr1扩展，所以我建议您至少为编译启用C++11语法，并从std::获取曾经是tr1的内容。