使用 std::tuple 作为 std::unordered_map 的键

Using a std::tuple as key for std::unordered_map(使用 std::tuple 作为 std::unordered_map 的键)

问题描述

使用下面的代码，我在 MSVC 中遇到了一个非常令人困惑的错误，它似乎表明密钥类型(std::tuple)正在转换为 std::string.

With the code below, I get a very confusing error in MSVC that seems to suggest the key type (an std::tuple) is being converted to an std::string.

#include <iostream>
#include <string>
#include <tuple>
#include <utility>
#include <unordered_map>

typedef std::tuple<std::string,int,char> key_t;

struct key_hash : public std::unary_function<key_t, std::size_t>
{
   std::size_t operator()(const key_t& k) const
   {
      return std::get<0>(k)[0] ^ std::get<1>(k) ^ std::get<2>(k);
   }
};

struct key_equal : public std::binary_function<key_t, key_t, bool>
{
   bool operator()(const key_t& v0, const key_t& v1) const
   {
      return (
               std::get<0>(v0) == std::get<0>(v1) &&
               std::get<1>(v0) == std::get<1>(v1) &&
               std::get<2>(v0) == std::get<2>(v1)
             );
   }
};

struct data
{
   std::string x;
};

typedef std::unordered_map<key_t,data,key_hash,key_equal> map_t;


int main()
{
   map_t m;
   data d;
   d.x = "test data";
   m[std::make_tuple("abc",1,'X')] = d;
   auto itr = m.find(std::make_tuple(std::string("abc"),1,'X'));
   if (m.end() != itr)
   {
      std::cout << "x: " << itr->second.x;
   }
   return 0;
}

错误:

Error   1   error C2664: 'std::basic_string<_Elem,_Traits,_Ax>::basic_string(const std::basic_string<_Elem,_Traits,_Ax> &)' : cannot convert parameter 1 from 'const std::tr1::tuple<_Arg0,_Arg1,_Arg2>' to 'const std::basic_string<_Elem,_Traits,_Ax> &'  c:Program Files (x86)Microsoft Visual Studio 10.0VCinclude	uple    127 1   

编译器:MS Visual Studio 2010

Compiler: MS Visual Studio 2010

在 ideone 上，我得到以下更令人费解的错误:

On ideone, I get the following even more convoluted error:

http://ideone.com/yEv2j

我似乎想不通我哪里出错了.

I can't seem to figure out where I've gone wrong.

推荐答案

ideone 的问题是 key_t 已经存在:

The problem for ideone is that key_t already exists:

prog.cpp:7:42: error: conflicting declaration 'typedef class std::tuple<std::basic_string<char>, int, char> key_t'
/usr/include/sys/types.h:123:17: error: 'key_t' has a previous declaration as 'typedef __key_t key_t'

将您的 key_t 重命名为其他名称，或将其放入某些命名空间.

Rename your key_t to something else, or put it into some namespaces.

此更改后您的代码在 g++ 和 clang++ 中都有效.我相信这是 MSVC 中的一个错误.

Your code works after this change in both g++ and clang++. I believe this is a bug in MSVC.

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