当 std::unordered_map 包装在一个类中时，它不接受 std::thread

Posted 2023-02-17

【中文标题】当 std::unordered_map 包装在一个类中时，它不接受 std::thread

【英文标题】：std::unordered_map doesn't accept std::thread when it is wrapped in a class

【发布时间】：2018-01-03 20:39:57

【问题描述】：

如果我尝试将int 和std::thread 保存在std::unordered_map<int, std::thread> 中，一切似乎都正常。

但是如果我包装std::unordered_map，那么我会在标准库中得到一个错误。

错误是：no matching constructor for initialization of '_Mypair'

有没有办法解决这个问题并使其在包装后工作？

这行得通：

Source.cpp

#include <thread>
#include <unordered_map>

void display() 

int main()

    std::unordered_map<int, std::thread> map_;
    std::thread tempThread(&display);
    map_.emplace(0, std::move(tempThread));

    return 0;

这不起作用：

MapperWrapper.h

#ifndef MAPPERWRAPPER_H
#define MAPPERWRAPPER_H

#include <unordered_map>
#include <utility>

template <typename KeyType, typename valueType> class MapperWrapper

    public:
        void add(KeyType key, valueType value)
        
            mapper.emplace(std::make_pair(key, value));
        

    protected:
        std::unordered_map<KeyType, valueType> mapper;
;
#endif // MAPPERWRAPPER_H

Source.cpp

#include <thread>
#include "MapperWrapper.h"

void display() 

int main()

    MapperWrapper<int, std::thread> map_;
    std::thread tempThread(&display);
    map_.add(0, std::move(tempThread));

    return 0;

这会产生错误：

||=== Build: Debug in Testing (compiler: LLVM Clang Compiler) ===|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|327|error: no matching constructor for initialization of '_Mypair' (aka 'pair<int, std::thread>')|
C:\Users\usr\Desktop\Project2\Project2\MapperWrapper.h|12|in instantiation of function template specialization 'std::make_pair<int &, std::thread &>' requested here|
C:\Users\usr\Desktop\Project2\Project2\Source.cpp|12|in instantiation of member function 'MapperWrapper<int, std::thread>::add' requested here|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|96|note: candidate template ignored: requirement 'is_copy_constructible<thread>::value' was not satisfied [with _Uty1 = int, _Uty2 = std::thread]|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|107|note: candidate template ignored: requirement 'is_copy_constructible<thread>::value' was not satisfied [with _Uty1 = int, _Uty2 = std::thread]|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|167|note: candidate template ignored: requirement 'is_constructible<thread, thread &>::value' was not satisfied [with _Other1 = int &, _Other2 = std::thread &]|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|181|note: candidate template ignored: requirement 'is_constructible<thread, thread &>::value' was not satisfied [with _Other1 = int &, _Other2 = std::thread &]|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|85|note: candidate constructor template not viable: requires 0 arguments, but 2 were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|121|note: candidate constructor template not viable: requires single argument '_Right', but 2 arguments were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|132|note: candidate constructor template not viable: requires single argument '_Right', but 2 arguments were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|150|note: candidate constructor template not viable: requires 4 arguments, but 2 were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|157|note: candidate constructor template not viable: requires 3 arguments, but 2 were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|195|note: candidate constructor template not viable: requires single argument '_Right', but 2 arguments were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|209|note: candidate constructor template not viable: requires single argument '_Right', but 2 arguments were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|112|note: candidate constructor not viable: requires 1 argument, but 2 were provided|
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\utility|113|note: candidate constructor not viable: requires 1 argument, but 2 were provided|
||=== Build failed: 1 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

【参考方案1】：

您已经按值接受值类型。由于函数参数是本地的，你可以移动它（也可以移动键）

void add(KeyType key, valueType value)

    mapper.emplace(std::move(key), std::move(value));

也无需使用std::make_pair 置入地图。

如果你想更精致一点，可以给add添加一些完善的转发支持，避免创建多余的中间对象。

template<typename... Args>
void add(Args&&... args)

    mapper.emplace(std::forward<Args>(args)...);

【讨论】：

签名读取void add(KeyType&& key, valueType&& value)会有什么好处/坏处吗？

@Questionable - 除了强制所有类型的调用方移动之外，没有。如果你想添加完美转发，我更新了我的答案。