C++20中std::future的类型要求？MSVC2019编译报错解析

cppreference未列出std::future的任何类型要求，但使用Visual Studio 2019编译以下代码时会报错，而GCC和Clang的libstdc++编译则可通过。

测试代码 #

#include <future>

struct A {
    A(int&){}

    A(A&&) = default;
    A(A const&) = delete;

    A& operator=(A&&) = delete;
    A& operator=(A const&) = delete;
};

int main(){
    int v;
    std::async([&v]{
        return A(v);
    }).get();
}

MSVC2019编译错误信息 #

example.cpp
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\future(314): error C2280: 'A &A::operator =(A &&)': attempting to reference a deleted function
<source>(9): note: see declaration of 'A::operator ='
<source>(9): note: 'A &A::operator =(A &&)': function was explicitly deleted
C:/data/msvc/14.29.30151/include\future(309): note: while compiling class template member function 'void std::_Associated_state<_Ty>::_Set_value_raw(_Ty &&,std::unique_lock<std::mutex> *,bool)'
        with
        [
            _Ty=A
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\future(305): note: see reference to function template instantiation 'void std::_Associated_state<_Ty>::_Set_value_raw(_Ty &&,std::unique_lock<std::mutex> *,bool)' being compiled
        with
        [
            _Ty=A
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\future(774): note: see reference to class template instantiation 'std::_Associated_state<_Ty>' being compiled
        with
        [
            _Ty=A
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\future(769): note: while compiling class template member function '_Ty &std::_State_manager<_Ty>::_Get_value(void) const'
        with
        [
            _Ty=A
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\future(884): note: see reference to function template instantiation '_Ty &std::_State_manager<_Ty>::_Get_value(void) const' being compiled
        with
        [
            _Ty=A
        ]
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30133\include\future(860): note: see reference to class template instantiation 'std::_State_manager<_Ty>' being compiled
        with
        [
            _Ty=A
        ]
<source>(17): note: see reference to class template instantiation 'std::future<A>' being compiled
Compiler returned: 2

问题分析与解答 #

从错误信息来看，MSVC2019的标准库实现尝试调用A的移动赋值运算符，而该运算符已被显式删除。

根据C++标准，std::future存储的对象仅要求满足可移动构造（MoveConstructible），并不强制要求可移动赋值（MoveAssignable）。异步任务返回的对象应该直接在std::future的内部状态中通过移动构造完成初始化，而非先默认构造再执行赋值操作。

你的A类已经显式默认了移动构造函数，完全符合标准要求。GCC和Clang的libstdc++实现遵循了这一标准，因此可以正常编译。而MSVC2019的标准库实现中，_Associated_state::_Set_value_raw函数内部错误地使用了移动赋值来存储结果，这属于实现不符合标准的问题。

内容的提问来源于stack exchange，提问作者Benjamin Buch