STD :: Promise Store const类型如何在MSVC，GCC和Clang中类型？

发布于 2025-02-13 20:32:39 字数 1817 浏览 0 评论 0原文

最近，我已经开始使用MSVC编译了总是用GCC和Clang编译的代码。此代码的某些部分产生了一个有趣的汇编错误（截断）：

C:/data/msvc/14.33.31424-Pre/include\future(311) error C2678: binary '=': no operator found which takes a left-hand operand of type 'const Result' (or there is no acceptable conversion)
<source>(7): note: could be 'Result &Result::operator =(Result &&)'
<source>(7): note: or       'Result &Result::operator =(const Result &)'

此代码的最小示例：

#include <future>
#include <iostream>

struct Result {
    int data{0};
};

// getting rid of constness resolves the problem for MSVC
using result_t = const Result; 
using promise_result_t = std::promise<result_t>;

auto compute_result(promise_result_t result)
{
    Result some_result{10};
    result.set_value(std::move(some_result));
}

int main()
{
    promise_result_t my_promise;
    auto my_future = my_promise.get_future();
    std::thread thread(compute_result, std::move(my_promise));
    std::cout << "result: " << my_future.get().data;
    thread.join();
    return 0;
}

用clang 11的成功编译11： https://godbolt.org/z/o6ge85rxg
成功用GCC 12：
https：//godbolt.org/z/x1rhqhfgt href =“ https://godbolt.org/z/4jmyhtj3n” rel =“ noreferrer”> https://godbolt.org/z/4jmyhtj3n

很明显，清除const const 对于std :: Promise存储解决问题，但是我很好奇是否有人试图深入研究针对不同编译器的std :: Promise的实现。

允许此代码可以使用GCC和Clang进行编译和工作的区别，而在使用MSVC时会产生汇编错误？（通过指针在GCC和Clang中存储的数据操作？）

原文

Recently, I've started using MSVC to compile a code which was always compiled with GCC and Clang. Some piece of this code produces an interesting compilation error (truncated):

C:/data/msvc/14.33.31424-Pre/include\future(311) error C2678: binary '=': no operator found which takes a left-hand operand of type 'const Result' (or there is no acceptable conversion)
<source>(7): note: could be 'Result &Result::operator =(Result &&)'
<source>(7): note: or       'Result &Result::operator =(const Result &)'

Minimal example of this piece of code:

#include <future>
#include <iostream>

struct Result {
    int data{0};
};

// getting rid of constness resolves the problem for MSVC
using result_t = const Result; 
using promise_result_t = std::promise<result_t>;

auto compute_result(promise_result_t result)
{
    Result some_result{10};
    result.set_value(std::move(some_result));
}

int main()
{
    promise_result_t my_promise;
    auto my_future = my_promise.get_future();
    std::thread thread(compute_result, std::move(my_promise));
    std::cout << "result: " << my_future.get().data;
    thread.join();
    return 0;
}

Successful compilation with Clang 11: https://godbolt.org/z/o6Ge85rxG
Successful compilation with GCC 12: https://godbolt.org/z/x1rhqhfGT
Error with MSVC 19: https://godbolt.org/z/4jMYhTj3n

It is quite clear that removal of const for std::promise storage solves the problem, but I'm curious if anyone has tried to dive deep into the implementations of std::promise for different compilers.

What might be the difference that allows this code to compile and work using GCC and Clang, yet produce a compilation error when using MSVC? (Operating via pointers on stored data in GCC and Clang?)

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