CODEWITHSUNDEEP
c++gccvisual-c++clangmsvc12
Eugene Kuznetsov
Eugene Kuznetsov

Reputation: 91

How does std::promise store const types in MSVC, GCC, and Clang?

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;
}

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?)

Upvotes: 9

Views: 155

Answers (1)

Zag
Zag

Reputation: 51

Looks like an MSVC issue, reported here:

https://developercommunity.visualstudio.com/t/std::promise-works-incorrectly-for-const/10092031

and linked to the issue on GitHub:

https://github.com/microsoft/STL/issues/2599

Upvotes: 5

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