<future>: promise default constructs the value type #2488
Description
Describe the bug
The value type T of a promise<T> is default constructed when the promise is constructed, which is surprising. I am not sure what the requirements of the standard are. The current implementation requires T to be default constructible and assignable. It is possible to implement promise with none of these two requirements.
Command-line test case
C:\Temp>type repro.cpp
#include <future>
#include <iostream>
struct test
{
test()
: result_{-1}
{
std::cout << "Default constructor" << std::endl;
}
explicit test(int i)
: result_{i}
{
}
int result() const
{
return result_;
}
private:
int result_;
};
int main()
{
std::promise<test> prm;
std::future<test> fut = prm.get_future();
prm.set_value(test{0});
return fut.get().result();
}
C:\Temp>cl /EHsc /W4 /WX .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30139 for x64
Copyright (C) Microsoft Corporation. All rights reserved.
repro.cpp
Microsoft (R) Incremental Linker Version 14.29.30139.0
Copyright (C) Microsoft Corporation. All rights reserved.
/out:repro.exe
repro.obj
C:\Temp>.\repro.exe
Default constructor
Expected behavior
The line "Default constructor" should not be printed. Moreover, replacing the default constructor test() by
test() = delete;
test(const test&) = default;
test& operator=(const test&) = delete;should still compile.
STL version
Microsoft Visual Studio Professional 2019
Version 16.11.9
vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.