Why is there still no std::thread, std::promise and std::future in MinGW? And what's the alternative to promise and futures in win32?

Question

So I've been trying to get the following code to compile and run on Windows by using a MinGW compiler.

#include <iostream>
#include <thread>
#include <future>

void ThreadWork(std::promise<int> &&p)
{
    // Pre-return from thread, do work after returning a value
    /* if else and things */ p.set_value(0);

    std::cout << "From thread" << std::endl;
    // More work
}

int main()
{
    std::promise<int> p;
    std::future<int> f = p.get_future();
    std::thread th(ThreadWork, std::move(p));

    // Get return value from thread
    int ret = f.get();
}

The above gives the error:

 error: aggregate 'std::promise<int> p' has incomplete type and cannot be defined
  75 |         std::promise<int> p;

error: variable 'std::future<int> f' has initializer but incomplete type
   76 |         std::future<int> f = p.get_future();

error: 'thread' is not a member of 'std'
   77 |         std::thread th(readInThread, callback, std::move(p));

Tried using official MinGW build, Tried using meganz/mingw-std-threads (that seem to throw errors like _WIN32_WININT is not set even though it is set to 0x0A00, if I try to set it manually compilation fails with redeclaration), and many more, succeed in none...

I also tried to link with pthreads: -lpthread no luck :(

Why are C++11 standard features still not implemented in MinGW? And what's the alternative for them currently?

@TedLyngmo sorry I did added that, but still no luck, same error. — Animesh Sahu, Jan 22 '21 at 12:51
To link `-lpthread` make sure it is installed . If you are using cmake try `find_package(Threads REQUIRED)` first — Umar Farooq, Jan 22 '21 at 12:57
What compiler are you using? g++ 7.3.0 compiles this without errors. — TonyK, Jan 22 '21 at 13:00
@TonyK g++ 9.2.0 @UmarFarooq doesn't symbols in `std::` namespace included in stdlib? — Animesh Sahu, Jan 22 '21 at 13:02
You have some other problems in the code that may distract people from your actual problem. [demo](https://godbolt.org/z/rxY8Mx) - Also, if you ever need to link with `pthread`, use the `-pthread` option, not the `-lpthread` link instruction. — Ted Lyngmo, Jan 22 '21 at 13:11

Benjamin Bihler · Answer 1 · 2021-01-22T13:15:47.370

futures and promises have been working for a long time in MinGW-w64. I am not content with the official binary distributions. Therefore I build MinGW-w64 on my own using https://github.com/niXman/mingw-builds.

With a Unix-like console (MSYS2) the steps after cloning the repository are the following:

cd /c/mingw-builds/
mkdir BuildRoot
git pull origin develop
git checkout develop
cd BuildRoot
../build --mode=gcc-10.2.0 --arch=x86_64 --buildroot=/c/mingw-builds/BuildRoot --update-sources --exceptions=seh --threads=posix --enable-languages=c++ --jobs=48 --rt-version=v7

#(https://stackoverflow.com/questions/32221221/mingw-x64-windows-plugin-needed-to-handle-lto-object)
mkdir mingw64/lib/bfd-plugins
cp mingw64/libexec/gcc/x86_64-w64-mingw32/10.2.0/liblto_plugin-0.dll mingw64/lib/bfd-plugins/liblto_plugin-0.dll

Then use the compiler, it should work like a charm and support futures and promises.

The MSYS2 GCC packages should have std::thread support... Those are based on MinGW-w64 of course, not MinGW.org, which a far as I know, does not have support for a lot more than only std::thread. — rubenvb, Jan 22 '21 at 13:14

Why is there still no std::thread, std::promise and std::future in MinGW? And what's the alternative to promise and futures in win32?

1 Answers1