c++ - c++11 std::async doesn't work in mingw

Question

Running this code from Herb Sutter's presentation. This works fine in linux under gcc 4.6.3. I'm thinking that future.h isn't supported in mingw, but the error is really hard to understand!

#include <iostream>
#include <vector>
#include <string>
#include <future>
#include <algorithm>

using namespace std;

string flip( string s ) {
  reverse( begin(s), end(s) );
  return s;
}

int main() {
  vector<future<string>> v;

  v.push_back( async([]{ return flip(" ,olleH"); }) );
  v.push_back( async([]{ return flip(".gnaL"); }) );
  v.push_back( async([]{ return flip("
!TXEN"); }) );

  for( auto& e: v ) {
    cout << e.get();
  }
}

Here is the error:

$ x86_64-w64-mingw32-g++.exe -std=c++0x -pthread swap.cpp
swap.cpp: In function 'int main()':
swap.cpp:17:51: error: invalid use of incomplete type 'std::__async_sfinae_helper<main()::<lambda()>, main()::<lambda()> >::type {aka class std::future<std::basic_string<char> >}'
In file included from swap.cpp:4:0:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:111:11: error: declaration of 'std::__async_sfinae_helper<main()::<lambda()>, main()::<lambda()> >::type {aka class std::future<std::basic_string<char> >}'
swap.cpp:18:49: error: invalid use of incomplete type 'std::__async_sfinae_helper<main()::<lambda()>, main()::<lambda()> >::type {aka class std::future<std::basic_string<char> >}'
In file included from swap.cpp:4:0:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:111:11: error: declaration of 'std::__async_sfinae_helper<main()::<lambda()>, main()::<lambda()> >::type {aka class std::future<std::basic_string<char> >}'
swap.cpp:19:51: error: invalid use of incomplete type 'std::__async_sfinae_helper<main()::<lambda()>, main()::<lambda()> >::type {aka class std::future<std::basic_string<char> >}'
In file included from swap.cpp:4:0:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:111:11: error: declaration of 'std::__async_sfinae_helper<main()::<lambda()>, main()::<lambda()> >::type {aka class std::future<std::basic_string<char> >}'
swap.cpp:22:14: error: invalid use of incomplete type 'class std::future<std::basic_string<char> >'
In file included from swap.cpp:4:0:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:111:11: error: declaration of 'class std::future<std::basic_string<char> >'
In file included from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_algobase.h:68:0,
                 from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/char_traits.h:41,
                 from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/ios:41,
                 from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/ostream:40,
                 from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/iostream:40,
                 from swap.cpp:1:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_iterator.h: In instantiation of '__gnu_cxx::__normal_iterator<_Iterator, _Container>& __gnu_cxx::__normal_iterator<_Iterator, _Container>::operator++() [with _Iterator = std::future<std::basic_string<char> >*; _Container = std::vector<std::future<std::basic_string<char> > >; __gnu_cxx::__normal_iterator<_Iterator, _Container> = __gnu_cxx::__normal_iterator<std::future<std::basic_string<char> >*, std::vector<std::future<std::basic_string<char> > > >]':
swap.cpp:21:17:   required from here
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_iterator.h:750:2: error: cannot increment a pointer to incomplete type 'std::future<std::basic_string<char> >'
In file included from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/vector:65:0,
                 from swap.cpp:2:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_vector.h: In instantiation of 'std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = std::future<std::basic_string<char> >; _Alloc = std::allocator<std::future<std::basic_string<char> > >]':
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_vector.h:247:15:   required from 'std::vector<_Tp, _Alloc>::vector() [with _Tp = std::future<std::basic_string<char> >; _Alloc = std::allocator<std::future<std::basic_string<char> > >]'
swap.cpp:15:26:   required from here
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_vector.h:161:9: error: invalid use of incomplete type 'class std::future<std::basic_string<char> >'
In file included from swap.cpp:4:0:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:111:11: error: declaration of 'class std::future<std::basic_string<char> >'
In file included from c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/vector:63:0,
                 from swap.cpp:2:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_construct.h: In instantiation of 'void std::_Destroy(_ForwardIterator, _ForwardIterator) [with _ForwardIterator = std::future<std::basic_string<char> >*]':
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_construct.h:155:7:   required from 'void std::_Destroy(_ForwardIterator, _ForwardIterator, std::allocator<_T2>&) [with _ForwardIterator = std::future<std::basic_string<char> >*; _Tp = std::future<std::basic_string<char> >]'
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_vector.h:403:9:   required from 'std::vector<_Tp, _Alloc>::~vector() [with _Tp = std::future<std::basic_string<char> >; _Alloc = std::allocator<std::future<std::basic_string<char> > >]'
swap.cpp:15:26:   required from here
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/bits/stl_construct.h:128:7: error: invalid use of incomplete type '_Value_type {aka class std::future<std::basic_string<char> >}'
In file included from swap.cpp:4:0:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:111:11: error: declaration of '_Value_type {aka class std::future<std::basic_string<char> >}'
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future: At global scope:
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:187:5: error: 'typename std::__async_sfinae_helper<typename std::decay<_Func>::type, _Fn, _Args ...>::type std::async(_Fn&&, _Args&& ...) [with _Fn = main()::<lambda()>; _Args = {}; typename std::__async_sfinae_helper<typename std::decay<_Func>::type, _Fn, _Args ...>::type = std::future<std::basic_string<char> >]', declared using local type 'main()::<lambda()>', is used but never defined [-fpermissive]
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:187:5: error: 'typename std::__async_sfinae_helper<typename std::decay<_Func>::type, _Fn, _Args ...>::type std::async(_Fn&&, _Args&& ...) [with _Fn = main()::<lambda()>; _Args = {}; typename std::__async_sfinae_helper<typename std::decay<_Func>::type, _Fn, _Args ...>::type = std::future<std::basic_string<char> >]', declared using local type 'main()::<lambda()>', is used but never defined [-fpermissive]
c:mingw64in../lib/gcc/x86_64-w64-mingw32/4.7.0/include/c++/future:187:5: error: 'typename std::__async_sfinae_helper<typename std::decay<_Func>::type, _Fn, _Args ...>::type std::async(_Fn&&, _Args&& ...) [with _Fn = main()::<lambda()>; _Args = {}; typename std::__async_sfinae_helper<typename std::decay<_Func>::type, _Fn, _Args ...>::type = std::future<std::basic_string<char> >]', declared using local type 'main()::<lambda()>', is used but never defined [-fpermissive]

I'm using GCC 4.7 in windows.

$ g++ -v
Using built-in specs.
COLLECT_GCC=c:mingw64inx86_64-w64-mingw32-g++.exe
COLLECT_LTO_WRAPPER=c:/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/4.7.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../gcc-4.7.0/configure --build=x86_64-w64-mingw32 --enable-targets=all --disable-multilib --enable-64bit --prefix=/mingw64 --with-sysroot=/mingw64 --disable-shared --enable-static --disable-nls --enable-version-specific-runtime-libs --disable-win32-registry --without-dwarf2 --enable-sjlj-exceptions --enable-fully-dynamic-string --enable-languages=c,ada,lto,c++,objc,obj-c++,fortran --enable-libgomp --enable-lto --enable-libssp -enable-gnattools --disable-bootstrap --with-gcc --with-gnu-as --with-gnu-ld --with-stabs --enable-interwork --with-mpfr-include=/home/beta/gcc-build/../gcc-4.7.0/mpfr/src --with-mpfr-lib=/home/beta/gcc-build/mpfr/src/.libs
Thread model: win32
gcc version 4.7.0 (GCC)

Answer 1

Maybe it's easier for me to interpret because I wrote that code, but it's not so hard, just look at the first line of the error output:

swap.cpp:22:14: error: invalid use of incomplete type 'class std::future<std::basic_string<char> >'

That's telling you that std::future is an incomplete type, i.e. it is declared but not defined. The next line tells you exactly where it's declared (then all the other errors are caused by trying to use that incomplete type in different ways.)

If you look in GCC's <future> header you'll see that the types are declared near the top, but then the definitions are dependent on this preprocessor condition:

#if defined(_GLIBCXX_HAS_GTHREADS) && defined(_GLIBCXX_USE_C99_STDINT_TR1) 
  && (ATOMIC_INT_LOCK_FREE > 1)

If you compile some simple tests with those macros you'll find out that _GLIBCXX_HAS_GTHREADS is not defined, and that's because your gcc -v shows

Thread model: win32

No one has provided the necessary code to make the C++11 thread features work on Windows yet.

Making <future> work would be harder, but it's not actually that difficult to enable <thread> and <mutex> but no one has stepped up to do the work yet. Yesterday I posted some ideas for how to enable the C++11 thread features for the win32 thread model: http://gcc.gnu.org/ml/libstdc++/2012-05/msg00020.html