I tried the following code but doesn't compile.
template <class T, class... A>
void tpool::enqueue(T&& func, A&&... args) {
std::function<void()> task([func, args] () {
//...
});
}
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2 Answers
Just use the ellipses. Per paragraph 5.1.2/23 of the C++11 Standard:
A capture followed by an ellipsis is a pack expansion (14.5.3). [ Example:
template<class... Args> void f(Args... args) { auto lm = [&, args...] { return g(args...); }; lm(); }—end example ]
Note: Interestingly enough, GCC is refusing to compile this (see the live example):
template <class T, class... A>
void foo(T&& func, A&&... args) {
std::function<void()> task([func, args...] () {
//...
});
}
But considering the above example from the Standard, this is definitely a compiler issue.
5 Comments
maverik
Amazing, do you hold in memory the whole standard or just do a quick look?
Marc Glisse
Looks like this gcc bug: gcc.gnu.org/bugzilla/show_bug.cgi?id=41933 (your quote was a rather late addition to the standard)
Andy Prowl
@MarcGlisse: Oh, interesting. Thank you for the information! Not sure if that's related, but this also fails to compile...
Marc Glisse
I assume it is the same issue (the capture is just implicit), but if you have a doubt, feel free to add a small testcase to that bug report (that might also remind people that the bug is still there). And don't hesitate to report other bugs when you find them.
Walter
@MarcGlisse fyi: both compile find under clang (llvm 3.2).
When you use args in the capture, you need ellipsis:
template <class T, class... A>
void tpool::enqueue(T&& func, A&&... args) {
std::function<void()> task([func, args...] () {
//...
});
}
answered May 13, 2013 at 17:21
Some programmer dude
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4 Comments
Mpac
I've tried your code but doesn't compile again. I get the following error:
error: expected ',' before '...' token error: expected identifier before '...' token error: parameter packs not expanded with '...'Some programmer dude
@MarcoPacini Are you using GCC? In that case there seems to be a bug in the compiler (as in the answer by Andy Prowl).
Mpac
Yes, I use GCC 4.7.2 installed with macport.
Isac Casapu
Same issue with G++ 4.8.4. The G++ ticket appears to hint that a resolution is planned for the 4.9 time-frame.
