Question

It would be nice if I could do the following:

template <class RT, class... PT>
class Event 
{
    ...
    void operator()(PT... args)
    {
        std::for_each(
            l.begin(), l.end(), [args...](Handler *p) { (*p)(args...); }
        );
    }
    ...
};

Unfortunately I couldn’t make it compile with g++ 4.7.2 (-std=c++0x):

evtempl.hh: In member function ‘void elt::Event::operator()(PT …)’:
evtempl.hh:75:54: error: expected ‘,’ before ‘…’ token
evtempl.hh:75:54: error: expected identifier before ‘…’ token
evtempl.hh:75:57: error: parameter packs not expanded with ‘…’:
evtempl.hh:75:57: note: ‘args’
evtempl.hh: In lambda function:
evtempl.hh:76:26: error: expansion pattern ‘args’ contains no argument packs
evtempl.hh: In instantiation of ‘void elt::Event::operator()(PT …) [with RT = void; PT = {int}]’:
testevtempl.cc:28:9: required from here
evtempl.hh:74:9: error: using invalid field ‘elt::Event::operator()(PT …)::::Handler*)>::__args’
evtempl.hh: In instantiation of ‘void elt::Event::operator()(PT …) [with RT = void; PT = {int, const char*}]’:
testevtempl.cc:29:20: required from here
evtempl.hh:74:9: error: using invalid field ‘elt::Event::operator()(PT …)::::Handler*)>::__args’

instead, I have to change that lambda to the old, mundane syntax:

for (itr = l.begin(); itr != l.end(); ++itr)
     (*(*itr))(args...);

This one compiles and works fine.

I wonder why the lambda syntax doesn’t work.

1. Did I do something wrong or miss something?
2. Is such thing outlawed in the c++11 standard?
3. or, this is allowed by the standard, but it’s
   a problem of the current compiler?

I tried

[=](Handler *p) { (*p)(args...); }

it gives the same error as if you did:

[args](Handler *p) { (*p)(args...); }

complaining parameter packs not expanded

Answer 1

It’s a bug in gcc. See [c++0x]lambdas and variadic templates don’t work together or perhaps [C++11] Pack expansion fails in lambda expressions.