Question

This question on the object generator pattern got me thinking about ways to automate it.

Essentially, I want to automate the creation of functions like std::make_pair, std::bind1st and std::mem_fun so that instead of having to write a different function for each template class type, you could write a single variadic template template function that handles all cases at once. Usage of this function would be like:

make<std::pair>(1, 2);         // equivalent to std::make_pair(1, 2)
make<std::binder2nd>(&foo, 3); // equivalent to std::bind2nd(&foo, 3);

Is it possible to write this function make? I have tried this, but it doesn’t work in GCC 4.5 or 4.6:

template <template <typename...> class TemplateClass, typename... Args>
TemplateClass<Args...> make(Args&&... args)
{
    return TemplateClass<Args...>(std::forward<Args>(args)...);
}

If I try to call (e.g) make<std::pair>(1, 2) I just get

error: no matching function for call to 'make(int, int)'

Have I got the syntax wrong anywhere here?
Or is this right and GCC is wrong?
Or is this just fundamentally impossible in C++0x?

[edit]

Proposal N2555 seems to suggest that this is allowed and GCC claims to have implemented it in GCC4.4.

Answer 1

That’s exactly right. I would expect it to work. So I think that GCC is in error with rejecting that. FWIW:

#include <utility>

template <template <typename...> class TemplateClass, typename... Args>
TemplateClass<Args...> make(Args&&... args)
{
    return TemplateClass<Args...>(std::forward<Args>(args)...);
}

int main() {
  make<std::pair>(1, 2);
}


// [js@HOST2 cpp]$ clang++ -std=c++0x main1.cpp
// [js@HOST2 cpp]$