Deduce variadic args and return type from functor template parameter (MSVC-specific)

Question

The function invoke in the following code is a simple wrapper for invoking another function / functor / lambda, such that invoke(f,args...) equals f(args...). (The reason behind this is to also have overloads for member functions, allowing a generalized syntax for both usages.)

This implementation works in g++:

template <class Fn, class ...Args>
auto invoke(Fn f, Args ...args) -> decltype(f(args...)) {
    return f(args...);
}

int y = invoke([](int x){ return x+1; }, 42);   // y = 43

Demo

However, MSVC 2012 (Nov CTP) complains:

Error: C2893: Failed to specialize function template 'unknown-type invoke(Fn,Args...)'
With the following template arguments:
'main::<lambda_687537b292ffb7afcfd8f8fc292d6e73>'
'int'

(Note that the int in the error message comes from the 42 being passed to invoke, not from the lambda's argument. I checked this by passing something different to invoke.)

How can I fix this to work with MSVC 2012 (Nov CTP)?

Things I tried:

• When I change the variadic template arguments to a single argument (but still a template argument, i.e. remove all ... in the code above), the code compiles. But I need to Support any number of arguments.

• It also compiles if I replace the trailing return type with a non-generic int return type. But obviously I want to support generic return types.

• The behavior doesn't change if I pass a function pointer instead of a lambda. So it isn't related to lambdas. Example error message:

  Error: C2893: Failed to specialize function template 'unknown-type invoke(Fn,Args...)'
  With the following template arguments:
  'int (__cdecl *)(int)'
  'double'
  

Answer 1

Try using typename result_of<F(Args...)>::type. (and I do mean the case: use types not values).

This does not support SFINAE under the standard, but if might work in half compliant C++11 compilers.

Also use Args&&... and perfect forward if your compiler does not blow up.