Specializing a template on a lambda in C++0x
- by Tony A.
I've written a traits class that lets me extract information about the arguments and type of a function or function object in C++0x (tested with gcc 4.5.0). The general case handles function objects:
template <typename F>
struct function_traits {
template <typename R, typename... A>
struct _internal { };
template <typename R, typename... A>
struct _internal<R (F::*)(A...)> {
// ...
};
typedef typename _internal<decltype(&F::operator())>::<<nested types go here>>;
};
Then I have a specialization for plain functions at global scope:
template <typename R, typename... A>
struct function_traits<R (*)(A...)> {
// ...
};
This works fine, I can pass a function into the template or a function object and it works properly:
template <typename F>
void foo(F f) {
typename function_traits<F>::whatever ...;
}
int f(int x) { ... }
foo(f);
What if, instead of passing a function or function object into foo, I want to pass a lambda expression?
foo([](int x) { ... });
The problem here is that neither specialization of function_traits<> applies. The C++0x draft says that the type of the expression is a "unique, unnamed, non-union class type". Demangling the result of calling typeid(...).name() on the expression gives me what appears to be gcc's internal naming convention for the lambda, main::{lambda(int)#1}, not something that syntactically represents a C++ typename.
In short, is there anything I can put into the template here:
template <typename R, typename... A>
struct function_traits<????> { ... }
that will allow this traits class to accept a lambda expression?
