Member function call in decltype

Question

The following code:

struct A
{
    int f(int);
    auto g(int x) -> decltype(f(x));
};

Fails to compile with the error:

error: cannot call member function 'int B::f(int)' without object

If I change it to:

struct A
{
    int f(int);
    auto g(int x) -> decltype(this->f(x));
};

I get another error:

error: invalid use of 'this' at top level

What is wrong with either of these? I am using gcc 4.6

Answer 1

Here are the magic words:

struct A
{
    int f(int);
    auto g(int x) -> decltype((((A*)0) ->* &A::f)(x)) ;
};

Edit I see from Mikael Persson's answer that this is how it's done in boost.

Answer 2

result_of and decltype in combination can give the the return type for a member function

#include <type_traits>
using namespace std;

struct A
{
    int f(int i) { return i; } 
    auto g(int x) -> std::result_of<decltype(&A::f)(A, int)>::type
    { 
        return x;
    }
};


int main() {
    A a;
static_assert(std::is_same<decltype(a.f(123)), 
                  decltype(a.g(123))>::value, 
                  "should be identical");
return 0;
}