Function alias vs Function pointer alias

Last weekend (4/4/2021) I was sitting down and working on my San Diego C++ agenda for the 25th upcoming session.

One of things I really like presenting is few riddles from cppquiz.org

I stumbled upon this one: https://cppquiz.org/quiz/question/227 . Simple and easy one.

Here is the code in question:

#include <iostream>

using Func = int();

struct S {

Func f;

int S::f() { return 1; }

int main() {

std::cout << s.f();

And the question is what is the output.

So clearly the question is what does the following syntax mean?

Func f;

Initially, the “using” part looked like an innocent typedef of a function pointer. But is it?

A quick check with cppinsights.io shows the following

using Func = int();

struct S

int f();

Clearly this is not a function pointer typedef, and we are not defining any “f” as a data member.

Instead, we are actually defining a function named as “f” that returns “int” and takes no params.

So when does it look like a function pointer and when it is not?

Here is a small playground cppinsights.io.

And the gist of it is described in the following snippet:

using Func = int(); // function typedef

using Funcp_t = int(*)(); // function pointer typedef

using bFuncp_t = std::add_pointer_t<int()>; // Modern C++ function pointer typedef

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