C++ -- Questions about function pointers

Question

I have written the following code:

#include "stdafx.h"
#include <iostream>
using namespace std;

double funcA()
{
    return 100.0;
}

int g(double (*pf)())
{
    cout << (*pf)() << endl;
    return 0;
}

int g2(double pf())
{
    cout << pf() << endl;
    return 0;
}

int _tmain(int argc, _TCHAR* argv[])
{
    g(&funcA);  // case I
    g(funcA);   // case II

    g2(funcA);  // case III
    g2(&funcA); // case IV
    return 0;
}

I have run the above code on VS2008 and each function call returns '100'. Here is the question:

Q1> Is there any problem in the code?

Q2> It seems that C++ doesn't make difference between *pf and pf. Is that correct?

Thank you

Answer 1

C++ does, in fact, make a distinction between the types double() and double(*)(), but the difference is subtle. When you pass a function type as an argument to a function, the function-type automatically "degrades" to a function pointer. (This is similar, I suppose, to how an array type degrades to a pointer type when passed as a function argument.)

However, a function type and a function-pointer type are still different types, according to the C++ type-system. Consider the following case:

void g() { }

template <class F>
struct Foo
{
    Foo(const F& f) : func(f)
    { }

    void operator()() { func(); }

    F func;
};


int main ()
{
    Foo<void()> f(g);
    f();
}

This should fail to compile, since you cannot declare a function type as an automatic variable. (Remember, functions are not first-class objects in C++.) So the declaration F func; is invalid. However, if we change our main function to instead instantiate the template using a function pointer, like so:

int main ()
{
    typedef void(*function_pointer)();
    Foo<function_pointer> f(g);
    f();
}

...now it compiles.