Dereferencing a function pointer

[Ganado]

I'm exploring function pointers as I think it will be a good way to do some curve parameterization stuff.
Given the following code,

double func(int a, char c)
{
    return 3.14 + a + static_cast<int>(c);
}
#include <iostream>
int main()
{
    using namespace std;
    double (*pfunc) (int, char) = func;
    cout << *pfunc << endl;
    cout << pfunc(2, '\0') << endl;

    return 0;
}

Line 9 prints 1 to the console.

My question is just, what is happening here, and why does it happen to be 1?
Undefined behavior? It's also 1 on cpp.sh.

[MiiNiPaa]

You probably meant line 10.
Try this. It might shed some light on what happens here.

double func(int a, char c)
{
    return 3.14 + a + static_cast<int>(c);
}
#include <iostream>
int main()
{
    using namespace std;
    double (*pfunc) (int, char) = func;
    cout.setf(ios::boolalpha);
    cout << *pfunc << endl;
    cout << pfunc(2, '\0') << endl;
}

[Ganado]

Thanks (yeah line 10 woops), so dereferencing it like that returns a bool to represent it? Is there any useful point of this, and in what situation could it be false?

[MiiNiPaa]

so dereferencing it like that returns a bool to represent it?

No, it returns function which gets converted to the function pointer right away. Because there is no overload for << operator which takes function pointer it gets converted to nearest acceptable result: bool. Every pointer can be converted to bool, not only function pointers.

Is there any useful point of this, and in what situation could it be false?

Yes, there is. You can use it at the same situation you would use it with normal pointers — to check if pointer holds value:

1 2 3 4 5 6 7

void log(std:string s, std::ostream* out = nullptr) { if(out) //If out parameter is not null *out << s; else std::clog << s; }

void callback(int i, void(*)(int) foo = nullptr) { if(foo) //If we are provided with function foo(i); else default_handler(i); }

[Ganado]

Thanks, the reason why *pfunc looks like bool makes sense now.

Edit : Although actually, one more question:

    double (*pfunc) (int, char) = nullptr; // setting it to null instead of a function
    cout.setf(ios::boolalpha);
    cout << *pfunc << endl;

This is defined behavior to display false, then?
It just seems really weird to be dereferencing something that looks like it's NULL, but I guess it's okay to do it if it's a function pointer and not just a regular pointer.

[MiiNiPaa]

I guess it's okay to do it if it's a function pointer and not just a regular pointer.

No, it is not. You should not dereference null pointers. Actually you should not dereference function pointers at all: in most cases they would just revert back to pointers immideately and in the only case where they are not (calling function immediately), you can just call it though pointer immediately:

double func(int a, char c)
{
    return 3.14 + a + static_cast<int>(c);
}
#include <iostream>
int main()
{
    using namespace std;
    double (*pfunc) (int, char) = func;
    cout.setf(ios::boolalpha);
    cout << pfunc << endl; // No dereferencing: still bool context
    cout << pfunc(2, '\0') << endl; //No dereferencing, still calling correctly

    cout << (*pfunc)(2, '\0') << endl; //Calling though dereferenced pointer
    pfunc = nullptr;
    cout << pfunc; //No dereferencing
}

[Ganado]

Okay. So basically even though the language gives you the ability to do it, there is no reason to dereference a function pointer without using it at the same time.
Once again thanks for your time.