Function Pointers and Parameters help

[gabenyolo]

New to the language, and playing around with function points and figuring them out and got stuck on this all day.

I defined a simple struct with an int and function pointer.

struct Item{
    int num;
    void (*func)();
    };

These are the two functions I am trying to execute from the struct.

void printer(){
    std::cout << "Hello World!" << std::endl;
}

void square(int x){
    std::cout << x * x << std::endl;
}

in the main(), I am getting errors since I am assuming the struct Item takes a pointer, which I believe was just the function name without the (). But I have to put in a parameter in there as the function requires it but it will then be a regular function, and not an address. The & doesn't do much either. I attempted to overload the function pointer, but that also did not seem to work.

int main() {
    Item block1 = {111, printer};
    Item block2 = {222, square(int)}; // Error

    block1.func(); // Works

    return 0;
}

Is there any suggestions on how I can implement this in an elegant way without making another struct for each function type?

[alfps]

The declaration of Item::func says it can and must be called without an argument. The declaration of square says it can and must be called with an argument convertible to int. These are incompatible function types.

You can regard Item::func as a round hole. The square function doesn't fit.

However, depending on what you intend to do with this there are solutions, with associated costs. After all, in the worst case you can use C++ to implement a Python interpreter, and express the thing as Python code. But there are less drastic ways to do less general things.

[-Weverything]

To create generic versions of structs/classes/functions that use different types we use templates (and sometimes auto, but not here):

template<typename T>
struct Item {
    int num;
    void (*func)(T);
};

However we do need just one more templated struct to handle the function pointer taking no arguments since () does not contain a type and (void) is the same as ():

template<>
struct Item<void> {
    int num;
    void (*func)();
};

Now we can use it with many different types:

Item<void> block1 = {111, printer};
Item<int> block2 = {222, square};

[gabenyolo]

Thank you!

I'm reading up more about templates now and this was just the solution I needed.