Thread: Parameters in function pointers

Trying to get around the old "invalid use of void expression" is starting to annoy me now. How do I pass parameters into a function that is a parameter of another function? Such as here:

Code:

MainMenu->CreateMenuItem("Play",SDL_Color{0,255,0},SDL_Color{255,255,255},New(this));
            MainMenu->CreateMenuItem("High Scores",SDL_Color{255,0,0},SDL_Color{255,255,255},NULL);
            MainMenu->CreateMenuItem("Exit",SDL_Color{255,0,0},SDL_Color{255,255,255},(*exit)(0));

            while(MainMenu->Run(Display));

            exit(0);
        }
    }
}

void New(void* data)
{
    cGame* temp = static_cast<cGame*>(data);
    temp->Phase = G_NEW_GAME;
}

Attempting to pass any data to the function just results in that error. I looked it up and can't find anything that helps. Code is cutoff also. Ignore the brackets.

A void return type means "return nothing". It does not return a pointer.

Change the return type of New(). At a guess, you need to do this.

Code:

cGame *New(void *data)
{
    cGame* temp = static_cast<cGame*>(data);
    temp->Phase = G_NEW_GAME;
    return temp;
}

Since your caller is passing a "this" pointer, and the function is interpreting the argument as a pointer to cGame, it is probably appropriate to change the argument type of New() and eliminate the static_cast<>.

Code:

cGame *New(cGame *data)
{
    data->Phase = G_NEW_GAME;
    return data;
}

There are other things wrong with your code, but I assume you know how to fix those.

That made sense and I think I understand void pointers a little more than I did now. That however still didn't fix the problem as I'm getting the same error. As for the other warning-inducing problems, yes I know how to fix those although as for right now, I'm just testing something so they're not much of an issue. Any other fatal errors I'm missing?

Code:

            MainMenu->CreateMenuItem("Play",SDL_Color{0,255,0},SDL_Color{255,255,255},(*New)(this));
            MainMenu->CreateMenuItem("High Scores",SDL_Color{255,0,0},SDL_Color{255,255,255},NULL);
            MainMenu->CreateMenuItem("Exit",SDL_Color{255,0,0},SDL_Color{255,255,255},(*Exit)(0));

            while(MainMenu->Run(Display));

            exit(0);
        }

        if (Phase == G_NEW_GAME)
        {

        }
    }
}

cGame* New(cGame* data)
{
    data->Phase = G_NEW_GAME;
    return data;
}

If it's any correlation, here's how Menu itself is set up and how I'm using those pointers.

Code:

typedef void (*MENU_FUNC)(void*);

struct MenuItem
{
    string Str;
    int x,y,w,h;
    SDL_Color Inactive;
    SDL_Color Active;

    SDL_Color Drawn;

    bool PickedAndMouseDown;
    MENU_FUNC Option;
};

class Menu
{
    private:
        vector<MenuItem*> Items;
        TTF_Font* Font;
        SDL_Surface* Bitmap;
    public:
        Menu(TTF_Font* f);
        ~Menu(){ TTF_CloseFont(Font); SDL_FreeSurface(Bitmap); }
        bool ItemChosen(); // Run through Item locations and check mouse coords and mouse_button states

        bool Run(SDL_Surface* Display);
        void CreateMenuItem(string Text, SDL_Color, SDL_Color, MENU_FUNC);
};

I suggested you change the function implementation. You have also incorrectly changed the calling code.

The changes I suggested will work without changing any of the MainMenu->CreateMenuItem() code.

I did change the function impl. and I have just reverted the calling code. Still getting the same error and I don't see the problem besides my misunderstanding of how function pointers work. I feel like I'm being annoying and/or stupid now.

OK; in your previous post, I hadn't noted that your CreateMenuItem function accepts a pointer to a function.

All you need to do when calling it is

Code:

MainMenu->CreateMenuItem("Play",SDL_Color{0,255,0},SDL_Color{255,255,255},New);

You don't need to call New(). That is presumably happening in the implementation of CreateMenuItem(). For this to work, the implementation of New() needs to be what you had it in your original post.

Code:

void New(void* data)
{
    cGame* temp = static_cast<cGame*>(data);
    temp->Phase = G_NEW_GAME;
}

That will get your code to compile.

Now, the problem is that somewhere in the implementation of CreateMenuItem() there needs to be a line that calls this function, and provides a pointer to a cGame to it. For example;

Code:

void Menu::CreateMenuItem(string Text, SDL_Color, SDL_Color, MENU_FUNC func)
{
     // other stuff

     cGame *some_object = retrieve_address_of_a_cGame();

     func(some_object);
}

If CreateMenuItem() does not do this, then the code will compile but not run correctly.

Note that I'm guessing as you haven't provided complete context (what your code is doing).

There are differences between a pointer to void, and a pointer to a function.

Ah. And that's why most things like this have other arguments that supply parameters to pass to the function. Thank you for the information. Context doesn't matter too much. Just wanted to know a certain something.

I don't think static_cast works here.
You may need reinterpret_cast for pointers.