C++/SDL 'void*' is not a point-to-object type

Question

Hello there,

I'm new on C++ and I'm trying to make some testing with C++ and SDL and in SDL we have a function:

SDL_TimerID SDL_AddTimer(Uint32 interval, SDL_NewTimerCallback callback, void *param);

which I can pass a callback for the timer created. But apparently it converts my instance this to *void so I can't retrieve it again on the update method which is static, and it's interesting but the the SDL_AddTime doesn't work on a non static callback function.

Well, so my problem is that when trying to call the public method render through the void* param argument It complains about not being a pointer-to-object-type...

Is there any way I can get the Character instance again inside the update method since I don't have control over the SDL_AddTime function and I have to pass the required parameters?

Thanks

#include "Character.h"

Character::Character(void)
{
  timer = SDL_AddTimer(33, update, this);
  this->render(); // is called without problem
}

//static method
Uint32 Character::update(Uint32 interval,void* param)
{
  param->render(); // yields: 'void*' is not a pointer-to-object type;

  SDL_Event event;

  event.type = SDL_USEREVENT;
  event.user.code = 1020;
  event.user.data1 = param;

  SDL_PushEvent(&event);

  return interval;
}

void Character::render(void)
{
  printf("rendering character \n");
}

Answer 1

Cast your param pointer to a Character:

Character * charPtr = reinterpret_cast<Character *>(param);
charPtr->render();

Answer 2

The reason is that C++ is a strong typed language. To change one type to another, you need to cast it first:

Uint32 Character::update(Uint32 interval, void* param)
{
    reinterpret_cast<Character* >(param)->render();

    /* ... */
}

Answer 3

You don't need a reinterpret_cast - a static_cast should be OK:

Character * cp = static_cast <Character *>( param );

You should avoid reinterpret_cast - it is almost always implementation specific, and may hide problems - just like old-style C casts.