Should C++ function default argument values be specified in headers or .cpp source files?

Question

I am kind of new to C++. I am having trouble setting up my headers. This is from functions.h

extern void apply_surface(int, int, SDL_Surface *, SDL_Surface *,SDL_Rect *); 

And this is the function definition from functions.cpp

void apply_surface(int x, int y, SDL_Surface * source, SDL_Surface * destination,SDL_Rect *clip = NULL) {     ... } 

And this is how I use it in main.cpp

#include "functions.h" int main (int argc, char * argv[]) {     apply_surface(bla,bla,bla,bla); // 4 arguments, since last one is optional. } 

But, this doesn't compile, because, main.cpp doesn't know last parameter is optional. How can I make this work?

Answer 1

You make the declaration (i.e. in the header file - functions.h) contain the optional parameter, not the definition (functions.cpp).

//functions.h extern void apply_surface(int, int, SDL_Surface *, SDL_Surface *,SDL_Rect * clip = NULL);  //functions.cpp void apply_surface(int x, int y, SDL_Surface * source, SDL_Surface * destination,SDL_Rect *clip /*= NULL*/) {     ... } 

Answer 2

The default parameter value should be in the function declaration (functions.h), rather than in the function definition (function.cpp).