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?

have you tried adding to the header?

PlasmaHH
– PlasmaHH

2012-02-13 12:08:54 +00:00
Commented Feb 13, 2012 at 12:08 — PlasmaHH
– PlasmaHH, Commented Feb 13, 2012 at 12:08

rayryeng · Accepted Answer · 2017-02-03 21:54:09Z

182

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*/)
{
    ...
}

edited Feb 3, 2017 at 21:54

rayryeng

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answered Feb 13, 2012 at 12:08

Luchian Grigore

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2 Comments

Björn Pollex Over a year ago

To clarify, the declaration is the part in the header.

clockw0rk Over a year ago

this is something i always confuse when setting up an empty c++ application, i really need to start using default-application setups

Didier Trosset · Accepted Answer · 2012-02-13 12:14:30Z

25

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

edited Feb 13, 2012 at 12:14

answered Feb 13, 2012 at 12:09

Didier Trosset

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Comments

Michel Keijzers · Accepted Answer · 2012-02-13 12:10:00Z

3

Use:

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

(note I can't check it here; don't have a compiler nearby).

answered Feb 13, 2012 at 12:10

Michel Keijzers

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Comments

Pupper Gump · Accepted Answer · 2022-02-23 02:01:38Z

-4

Strangely enough, it works fine for me if I have a virtual function without a default parameter, and then inheritors in .h files without default parameters, and then in their .cpp files I have the default parameters. Like this:

// in .h
class Base {virtual void func(int param){}};
class Inheritor : public Base {void func(int param);};
// in .cpp
void Inheritor::func(int param = 0){}

Pardon the shoddy formatting

answered Feb 23, 2022 at 2:01

Pupper Gump

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1 Comment

BobMorane Over a year ago

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

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