Function pointer and default argument

Question

Let's assume, I have a function with a prototype like (in function.h)

void function(int argument, bool flag);

This function has two variant behaviours, dependening on flag.

Early on, in a module (module.h, module.c) I called that function:

#include "function.h"

function(argument1, true);
function(argument2, false);

Let's further assume, I want to pass this function into a module, inverting that dependencies:

void (* funcPtr)(int argument, bool flag);

void setFuncPtr(void (* func)(int argument, bool flag))
{
  funcPtr = func;
}

Now I can have in function.c:

#include "module.h"

setFuncPtr(function);

And in module.h:

funcPtr(argument1, true);
funcPtr(argument2, false);

Is there a way to have two pointers, each pointing to the function, but with different, hardcodes values for flag? Like:

void (* funcPtrTrue)(int argument, bool flag);
void (* funcPtrFals)(int argument, bool flag);
/* Or, even better
void (* funcPtrTrue)(int argument);
void (* funcPtrFals)(int argument);
*/

void setFuncPtr(void (* func)(int argument, bool flag))
{
  funcPtrTrue  = func(argument, true);
  funcPtrFalse = func(argument, false);

}

(I now, this is no correct code, but should illustrated the desired functionality)

After Olis proposal, let's complicate things. bool is a typedef in function.h:

typedef enum {
  FALSE,
  TRUE
} bool;

Now I have to include function.h again to module.c in order to make it work again.

#include "function.h"

void functionTrue(int argument)
{
  fktPtr(argument, TRUE);
}

Nope, not possible. A function pointer is an address. It doesn't hold any more information in it, as argument checking is compile-time, not run-time. Use wrappers, like Oli suggested. — Nikos C.
– Nikos C., Commented Mar 25, 2013 at 12:40

Oliver Charlesworth · Accepted Answer · 2013-03-25 12:40:20Z

4

C doesn't have default arguments.

I assume there's some good reason you want two separate function pointers, rather than just directly calling funcPtr with the required flag parameter. If so, why not just have two wrapper functions inside your module?

void (* funcPtr)(int argument, bool flag);

void setFuncPtr(void (* func)(int argument, bool flag)) {
  funcPtr = func;
}

void funcTrue(int argument)  { funcPtr(argument, true); }
void funcFalse(int argument) { funcPtr(argument, false); }

answered Mar 25, 2013 at 12:40

Oliver Charlesworth

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7 Comments

Oliver Over a year ago

Okay, that helps alot. Just one more point. Suppose, flag is not a bool but a typedef enum. Now I need to include that typedef somehow, but with the function-pointer setter I wanted to invert exact that dependency..

Oliver Charlesworth Over a year ago

@Oliver: So do you want your module to have a public API function along the lines of setFuncPtr(funcPtr, argToUse)?

Oliver Over a year ago

I'm not sure I understand what you mean. I want to hide enumeration, that controls the function. function.h should be hidden and the API of module.h should be usable without function.h. I want to be able to replace function.h without changing the interface of module.h.

Oliver Charlesworth Over a year ago

@Oliver: What I mean is, how do you intend to inject the value of flag?

Oliver Over a year ago

Ah, I see. As for the problem, where the question rised: Not at all. I only need one behavior of the function, therefore only one value for flag. This is why I didn't want to expose the whole interface with the enumeration to the "outside".

|

MOHAMED · Accepted Answer · 2013-03-25 13:30:58Z

1

you can use structures

struct funcstruct {
    void (*func)(int arg, bool flag);
    bool default_flag;
} funcstruct;

struct funcstruct funcPtrTrue, funcPtrFalse;

void setFuncPtr(void (* func)(int argument, bool flag))
{
  funcPtrTrue.func = func; funcPtrTrue.default_flag = true;
  funcPtrFals.func = func; funcPtrFals.default_flag = false;

}

edited Mar 25, 2013 at 13:30

answered Mar 25, 2013 at 13:02

MOHAMED

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