Thread: Pointer to function problem

Hi,

So I have three similar working functions:

Code:

char *ltrim(char *);
char *rtrim(char *);
char *trim(char *);

And i want to write a function that can test these functions and takes a pointer to function as its argument.

I tried this first

Code:

void test_func(char *(*func)(char *in))
{
    printf("in = |%s|\nin.length = %d\nout = |%s|\nout.length = %d\n",
	    in,
	    strlen(in),
	    (*func)(in),
	    strlen((*func)(in)));
}

But this won't compile and the compiler says 'in' is undeclared.

Then I changed it to this

Code:

void test_func(char *(*func)(char *),char *in)
{
    printf("in = |%s|\nin.length = %d\nout = |%s|\nout.length = %d\n",
	    in,
	    strlen(in),
	    (*func)(in),
	    strlen((*func)(in)));
}

And it seemed to work since i could print out the expected results with this

Code:

test_func(ltrim, str);

So my problem is
1) why the first piece of code won't compile (to be more specific, I'd expected 'in' to be an argument passed to the function that is pointed to. What does the compiler *think* of this variable to be)
2) Is the second piece of code the right way to do this?

Thanks,

1) In the first case 'in' is just part of the function prototype (you aren't actually invoking the function at that point, after all, so how could 'in' actually get passed in?). Think of it as if you had created a typedef:

Code:

typedef char* (* function )( char* in );

void test( function fun )
{
	fun( "ok" );
}

2) That's fine, but you don't actually have to dereference the pointer (see example above). Either way works, though.

Thank you guys for reply. Still i have a question.

Originally Posted by King Mir

char *(*func)(char *) is a function pointer. It refers to a function by itself. It does not include any arguments. So you can use the same function pointer multiple times with different arguments.

So King Mir what do you mean by "It does not include any arguments."?
In my understanding char *(*func)(char *) is a pointer to function that takes type char * as argument and returns type char *.

For the dereferencing part, yes, that's quite cool and also Sebastiani's typedef solution looks neat. I'll try those and thanks for these good suggestions.

Thanks laserlight. I get it now.

also, you can just do

Code:

void test_func(char *func(char *),char *in)