Passing function as argument

**NeMewSys** · 06-11-2008

I cant find any example that shows how it works and what are is advantages.

What you mean by passing a function as argument to another function? How does it work?

Thanks!

**matsp** · 06-11-2008

Look up the standard qsort function in the help/google. It takes a compare function.

As to why you would want to do that, there are plenty of different reasons why you would want that:
1. Flexibility - in the qsort case, it can sort ANY type of data that you can compare in an orderable fashion (and that is entirely up to you - it can be integers, strings, dates (in any format), book structures by number of pages, book structures by ISBN, book structure by author, etc, etc).

2. User supplied functions - for example the Windows API supports "callback" functions, which is called under defined circumstances, e.g. certain events happen.

3. Table-driven programming - a function is used to supply what to do when (for example) a value in a table matches some input data (e.g. parsing commands).

4. Dynamic support for different options (e.g. different formatting, different hardware devices, etc, etc).

This is not a conclusive list of what function pointers/functions as parameters are used for.

--
Mats

**Elysia** · 06-11-2008

How do you do it? Passing is easy:
func(&function);

Declaring a function to take a pointer to a function is less trivial, but something along the lines of:
typedef return_type (name)(argument_type argument_name);
void foo(name* pFunction);

**Thantos** · 06-11-2008

Originally Posted by Elysia

How do you do it? Passing is easy:
func(&function);

and wrong (or at least your example is).

**Elysia** · 06-11-2008

Oh? Do elaborate.

**Thantos** · 06-11-2008

You don't take the address of function names.

**Elysia** · 06-11-2008

It works either way and if you have to do it with members of a class, then I'd rather do it with global functions, as well (if I'm reading you correctly).

Code:

void foo2();
typedef void (fooptr)();
void foo(fooptr* p) { }
int main()
{
    foo2(&foo);
    foo2(foo);
}

Both work.

**Thantos** · 06-11-2008

What is this class thing you speak of?

**Elysia** · 06-11-2008

C++-ish:

Code:

class CFoo
{
public:
    void foo();
}

typedef void (CFoo::* fooptr)();
void foo(fooptr p) { }
int main()
{
    foo(&CFoo::foo); // OK
    foo(CFoo::foo); // Illegal.
}

**Thantos** · 06-11-2008

Hmm that doesn't look like legal c code...

**Elysia** · 06-11-2008

Indeed not, it's C++.
But the point was that if I have to use & on variables, and on member functions in classes, then why should I not do it on global functions?
Both the examples with & and without compiles in both C and C++, so I prefer to use the &.

**iMalc** · 06-11-2008

Older versions of VC didn't allow the & in front of the function name, but newer versions do, and I believe it has to do with standards compliance.

In C++ you can use function references. Much nicer since you never need to check for NULL.

**Elysia** · 06-11-2008

I believe the ability to pass a function without & is for backwards compatibility only.

**King Mir** · 06-11-2008

Actually, I think the requirement of & was originally present, then removed because a function name by itself has no other meaning.

**Elysia** · 06-11-2008

Well, one of them is there for backwards compatibility, but as long as it works...

Passing function as argument

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