Passing function as argument

This is a discussion on Passing function as argument within the C Programming forums, part of the General Programming Boards category; I cant find any example that shows how it works and what are is advantages. What you mean by passing ...

  1. #1
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    Passing function as argument

    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!

  2. #2
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    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.

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  3. #3
    C++まいる!Cをこわせ!
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    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);
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  4. #4
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    Quote Originally Posted by Elysia View Post
    How do you do it? Passing is easy:
    func(&function);
    and wrong (or at least your example is).

  5. #5
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    Oh? Do elaborate.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
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    You don't take the address of function names.

  7. #7
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    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.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  8. #8
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    What is this class thing you speak of?

  9. #9
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    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.
    }
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #10
    & the hat of GPL slaying Thantos's Avatar
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    Hmm that doesn't look like legal c code...

  11. #11
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    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 &.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  12. #12
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    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.
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  13. #13
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    I believe the ability to pass a function without & is for backwards compatibility only.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  14. #14
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    Actually, I think the requirement of & was originally present, then removed because a function name by itself has no other meaning.
    It is too clear and so it is hard to see.
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    He could have cooked his rice much sooner.

  15. #15
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    Well, one of them is there for backwards compatibility, but as long as it works...
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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