Define new function as product of two functions

This is a discussion on Define new function as product of two functions within the C Programming forums, part of the General Programming Boards category; Hello everyone, I am trying to write a function in C which takes in two (arbitrary) function pointers and returns ...

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    Define new function as product of two functions

    Hello everyone, I am trying to write a function in C which takes in two (arbitrary) function pointers and returns a pointer to a function which is the product of the two initial (arbitrary) functions. For example, if I have two arbitrary functions f(x) and g(x), my new function h(x) should equal f(x)*g(x). I do not want to evaluate the functions at any point, only after I have defined the new function h(x). Can anyone help me?

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    Code:
    int DynaFunc(void *func1, void *func2, int x)
      { return func1(x) * func2(x); }
    
    
    //call as
    
    z = Dynafunc(&f,&g, 31);

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    It is not possible to create new functions in C at run time.

    The closest you can come is to store the supplied f() and g() in some static variables (of type pointer to function) that are accessible by a function named h(). Implement h() so it uses those two static function pointers, multiplies their results together, and returns that. There is a trade-off that, if you call your function with different another_f() and another_g(), that a previously returned h(x) will quietly start returning the value of another_f(x)*another_g(x) rather than the original f(x)*g(x). That is a consequence of using static variables.
    Right 98% of the time, and don't care about the other 3%.

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    CommonTater's code is not going to work. You can't pass a void* and call it as if it was a function (at least not without casting the pointer).

    You can't create a function that takes a function as an input and returns a new one because C hasn't support for "anonymous functions". You can simulate it by using a struct that wraps the input functions or by having global state, though.
    Stick close to your desks and never program a thing,
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    Quote Originally Posted by Ronix View Post
    CommonTater's code is not going to work. You can't pass a void* and call it as if it was a function (at least not without casting the pointer).

    You can't create a function that takes a function as an input and returns a new one because C hasn't support for "anonymous functions". You can simulate it by using a struct that wraps the input functions or by having global state, though.
    You're right... my bad. I was just wanting to show an example, and should have tested it first.

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    Quote Originally Posted by Ronix View Post
    You can't create a function that takes a function as an input and returns a new one because C hasn't support for "anonymous functions". You can simulate it by using a struct that wraps the input functions or by having global state, though.
    In C++, I would have suggested using a (possibly templated) functor. That is more or less the same as a wrapper struct, except that a functor can support the same calling syntax, but a wrapper struct in C cannot.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy in reply to you, it is likely you deserve it. Suck it up, sunshine, and read this, this, and this before posting again.

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