Pointer to Function(names)

This is a discussion on Pointer to Function(names) within the C++ Programming forums, part of the General Programming Boards category; Hi, in my c++-book is an example how to use pointer to functions: Code: compare(double, double); (funcptr*)(double, double); funcptr = ...

  1. #1
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    Pointer to Function(names)

    Hi,

    in my c++-book is an example how to use pointer to functions:

    Code:
    compare(double, double);
    (funcptr*)(double, double);
    
    funcptr = compare;
    (funcptr*)(double, double);
    this would call compare. (I don't have the book here, so i am not 100% sure if the * belongs in front or at the end of funcptr.

    My problem is now in my case:
    Code:
    string funcname = "compare";
    funcptr = funcname;
    Of course i can't just write it the same like above, since one is a pointer and one is a string object.
    Anyone could help me, how i could make it work?


    Thanks

    Hork83


    PS: Why I need this? i'd like to write less code due to the large if/case i would need to write for any possible option.

  2. #2
    Deathray Engineer MacGyver's Avatar
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    You got all of your function pointer stuff confused, so you might want to review it all.

    As far as the names go, if you're using a .dll, you obtain a function pointer to the function in the DLL by means of GetProcAddress(), I think, which takes a C string. I've heard of cases of doing this on the .exe itself, but I don't think it's officially supported, so your milage may vary.

    In terms of *nix dynamic libraries, I believe there is a similar way to go about it, but I don't know the specific way to do it.

    Otherwise, you could build your own table of function names and pointers in a function, and then obtain the right function pointer from that function, given the name.

    Code:
    somefuncptr getFunction(string s)
    {
            if(s == "bleh")
                    return bleh_ptr;
            ....
    }

  3. #3
    Kernel hacker
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    The "syntax" for function pointer declaration is
    Code:
    returnType (*funcptr)(parameter(s));
    
    e.g.
    
    double (*compptr)(double, double);
    The parenthesis is necessary to differentiate between pointer to a type and function pointers - it ties the pointer to the parenthesis after, rather than a function that returns a pointer to a "returnType".

    What I'd like to understand is what you actually want to achieve with your string to function pointer conversion. But perhaps this is a way:

    Code:
    struct {
       char *str;
       int (*funcptr)(double, double);
    } table[] = { { "compare", compare },
                         { "add", add },
                         { "sub", sub } };
    Then just loop through table and compare the string with the "str" in the struct. Or if you need to do it "more clever", you build a hash-table or a sorted list so that you can search more quickly - assuming there is enough table entries and searches to require this.

    --
    Mats

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Note that you can assign pointers with two syntaxes:
    Code:
    int (*funcptr)(double, double) = compare;
    int (*funcptr)(double, double) = &compare;
    You can also call a function pointer with two syntaxes:
    Code:
    (*funcptr)(x, y);
    funcptr(x, y);
    There are arguments for both, each of which are valid, which is why they both exist. I don't feel like typing them out here; search the board or the net if you're interested.

    I prefer assigning with compare and calling with (*funcptr) myself, but everyone has their own programming style.
    dwk

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