Function pointer with different number of arguments

I am trying to create a pointer to a function that only requires some of the arguments when called and specifies the others at the time the pointer is created. For example, see the code below.

Code:

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int main() {
    // The function bar only takes one argument -> a
    // b and c are specified as 1 and 2 (respectively) at time of function creation
    int (*bar)(int a) = &foo( int, 1, 2);

    cout << bar(3) << endl;
    // should print 3

    int (*bar)(int a) = &foo( int, 4, 5);

    cout << bar(3) << endl;
    // should print 12
}

// foo takes 3 arguments, but b and c are only necessary when first called
int foo(int a, int b, int c) {
  static int d = b + c;
  return a + d;
}

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Of course, as written this code doesn't compile, but I think it should help you understand what I'm trying to do. I have programmed in lisp and this is very easy to do, I would think C++ should have a way of doing it.

Thanks in advance for any help.

It sounds to me like you're looking for boost::bind.

Basically what it does is create a functor which passes some arguments through to the underlying function, and hides others using values specified at binding time. You can store the functor in a boost::function if you need a variable to hold it; otherwise, just use a template so that the specific type isn't a problem.

Also, static local variables should be avoided as they create bad habits.

Quote:

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Originally Posted by Lindley

It sounds to me like you're looking for boost::bind.

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Thanks, Lindley. I looked up boost::bind and you're right, I think it would solve my problem. Also, it's been a while for me since I've used C++, so I forgot about functors. Unfortunately this is a school project and I can't just add non-standard libraries to my project. It's a graphics project, so I was just trying to find a way to make it more efficient because I would be calling the same function over and over with the same b and c values where only a is changing. For now I just have it calling the function with all three arguments, so it works. But if anybody knows a way to do this with the standard Visual C++ libraries, let me know.

Quote:

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Originally Posted by Lindley

Also, static local variables should be avoided as they create bad habits.

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I didn't know there was anything bad about static local variables. But the static was just for the example, the real code is much more complex than that and includes no statics.

Generally if you have values which only need to be set once and other values that need to be passed every time, the appropriate design is to build a class rather than a simple function. You put the one-time arguments in the constructor and give the object a public method which does the work.

The problem with statics becomes apparent when you start thinking about multi-threading. The key aspect of thread safety is the use of local variables, since each thread will have its own stack. When you declare a local variable static, then it becomes just as dangerous as a global variable as far as MT goes.

Thanks for the help, Lindley. I think making the class is the best solution.

As far as the statics go, you're right; it's obvious how it could be a problem as soon as you think about multi-threading.