Function pointer with different number of arguments

[dmham1586]

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.

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;
}

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.

[Lindley]

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.

[dmham1586]

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

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.

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

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.

[Lindley]

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.

[dmham1586]

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.