CODEWITHSUNDEEP
c++functionparameters
Arlington
Arlington

Reputation: 79

C++ passing a user-defined function to another function

I'm trying to write a function (we'll call it function A) which can receive a user-defined function as input (for example, a user defined function which takes any number of variables and returns an integer -- We'll call this function B). I want to pass function B as a parameter for function A, only modify some of the variables in function B and call function B as many times as I need to from within function A.

I was thinking I can get this to work by making the variables of function B pointers and modifying the pointers at each call, however I don't know how to make it so I can call function B from within function A if the number of variables to be passed to function B are unknown.

Does anyone have any suggestions as to how I can pass a user-specified function as a parameter to another function?

Upvotes: 1

Views: 7667

Answers (2)

Scott Deng
Scott Deng

Reputation: 119

Since you mentioned that you may modify variables in function B, function pointer is not a good choice,especially when C++ introduce the callable object, or std::function. By constructing a class with operator () overloaded, you are able to treat the object of such class like a object and like a function pointer.

Another alternative is lambda introduced in C++11. You can simply write [A_variable_1, A_variable_2](auto parameter){ your code} to use the variable in A and your own variable in B.

Upvotes: 3

Peter
Peter

Reputation: 36597

What you want is a pointer to a function.

For example, to pass a function that accepts an int and returns a double.

 #include <iostream>
 double B(int x)    // sample function that we call
 {
      return x*42.0;
 }

 void A(double (*f)(int))
 {
     for (int i = 0; i < 25; ++i)
        std::cout << f(i) << '\n';
 }

 int main()
 {
    A(B);
 }

If you want to pass a function that accepts a variable argument list, then modify the above. Look up in formation on the standard header <cstdarg> for pointers on how to do that.

There are also techniques involving templates (specifically, templated functors) and lambdas (in C++11 and later). But using a pointer to a function is enough to get you started.

Upvotes: 1

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