C++ Class Member Function As Template Parameter

5 min read Jul 01, 2024

C++ Class Member Function As Template Parameter

C++ Class Member Functions as Template Parameters

Template parameters in C++ are extremely versatile. You can use them to parameterize types, values, and even functions. One powerful, yet often overlooked, use case is passing class member functions as template parameters. This allows you to write generic code that can work with various member functions from different classes.

Let's dive into how this works:

Why Use Member Functions as Template Parameters?

Imagine you have a collection of objects and want to apply a specific operation to each object. You could write separate functions for each operation, but that's repetitive and inefficient. By using member functions as template parameters, you can create a single, generic function that takes any member function as input, making your code more reusable and flexible.

Understanding the Syntax

Let's break down the syntax using a simple example:

#include 

class MyClass {
public:
  int value;

  int doubleValue() { return value * 2; }
  int tripleValue() { return value * 3; }
};

template 
void applyMemberFunction(T& obj, F function) {
  std::cout << "Result: " << function(obj) << std::endl;
}

int main() {
  MyClass myObject;
  myObject.value = 5;

  // Pass doubleValue as the template parameter
  applyMemberFunction(myObject, &MyClass::doubleValue);

  // Pass tripleValue as the template parameter
  applyMemberFunction(myObject, &MyClass::tripleValue);

  return 0;
}

In this code:

applyMemberFunction is a template function that takes two parameters:
- T: The type of the object (MyClass in this case).
- F: The type of the member function (a pointer to a member function).
We pass &MyClass::doubleValue and &MyClass::tripleValue as template parameters, providing applyMemberFunction with the specific member functions to call.
Inside the function, we call the passed member function using function(obj).

Key Points to Remember

Member function pointers: When passing member functions as template parameters, you use a pointer to the member function (e.g., &MyClass::doubleValue).
Calling member functions: You use the object.member_function_name() syntax to call a member function within the template function.
Type deduction: The compiler deduces the types T and F automatically based on the object and member function you provide.

Example Use Cases

Here are some practical use cases for member functions as template parameters:

Generic sorting algorithms: You can write a template sorting function that accepts a custom comparison function as a parameter, allowing users to sort objects based on different criteria.
Applying operations to collections: You can create a generic function that iterates over a collection and applies a specified member function to each element.
Dynamic behavior: By passing member functions as parameters, you can dynamically change the behavior of your code based on user input or runtime conditions.

Conclusion

Utilizing class member functions as template parameters empowers you to write flexible and reusable C++ code. It allows you to create generic functions that can adapt to various member functions from different classes, enhancing the maintainability and expressiveness of your applications. Remember to utilize member function pointers and the correct calling syntax to leverage this powerful technique.