Detecting Template Member Functions in C++ Using std::is_detected

A guide to detecting template member functions in C++ using `std::experimental::is_detected`, along with practical examples and solutions for common pitfalls.
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Detecting Template Member Functions in C++ Using std::is_detected

In C++ programming, particularly with template classes, it’s common to encounter situations where you need to check if a particular member function exists in a given class. This is especially crucial in template metaprogramming (TMP). Today, we will dive into how to accomplish this using std::experimental::is_detected, which simplifies checking for member functions and their signatures.

The Problem Statement

You may need to determine if your class has certain member functions, including template member functions that can take a variety of parameters. For instance, consider that you have a class called Descriptor, and you want to check for two member functions:

A non-template member function: int member1(int)

A template member function: template<typename Derived> int member2(Eigen::ArrayBase<Derived>&&)

The initial implementation you might have created for this might not work as expected due to specifics about how templates and type deductions work in C++.

The Solution: Using C++20 Concepts and std::experimental::is_detected

If C++20 is an option, using concepts can make your code much clearer and easier to manage. Here’s how you can approach this problem step by step.

Step 1: Define Your Classes

First, we create some example classes that we will use to test our member function checks.

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Step 2: Create Your Concept

Next, we define a concept called MyConcept that checks whether a class has both member functions.

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Here, requires allows us to specify the conditions for our concept.

Step 3: Utilize Static Assertions for Your Checks

In main(), you can use static_assert to verify if our classes conform to the concept:

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Step 4: Detecting Template Member Functions

If you want to stick to using std::experimental::is_detected, below is another way to implement it. Here’s a refined version that correctly checks for the template member function:

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Conclusion

By using std::experimental::is_detected or C++20 concepts, detecting member functions in a class becomes a straightforward process. You can expand these strategies to check for other members, ensuring your templates interact properly within your projects.

Make sure to explore these features to enhance your C++ programming techniques, especially if you're delving deeper into template metaprogramming! Happy coding!

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