In C++, templates are a powerful feature that allows you to write generic code that can be used with different types. One use case for templates is to define template concepts, which are abstractions that describe the requirements for a type to be used with a template.

In this blog post, we will explore how to implement recursive templates for template concepts in C++. Recursive templates allow us to define more complex concepts by building them up from simpler ones. This technique can be useful when you need to express relationships or constraints between multiple types.

Let’s start with a simple example of a template concept called Addable, which represents types that can be added together using the + operator.

template <typename T>
concept Addable = requires(T a, T b) {
    { a + b } -> std::same_as<T>;
};

In the above code, we define the Addable concept as a template that takes a type T. The concept specifies that T must support the + operator and the result of the addition must have the same type as T.

Now, let’s define a more complex concept called Sortable, which represents types that can be sorted in ascending order. The Sortable concept depends on the Addable concept, as we will use the addition of two elements to compare them.

template <typename T>
concept Sortable = requires(T a, T b) {
    { a + b } -> std::same_as<T>;
    { a < b } -> std::convertible_to<bool>;
};

In the Sortable concept, we extend the requirements from the Addable concept by also specifying that the types must be comparable using the less-than operator (<).

Recursive templates allow us to define more complex concepts by composing simpler ones. For example, we can define a concept called Numeric, which represents types that are both Addable and Sortable.

template <typename T>
concept Numeric = Addable<T> && Sortable<T>;

In the Numeric concept, we combine the Addable and Sortable concepts using the && operator. This means that a type must satisfy both Addable and Sortable in order to be considered Numeric.

Using these template concepts, we can write generic code that operates on types that satisfy specific requirements. For example, we can write a function that calculates the sum of a range of elements, given that the elements are Numeric.

template <Numeric T>
T sum(const std::vector<T>& nums) {
    T result = T(0);
    for (const auto& num : nums) {
        result = result + num;
    }
    return result;
}

The sum function takes a vector of Numeric elements and uses the addition operator to calculate the sum of the elements. This function will only compile if the elements satisfy the Numeric concept.

Recursive templates for template concepts in C++ allow us to express complex requirements for types in a concise and flexible way. By combining simpler concepts, we can define powerful abstractions that capture the behavior we need. Understanding and utilizing template concepts can greatly enhance the expressiveness and reusability of your C++ code.

#C++ #Templates