In C++, templates are a powerful feature that allows for generic programming. They provide a way to write code that can be used with different types without explicitly specifying those types. Template specialization is a technique used to provide a customized implementation of a template for a specific type or set of types. In some cases, you may need to define template specializations for a range of related types. Recursive templates can be a useful approach to achieve this.

What are recursive templates?

Recursive templates, as the name suggests, are templates that use recursion to generate specializations for a range of types. Instead of explicitly defining each specialization, recursive templates rely on the template instantiation process to generate the specialized versions.

Example: Recursive template specialization

Let’s consider an example where we have a templated printSize() function that prints the size of different types.

// Recursive template for generic specialization
template <typename T>
void printSize() {
    std::cout << "Size of T: " << sizeof(T) <<  bytes" << std::endl;
}

// Recursive template specialization for arrays
template <typename T, std::size_t N>
void printSize() {
    std::cout << "Size of T[" << N << "]: " << sizeof(T) * N <<  bytes" << std::endl;
    printSize<T>();
}

// Recursive template specialization for vectors
template <typename T, typename Allocator>
void printSize() {
    std::cout << "Size of std::vector<" << typeid(T).name() << ">: " << sizeof(std::vector<T, Allocator>) <<  bytes" << std::endl;
    printSize<T>();
}

In this example, the printSize() function is initially defined as a generic template that prints the size of a given type T. The recursive template specializations then handle specific cases like arrays and vectors. These recursive specializations call the base template with the same type T, ensuring the recursion continues until the appropriate specialization is reached.

Usage

To use the printSize() function, you can simply call it with the desired type or use it with arrays or vectors:

int main() {
    printSize<int>();  // Prints "Size of T: 4 bytes"
    printSize<float>();  // Prints "Size of T: 4 bytes"

    int arr[5];
    printSize<decltype(arr), sizeof(arr)>();  // Prints "Size of int[5]: 20 bytes"

    std::vector<double> vec;
    printSize<decltype(vec), std::allocator<double>>();  // Prints "Size of std::vector<double>: 24 bytes"

    return 0;
}

Conclusion

Recursive templates provide an elegant way to handle template specializations for a range of related types. By relying on recursion, you can avoid repetitive code and let the template instantiation process generate the desired specializations. This technique can be particularly useful when working with arrays, containers, or other scenarios where you need to provide different implementations for different types.

#CPP #Templates