Recursive templates for template code validation in C++

22 Sep 2023

When working with complex template code in C++, it can sometimes be challenging to validate its correctness. A simple typo or incorrect usage of template parameters can lead to compilation errors that are often difficult to debug. In this blog post, we will explore an advanced technique called recursive templates that can help validate template code at compile-time.

What is Recursive Templates?

Recursive templates is a programming technique in C++ where a template class or function makes use of recursive calls on itself to perform certain operations. This approach can be used to enforce constraints on the usage of template parameters and verify their validity during compilation.

Why Use Recursive Templates for Template Code Validation?

Recursive templates allow us to catch potential errors in template code at compile-time rather than at runtime. By performing validations recursively and utilizing compile-time type checking, we can identify and correct errors before the code gets executed. This can save significant debugging time and ensure the correctness of the code.

Example: Validating a Compile-Time List

Let’s consider an example where we want to validate a compile-time list using recursive templates. We’ll define a List class template that enforces constraints on the list’s length and ensures that only specific types are allowed. Here’s how the code looks:

template<typename... Types>
struct List;

template<typename Head, typename... Tail>
struct List<Head, Tail...> {
    static_assert(std::is_integral<Head>::value, "Only integral types allowed in the list.");
    static_assert(sizeof...(Tail) <= 10, "The maximum length of the list is 10.");

    template<typename... Types>
    using Append = List<Head, Tail..., Types...>;
};

template<>
struct List<> {
    static_assert(sizeof...(Types) <= 10, "The maximum length of the list is 10.");
};

In this example, the List class template contains two specializations: one for non-empty lists and another for empty lists.

The non-empty list specialization enforces that only integral types are allowed and limits the maximum length of the list to 10. The Append type alias allows for adding more types to the list.

The empty list specialization enforces the maximum length constraint.

With this implementation, we can now validate our compile-time list using recursive templates. For example, the following code will compile successfully:

using MyList = List<int, bool, short>;

int main() {
    MyList::Append<long, char, float>::Append<double> list;
    return 0;
}

However, if we try to add a non-integral type or exceed the maximum list length, a compile-time error will occur.

Conclusion

Recursive templates provide an advanced technique for validating template code in C++. By recursively checking the constraints and utilizing compile-time type checking, we can catch potential errors at compile-time. This helps ensure the correctness of the code and saves valuable debugging time.

#C++ #RecursiveTemplates #CodeValidation #CompileTime