Static assertions in C++ are a powerful mechanism to validate properties at compile-time. They allow us to catch errors early, reducing debugging time and improving code quality. In this blog post, we will explore a technique using recursive templates to create template static assertions in C++.

Introduction

Template static assertions are compile-time checks that enforce certain conditions on template parameters. Traditional static assertions are executed during the compilation process and result in a compilation error if the condition fails. However, standard static assertions have limited flexibility when it comes to template parameters.

By using recursive templates, we can overcome these limitations and perform more advanced checks on template parameters. This technique allows us to create powerful template static assertions that can be recursively extended and customized.

Recursive Template Static Assertions

To start, let’s define a basic template static assertion using a recursive template. We’ll use the is_same trait to check if two types are the same:

template<typename T, typename U>
struct is_same {
  static_assert(false, "Types are not the same");
};
 
template<typename T>
struct is_same<T, T> {
  static_assert(true, "Types are the same");
};

In the above code, the is_same struct contains a static assertion that always fails by default. However, when the types T and U are the same, the specialized template is used, leading to a successful static assertion.

This technique can be extended to perform more complex template static assertions. For example, let’s create a template static assertion to check if an integer is positive:

template<int N>
struct is_positive {
  static_assert(N > 0, "Integer is not positive");
};

In this case, the static_assertion will succeed if the integer template parameter N is greater than 0.

Customizing Recursive Template Static Assertions

The power of recursive templates lies in their ability to be extended and customized. We can define additional specialized templates to perform specific checks on template parameters.

For example, let’s create a template static assertion to check if a type is a pointer:

template<typename T>
struct is_pointer {
  static_assert(false, "Type is not a pointer");
};
 
template<typename T>
struct is_pointer<T*> {
  static_assert(true, "Type is a pointer");
};

With this additional specialized template, we can now perform static assertions on whether a type is a pointer or not.

Conclusion

Recursive templates provide a powerful way to create custom and flexible template static assertions in C++. By defining specialized templates and using static_assert within them, we can perform advanced compile-time checks on template parameters. These assertions help catch errors early, improve code quality, and reduce debugging time.

With the techniques discussed in this blog post, you can create recursive template static assertions to validate a wide range of conditions on template parameters. This enables you to write more robust and error-free code.

#cpp #templates #staticassertions