How to Correctly Declare Class Template Constructors in C 20?

Overcoming Redundant Template Parameters in Class Template Constructors in C 20

In C 20, the declaration of class template constructors has undergone a subtle but impactful change. Previously, constructors for class templates could optionally include the template parameter list within their declarator, as seen in the following code snippet:

template<typename T>
struct S {
    S<T>();
};

This practice, however, is now deprecated in C 20. The code above would now result in a compiler error in gcc trunk with -std=c 20. However, it would still compile in clang trunk with the same standard.

Rationale for the Change

The removal of redundant template parameters in class template constructors addresses a potential source of errors. By requiring the explicit use of the injected class name instead, the straightforward intent is preserved when declaring constructors within class templates.

Effect on Existing Code

This change may have implications for existing codebases that rely on the previously valid syntax. Valid C 2017 code like the example above may no longer compile under the C 20 standard.

Resolution for Affected Code

To accommodate this change, the following alternative syntax can be used:

template<typename T>
struct S {
    S();  // Constructor declaration using the injected class name
};

Alternatively, if specific template arguments are required, constructor overloading can be employed:

template<typename T>
struct S {
    S<T>();  // Constructor declaration with explicit template arguments
    S<int>();  // Overload for specific template argument
};

By adhering to these syntax guidelines, codebases can maintain compatibility with the updated C 20 standard.

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