How Can I Correctly Initialize a Constexpr Static Member Using a Constexpr Static Function in C ?

constexpr Initializing Static Member Using Static Function: A Walkthrough

Introduction

The scenario you presented involves initializing a constexpr static member within a class using a constexpr function. Unfortunately, attempts to achieve this with specific approaches have encountered challenges in different versions of gcc. In this article, we dissect these attempts, explore the underlying reasons for the errors, and provide possible solutions or clarifications.

First Attempt and its Error

Your initial attempt involved declaring both the constexpr function foo and the constexpr static member bar within the class body, but this led to compilation errors. They indicate that function calls cannot exist within constant expressions, and static class members cannot have initializers that are not constant.

Second Attempt and its Error

The second attempt moved the declarations outside the class body, but this still resulted in errors. Notably, the error messages mentioned that the static data member bar must have an initializer and that its declaration as constexpr contradicts its previous declaration outside the class.

Analyzing the Standard

The C Standard's section 9.4.2 states that static data members of literal type can have the constexpr specifier in their declaration. However, their initializer must be a brace-or-equal-initializer, and any assignment-expression within it must be a constant expression.

In both your attempts, the declaration of bar lacked a brace-or-equal-initializer. Therefore, neither of them meets the requirements set forth by the Standard.

Limitations and Solution

This situation, however, has an inherent limitation imposed by the Standard. It disallows initializing static constexpr data members in contexts where the class is complete. Hence, using the brace-or-equal-initializer rule is only applicable to non-static data members.

Consequently, the initialization of a static constexpr data member using a constexpr function that relies on the class's completion becomes impossible. This restriction is likely due to the requirement that constexpr variables must be available as compile-time constant expressions in the context of member functions. Therefore, the variable initializers must be fully defined before the function bodies, but the static data member's initializer leads to an undefined function invocation in this context, violating the constant expression requirement.

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