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

constexpr Initializing Static Member Using Static Function

Issue

The goal is to obtain a compile-time constant (constexpr value) calculated from a constexpr function, both within the same class namespace.

First Attempt

class C1 {
  constexpr static int foo(int x) { return x + 1; }
  constexpr static int bar = foo(sizeof(int));
};

However, this fails on both g -4.5.3 and g -4.6.3, citing the use of non-constant expressions in the initializer.

Second Attempt

class C2 {
  constexpr static int foo(int x) { return x + 1; }
  constexpr static int bar;
};
constexpr int C2::bar = C2::foo(sizeof(int));

While this compiles on g -4.5.3, g -4.6.3 shows inconsistency and redeclaration errors.

Explanation

According to the C Standard (section 9.4.2), a constexpr static data member can only be declared with a brace-or-equal-initializer in the class definition, where every initializer-clause must be a constant expression.

In the "second attempt," the declaration lacks the required brace-or-equal-initializer.

Conclusion

Contrary to initial assumptions, both attempts prove unsuccessful. The Standard prohibits initializing static constexpr data members in contexts where the class is complete.

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