constexpr specifier (since C23)
A scalar object declared with the constexpr storage-class specifier is a constant. It must be fully and explicitly initialized according to the static initialization rules. It still has linkage appropriate to its declaration and it exist at runtime to have its address taken; it simply cannot be modified at runtime in any way, i.e. the compiler can use its knowledge of the object’s fixed value in any other constant expression.
Additionally, the constant expression that is used for the initializer of such a constant is checked at compile time.
An initializer of floating-point type must be evaluated with the translation-time floating-point environment.
There are some restrictions on the type of an object that can be declared with constexpr. Namely, the following constructs are not allowed to be constexpr:
- Pointers (except that null pointers can be
constexpr), - Variably modified types,
- Atomic types,
-
volatiletypes, -
restrictpointers.
Keywords
Notes
Example
#include <fenv.h> #include <stdio.h> int main(void) { constexpr float f = 23.0f; constexpr float g = 33.0f; fesetround(FE_TOWARDZERO); constexpr float h = f / g; // is not affected by fesetround() above printf("%f\n", h); }
Output:
0.696969
References
- C23 standard (ISO/IEC 9899:2024):
- TBD TBD (p: TBD)
See also
| C++ documentation for
constexpr type specifier |