absl::is_constant

Detects whether the function call occurs within a constant‐evaluated context. Returns true if the evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant‐evaluated; otherwise returns false.

Synopsis

Declared in <absl/meta/type_traits.h>

constexpr
bool
is_constant_evaluated() noexcept;

Description

This function is implemented in terms of std::is_constant_evaluated for c++20 and up. For older c++ versions, the function is implemented in terms of __builtin_is_constant_evaluated if available, otherwise the function will fail to compile.

Applications can inspect ABSL_HAVE_CONSTANT_EVALUATED at compile time to check if this function is supported.

Example:

constexpr MyClass::MyClass(int param) { #ifdef ABSL_HAVE_CONSTANT_EVALUATED if (!absl::is_constant_evaluated()) { ABSL_LOG(INFO) << "MyClass(" << param << ")"; } #endif // ABSL_HAVE_CONSTANT_EVALUATED }

Upstream documentation:

http://en.cppreference.com/w/cpp/types/is_constant_evaluated http://gcc.gnu.org/onlinedocs/gcc/Other‐Builtins.html#:~:text=__builtin_is_constant_evaluated

Return Value

true if called within a constant‐evaluated context, false otherwise.

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