std::is_within_lifetime - cppreference.com (original) (raw)
| | | | | ---------------------------------------------------------------------------------- | | ------------- | | template< class T >consteval bool is_within_lifetime( const T* ptr ) noexcept; | | (since C++26) |
Determines whether the pointer ptr points to an object that is within its lifetime.
During the evaluation of an expression E as a core constant expression, a call to std::is_within_lifetime is ill-formed unless ptr points to an object
- that is usable in constant expressions, or
- whose complete object’s lifetime began within E.
[edit] Parameters
[edit] Return value
true if pointer ptr points to an object that is within its lifetime; otherwise false.
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
| __cpp_lib_is_within_lifetime | 202306L | (C++26) | Checking if a union alternative is active |
[edit] Example
std::is_within_lifetime can be used to check whether a union member is active:
#include // an optional boolean type occupying only one byte, // assuming sizeof(bool) == sizeof(char) struct optional_bool { union { bool b; char c; }; // assuming the value representations for true and false // are distinct from the value representation for 2 constexpr optional_bool() : c(2) {} constexpr optional_bool(bool b) : b(b) {} constexpr auto has_value() const -> bool { if consteval { return std::is_within_lifetime(&b); // during constant evaluation, // cannot read from c } else { return c != 2; // during runtime, must read from c } } constexpr auto operator*() -> bool& { return b; } }; int main() { constexpr optional_bool disengaged; constexpr optional_bool engaged(true); static_assert(!disengaged.has_value()); static_assert(engaged.has_value()); static_assert(*engaged); }