std::is_within_lifetime(3) C++ Standard Libary std::is_within_lifetime(3) NAME std::is_within_lifetime - std::is_within_lifetime Synopsis Defined in header <type_traits> template< class T > (since C++26) consteval bool is_within_lifetime( const T* ptr ) noexcept; 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 objectas lifetime began within E. Parameters p - pointer to detect Return value true if pointer ptr points to an object that is within its lifetime; otherwise false. Example std::is_within_lifetime can be used to check whether a union member is active: // Run this code #include <type_traits> // 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 evalua- tion, // 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); } http://cppreference.com 2024.06.10 std::is_within_lifetime(3)
NAME | Synopsis | Parameters | Return value | Example
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