< cpp‎ | types
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Defined in header <type_traits>
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

[edit] Parameters

p - pointer to detect

[edit] Return value

true if pointer ptr points to an object that is within its lifetime; otherwise false.

[edit] Example

std::is_within_lifetime can be used to check whether a union member is active:

#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 evaluation,
                                                // cannot read from c
            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);