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 object’s 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 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);
}