|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Defined in header <type_traits> | ||
template<class T> constevalbool is_within_lifetime(const T* ptr)noexcept; | (since C++26) | |
Determines whether the pointerptr points to an object that is within itslifetime.
During the evaluation of an expressionE as a core constant expression, a call tostd::is_within_lifetime is ill-formed unlessptr points to an object
Contents |
| p | - | pointer to detect |
true if pointerptr points to an object that is within its lifetime; otherwisefalse.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_is_within_lifetime | 202306L | (C++26) | Checking if a union alternative is active |
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 2constexpr optional_bool(): c(2){}constexpr optional_bool(bool b): b(b){} constexprauto 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}} constexprauto 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);}
