std::optional<T>::value_or

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std::optional

Member functions
optional::optional
optional::~optional
optional::operator=
Observers
optional::operator->optional::operator*
optional::operator booloptional::has_value
optional::value
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template<class U=std::remove_cv_t<T>> constexpr T value_or( U&& default_value)const&;	(1)	(since C++17)
template<class U=std::remove_cv_t<T>> constexpr T value_or( U&& default_value)&&;	(2)	(since C++17)

Returns the contained value if*this contains a value, otherwise returnsdefault_value.

1) Ifstd::is_copy_constructible_v<T>&&std::is_convertible_v<U&&, T> isfalse, the program is ill-formed.

2) Ifstd::is_move_constructible_v<T>&&std::is_convertible_v<U&&, T> isfalse, the program is ill-formed.

[edit]Parameters

default_value

the value to be returned if*this does not contain a value

[edit]Return value

1)has_value()?**this:static_cast<T>(std::forward<U>(default_value));

2)has_value()? std::move(**this):static_cast<T>(std::forward<U>(default_value))

[edit]Example

Run this code

#include <cstdlib>#include <iostream>#include <optional> std::optional<constchar*> maybe_getenv(constchar* n){if(constchar* x=std::getenv(n))return x;elsereturn{};} int main(){std::cout<< maybe_getenv("SHELL").value_or("(none)")<<'\n';std::cout<< maybe_getenv("MYPWD").value_or("(none)")<<'\n';}