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

optional::value_or

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

nullopt_t

in_place_t

bad_optional_access

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nullopt

in_place

 

template< class U > constexpr T value_or( U&& default_value ) const&;	(1)	(since C++17)
template< class U > constexpr T value_or( U&& default_value ) &&;	(2)	(since C++17)

Returns the contained value if *this has a value, otherwise returns default_value.

1) Equivalent to bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value)).

2) Equivalent to bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(default_value)).

Parameters

default_value	-	the value to use in case *this is empty
Type requirements
- T must meet the requirements of CopyConstructible in order to use overload (1).
- T must meet the requirements of MoveConstructible in order to use overload (2).
- U&& must be convertible to T.

Return value

The current value if *this has a value, or default_value otherwise.

Exceptions

Any exception thrown by the selected constructor of the return value T.

Example

Run this code

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

Possible output:

/usr/bin/zsh
(none)

See also

value

returns the contained value (public member function)