std::optional<T>::value_or

 

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std::optional

Member functions

optional::optional (C++17)

optional::~optional (C++17)

optional::operator= (C++17)

Observers

optional::operator->optional::operator* (C++17)(C++17)

optional::operator booloptional::has_value (C++17)(C++17)

optional::value (C++17)

optional::value_or (C++17)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace (C++17)

optional::swap (C++17)

optional::reset (C++17)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)

make_optional (C++17)

swap (C++17)

Deduction guides

Helper classes

hash (C++17)

nullopt_t (C++17)

in_place_t (C++17)

bad_optional_access (C++17)

Helper objects

nullopt (C++17)

in_place (C++17)

 

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 <optional>
#include <iostream>
#include <cstdlib>
 
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("MYPWD").value_or("(none)") << '\n';
}

Possible output:

(none)

See also

value

returns the contained value (public member function)