std::experimental::optional::value_or

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Technical specifications

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals 2 TS)

Extensions for parallelism (parallelism TS)

Extensions for concurrency (concurrency TS)

Concepts (concepts TS)

Ranges (ranges TS)

Special mathematical functions (special math TR)

 

Library fundamentals

experimental::optional

experimental::any

experimental::basic_string_view

experimental::sample

experimental::shared_ptr

experimental::weak_ptr

experimental::apply

experimental::invocation_type experimental::raw_invocation_type

experimental::search

experimental::default_searcher experimental::make_default_searcher

experimental::boyer_moore_searcher experimental::make_boyer_moore_searcher

experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher

type-erased and polymorphic allocators

variable templates for type traits

 

std::experimental::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator bool

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

Non-member functions

operator==operator!=operator<operator<=operator>operator>=

make_optional

swap

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

template< class U > constexpr T value_or( U&& default_value ) const&;		(library fundamentals TS)
template< class U > constexpr T value_or( U&& default_value ) &&;		(library fundamentals TS)

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 <experimental/optional>
#include <iostream>
#include <cstdlib>
 
std::experimental::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)