在线文档教程
首页 技术教程 开发文档 觅分类 ICON图标
技术教程 开发文档 觅分类 ICON图标
Bootstrap 4
Bootstrap 3
C
C++
算法 | Algorithm
原子性操作 | Atomic operations
概念 | Concepts
容器 | Containers
动态内存管理 | Dynamic memory management
文件系统 | Filesystem
输入/输出 | Input/output
迭代器 | Iterator
关键词 | Keywords
语言 | Language
本土化 | Localizations
数字 | Numerics
规律表达 | Regular expressions
标准库头文件 | Standard library header files
字符串 | Strings
线程支持 | Thread support
应用 | Utilities
assert
C Date and time utilities
C memory management library
C numeric limits interface
CLOCKS_PER_SEC
Date and time utilities
Dynamic memory management
errno
Error handling
Error numbers
EXIT_FAILURE
EXIT_SUCCESS
Fixed width integer types
FLT_EVAL_METHOD
FLT_ROUNDS
Function objects
Low level memory management
NULL
offsetof
operators (delete[])
operators (new[])
operators (std::bitset)
operators (std::bitset)
operators (std::bitset)
operators (std::chrono::duration)
operators (std::chrono::duration)
operators (std::error_condition)
operators (std::function)
operators (std::optional)
operators (std::pair)
operators (std::time_point)
operators (std::time_point)
operators (std::tuple)
operators (std::unique_ptr)
operators (std::variant)
Program support utilities
setjmp
SIGABRT
SIGFPE
SIGILL
SIGINT
SIGSEGV
SIGTERM
SIG_DFL
SIG_ERR
SIG_IGN
std::abort
std::addressof
std::add_const
std::add_cv
std::add_lvalue_reference
std::add_pointer
std::add_rvalue_reference
std::add_volatile
std::align
std::aligned_alloc
std::aligned_storage
std::aligned_union
std::alignment_of
std::align_val_t
std::allocate_shared
std::allocator_arg
std::allocator_arg_t
std::any
std::any::any
std::any::emplace
std::any::has_value
std::any::reset
std::any::swap
std::any::type
std::any_cast
std::apply
std::asctime
std::as_const
std::atexit
std::atomic_...<std::shared_ptr>
std::at_quick_exit
std::auto_ptr
std::auto_ptr::auto_ptr
std::auto_ptr::get
std::auto_ptr::operator auto_ptr<Y>
std::auto_ptr::operator->
std::auto_ptr::operators
std::auto_ptr::release
std::auto_ptr::reset
std::bad_alloc
std::bad_any_cast
std::bad_array_new_length
std::bad_array_new_length::bad_array_new_length
std::bad_cast
std::bad_cast::bad_cast
std::bad_exception
std::bad_exception::bad_exception
std::bad_exception::what
std::bad_function_call
std::bad_optional_access
std::bad_typeid
std::bad_typeid::bad_typeid
std::bad_variant_access
std::bad_weak_ptr
std::bad_weak_ptr::bad_weak_ptr
std::begin(std::initializer_list)
std::binary_function
std::binary_negate
std::bind
std::bind1st
std::bind2nd
std::binder1st
std::binder2nd
std::bitset
std::bitset::all
std::bitset::any
std::bitset::bitset
std::bitset::count
std::bitset::flip
std::bitset::none
std::bitset::operators
std::bitset::operators
std::bitset::operator[]
std::bitset::reference
std::bitset::reset
std::bitset::set
std::bitset::size
std::bitset::test
std::bitset::to_string
std::bitset::to_ullong
std::bitset::to_ulong
std::bit_and
std::bit_and<void>
std::bit_not
std::bit_not<void>
std::bit_or
std::bit_or<void>
std::bit_xor
std::bit_xor<void>
std::boyer_moore_horspool_searcher
std::boyer_moore_searcher
std::byte
std::calloc
std::chars_format
std::chrono::abs(std::chrono::duration)
std::chrono::ceil(std::chrono::duration)
std::chrono::ceil(std::chrono::time_point)
std::chrono::duration
std::chrono::duration::count
std::chrono::duration::duration
std::chrono::duration::max
std::chrono::duration::min
std::chrono::duration::operator-
std::chrono::duration::operator--
std::chrono::duration::operators
std::chrono::duration::operators (%=)
std::chrono::duration::operators (unary)
std::chrono::duration::zero
std::chrono::duration_cast
std::chrono::duration_values
std::chrono::duration_values::max
std::chrono::duration_values::min
std::chrono::duration_values::zero
std::chrono::floor(std::chrono::duration)
std::chrono::floor(std::chrono::time_point)
std::chrono::high_resolution_clock
std::chrono::high_resolution_clock::now
std::chrono::round(std::chrono::duration)
std::chrono::round(std::chrono::time_point)
std::chrono::steady_clock
std::chrono::steady_clock::now
std::chrono::system_clock
std::chrono::system_clock::from_time_t
std::chrono::system_clock::now
std::chrono::system_clock::to_time_t
std::chrono::time_point
std::chrono::time_point::max
std::chrono::time_point::min
std::chrono::time_point::operators (operator-)
std::chrono::time_point::time_point
std::chrono::time_point::time_since_epoch
std::chrono::time_point_cast
std::chrono::treat_as_floating_point
std::clock
std::clock_t
std::common_type
std::common_type(std::chrono::duration)
std::common_type(std::chrono::time_point)
std::conditional
std::conjunction
std::const_mem_fun1_ref_t
std::const_mem_fun1_t
std::const_mem_fun_ref_t
std::const_mem_fun_t
std::const_pointer_cast
std::cref
std::ctime
std::current_exception
std::decay
std::declare_no_pointers
std::declare_reachable
std::declval
std::default_delete
std::default_searcher
std::destroy
std::destroy_at
std::destroy_n
std::difftime
std::disjunction
std::divides
std::divides<void>
std::domain_error
std::dynamic_pointer_cast
std::enable_if
std::enable_shared_from_this
std::enable_shared_from_this::enable_shared_from_this
std::enable_shared_from_this::shared_from_this
std::enable_shared_from_this::weak_from_this
std::end(std::initializer_list)
std::equal_to
std::equal_to<void>
std::errc
std::error_category
std::error_category::default_error_condition
std::error_category::equivalent
std::error_category::error_category
std::error_category::message
std::error_category::name
std::error_category::operators
std::error_code
std::error_code::assign
std::error_code::category
std::error_code::clear
std::error_code::default_error_condition
std::error_code::error_code
std::error_code::message
std::error_code::operator bool
std::error_code::value
std::error_condition
std::error_condition::assign
std::error_condition::category
std::error_condition::clear
std::error_condition::error_condition
std::error_condition::message
std::error_condition::operator bool
std::error_condition::value
std::exception
std::exception::exception
std::exception::what
std::exception_ptr
std::exchange
std::exit
std::extent
std::float_denorm_style
std::float_round_style
std::forward
std::forward_as_tuple
std::free
std::from_chars
std::function
std::function::assign
std::function::function
std::function::operator bool
std::function::swap
std::function::target
std::function::target_type
std::generic_category
std::get
std::get(std::pair)
std::get(std::tuple)
std::getenv
std::get_deleter
std::get_if
std::get_new_handler
std::get_pointer_safety
std::get_temporary_buffer
std::get_terminate
std::get_unexpected
std::gmtime
std::greater
std::greater<void>
std::greater_equal
std::greater_equal<void>
std::hash
std::hash
std::hash <std::optional>
std::hash <std::unique_ptr>
std::hash <std::variant>
std::hash(std::shared_ptr)
std::hash::hash
std::hash<std::error_code>
std::hash<std::error_condition>
std::hash<std::type_index>
std::has_unique_object_representations
std::has_virtual_destructor
std::holds_alternative
std::ignore
std::initializer_list
std::initializer_list::begin
std::initializer_list::end
std::initializer_list::initializer_list
std::initializer_list::size
std::integer_sequence
std::integral_constant
std::invalid_argument
std::invoke
std::invoke_result
std::in_place
std::in_place_index
std::in_place_index_t
std::in_place_t
std::in_place_t
std::in_place_type
std::in_place_type_t
std::is_abstract
std::is_aggregate
std::is_arithmetic
std::is_array
std::is_assignable
std::is_base_of
std::is_bind_expression
std::is_class
std::is_compound
std::is_const
std::is_constructible
std::is_convertible
std::is_copy_assignable
std::is_copy_constructible
std::is_default_constructible
std::is_destructible
std::is_empty
std::is_enum
std::is_error_code_enum
std::is_error_code_enum<std::future_errc>
std::is_error_condition_enum
std::is_error_condition_enum<std::errc>
std::is_final
std::is_floating_point
std::is_function
std::is_fundamental
std::is_integral
std::is_invocable
std::is_invocable_r
std::is_literal_type
std::is_lvalue_reference
std::is_member_function_pointer
std::is_member_object_pointer
std::is_member_pointer
std::is_move_assignable
std::is_move_constructible
std::is_nothrow_assignable
std::is_nothrow_constructible
std::is_nothrow_copy_assignable
std::is_nothrow_copy_constructible
std::is_nothrow_default_constructible
std::is_nothrow_destructible
std::is_nothrow_invocable
std::is_nothrow_invocable_r
std::is_nothrow_move_assignable
std::is_nothrow_move_constructible
std::is_nothrow_swappable
std::is_nothrow_swappable_with
std::is_null_pointer
std::is_object
std::is_placeholder
std::is_pod
std::is_pointer
std::is_polymorphic
std::is_reference
std::is_rvalue_reference
std::is_same
std::is_scalar
std::is_signed
std::is_standard_layout
std::is_swappable
std::is_swappable_with
std::is_trivial
std::is_trivially_assignable
std::is_trivially_constructible
std::is_trivially_copyable
std::is_trivially_copy_assignable
std::is_trivially_copy_constructible
std::is_trivially_default_constructible
std::is_trivially_destructible
std::is_trivially_move_assignable
std::is_trivially_move_constructible
std::is_union
std::is_unsigned
std::is_void
std::is_volatile
std::jmp_buf
std::launder
std::length_error
std::less
std::less<void>
std::less_equal
std::less_equal<void>
std::literals::chrono_literals::operator""h
std::literals::chrono_literals::operator""min
std::literals::chrono_literals::operator""ms
std::literals::chrono_literals::operator""ns
std::literals::chrono_literals::operator""s
std::literals::chrono_literals::operator""us
std::localtime
std::logical_and
std::logical_and<void>
std::logical_not
std::logical_not<void>
std::logical_or
std::logical_or<void>
std::logic_error
std::longjmp
std::make_any
std::make_error_code(std::errc)
std::make_error_condition(std::errc)
std::make_exception_ptr
std::make_from_tuple
std::make_optional
std::make_pair
std::make_shared
std::make_signed
std::make_tuple
std::make_unique
std::make_unsigned
std::malloc
std::max_align_t
std::mem_fn
std::mem_fun
std::mem_fun1_ref_t
std::mem_fun1_t
std::mem_fun_ref
std::mem_fun_ref_t
std::mem_fun_t
std::minus
std::minus<void>
std::mktime
std::modulus
std::modulus<void>
std::monostate
std::move
std::move_if_noexcept
std::multiplies
std::multiplies<void>
std::negate
std::negate<void>
std::negation
std::nested_exception
std::nested_exception::nested_exception
std::nested_exception::nested_ptr
std::nested_exception::rethrow_nested
std::new_handler
std::not1
std::not2
std::nothrow
std::nothrow_t
std::not_equal_to
std::not_equal_to<void>
std::not_fn
std::nullopt
std::nullopt_t
std::nullptr_t
std::numeric_limits
std::numeric_limits::denorm_min
std::numeric_limits::digits
std::numeric_limits::digits10
std::numeric_limits::epsilon
std::numeric_limits::has_denorm
std::numeric_limits::has_denorm_loss
std::numeric_limits::has_infinity
std::numeric_limits::has_quiet_NaN
std::numeric_limits::has_signaling_NaN
std::numeric_limits::infinity
std::numeric_limits::is_bounded
std::numeric_limits::is_exact
std::numeric_limits::is_iec559
std::numeric_limits::is_integer
std::numeric_limits::is_modulo
std::numeric_limits::is_signed
std::numeric_limits::is_specialized
std::numeric_limits::lowest
std::numeric_limits::max
std::numeric_limits::max_digits10
std::numeric_limits::max_exponent
std::numeric_limits::max_exponent10
std::numeric_limits::min
std::numeric_limits::min_exponent
std::numeric_limits::min_exponent10
std::numeric_limits::quiet_NaN
std::numeric_limits::radix
std::numeric_limits::round_error
std::numeric_limits::round_style
std::numeric_limits::signaling_NaN
std::numeric_limits::tinyness_before
std::numeric_limits::traps
std::operator<<(std::error_code)
std::operators (std::error_code)
std::optional
std::optional::emplace
std::optional::has_value
std::optional::operator*
std::optional::operators
std::optional::operators
std::optional::optional
std::optional::reset
std::optional::swap
std::optional::value
std::out_of_range
std::overflow_error
std::owner_less
std::owner_less
std::pair
std::pair::pair
std::pair::swap
std::piecewise_construct
std::piecewise_construct_t
std::placeholders::_1
std::placeholders::_2
std::placeholders::_N
std::plus
std::plus<void>
std::pmr::get_default_resource
std::pmr::new_delete_resource
std::pmr::null_memory_resource
std::pmr::pool_options
std::pmr::set_default_resource
std::pointer_safety
std::pointer_to_binary_function
std::pointer_to_unary_function
std::pointer_traits
std::ptrdiff_t
std::ptr_fun
std::quick_exit
std::raise
std::range_error
std::rank
std::rbegin(std::initializer_list)
std::realloc
std::ref
std::reference_wrapper
std::reference_wrapper::get
std::reference_wrapper::operator T&
std::reference_wrapper::reference_wrapper
std::reinterpret_pointer_cast
std::rel_ops::operators
std::remove_all_extents
std::remove_const
std::remove_cv
std::remove_extent
std::remove_pointer
std::remove_reference
std::remove_volatile
std::rend(std::initializer_list)
std::result_of
std::rethrow_exception
std::rethrow_if_nested
std::return_temporary_buffer
std::runtime_error
std::set_new_handler
std::set_terminate
std::set_unexpected
std::shared_ptr
std::shared_ptr::get
std::shared_ptr::operator bool
std::shared_ptr::operator->
std::shared_ptr::operator<<
std::shared_ptr::operators
std::shared_ptr::operators (>=)
std::shared_ptr::operator[]
std::shared_ptr::owner_before
std::shared_ptr::reset
std::shared_ptr::shared_ptr
std::shared_ptr::swap
std::shared_ptr::unique
std::shared_ptr::use_count
std::signal
std::sig_atomic_t
std::size_t
std::static_pointer_cast
std::strftime
std::swap(std::any)
std::swap(std::function)
std::swap(std::optional)
std::swap(std::pair)
std::swap(std::shared_ptr)
std::swap(std::tuple)
std::swap(std::unique_ptr)
std::swap(std::variant)
std::swap(std::weak_ptr)
std::system
std::system_category
std::system_error
std::system_error::code
std::system_error::system_error
std::system_error::what
std::terminate
std::terminate_handler
std::throw_with_nested
std::tie
std::time
std::timespec
std::timespec_get
std::time_t
std::tm
std::to_chars
std::tuple
std::tuple::swap
std::tuple::tuple
std::tuple_cat
std::tuple_element<std::pair>
std::tuple_element<std::tuple>
std::tuple_size<std::pair>
std::tuple_size<std::tuple>
std::tx_exception
std::type_index
std::type_index::hash_code
std::type_index::name
std::type_index::operators
std::type_index::type_index
std::type_info
std::type_info::before
std::type_info::hash_code
std::type_info::name
std::type_info::operator!=
std::type_info::operators
std::unary_function
std::unary_negate
std::uncaught_exception
std::uncaught_exceptions
std::undeclare_no_pointers
std::undeclare_reachable
std::underflow_error
std::underlying_type
std::unexpected
std::unexpected_handler
std::uninitialized_copy
std::uninitialized_copy_n
std::uninitialized_default_construct
std::uninitialized_default_construct_n
std::uninitialized_fill
std::uninitialized_fill_n
std::uninitialized_move
std::uninitialized_move_n
std::uninitialized_value_construct
std::uninitialized_value_construct_n
std::unique_ptr
std::unique_ptr::get
std::unique_ptr::get_deleter
std::unique_ptr::operator bool
std::unique_ptr::operator*
std::unique_ptr::operator[]
std::unique_ptr::release
std::unique_ptr::reset
std::unique_ptr::swap
std::unique_ptr::unique_ptr
std::uses_allocator
std::uses_allocator<std::function>
std::uses_allocator<std::tuple>
std::uses_allocator<std::variant>
std::variant
std::variant::emplace
std::variant::index
std::variant::swap
std::variant::valueless_by_exception
std::variant::variant
std::variant_alternative
std::variant_alternative_t
std::variant_npos
std::variant_size
std::variant_size_v
std::va_list
std::visit
std::void_t
std::wcsftime
std::weak_ptr
std::weak_ptr::expired
std::weak_ptr::lock
std::weak_ptr::owner_before
std::weak_ptr::reset
std::weak_ptr::swap
std::weak_ptr::use_count
std::weak_ptr::weak_ptr
std::_Exit
Type support
Utility library
Variadic functions
va_arg
va_copy
va_end
va_start
Clojure 1.8
Codeigniter 3
CSS
Docker 17
Electron
Elixir 1.5
Erlang 20
Eslint
Express
Git
Go
HTML
HTTP
Immutable 3.8.1
JavaScript
Lodash 4
Lua 5.3
Nginx
PHP
Phpunit 6
Python
React
React native
Redis
Redux
Ruby 2.4
Sass
Scikit image
Socket.IO
Sqlite
SVG
TensorFlow Guide
Typescript
Underscore
Vue 2
Webpack
Xslt & Xpath
Yarn
RxJS 5
Rollup.js
Babel
Parcel
MobX
Koa
Angular
Gulp
Grunt
Stylelint
Standard JS
Element UI
iView UI
Lavas
Mint UI
PostCSS
ThinkJS
Nest
npm
Node.js教程
JSON教程
Groovy教程
vb.net教程
Storm入门教程
Hibernate 教程
Slick教程
MongoDB教程
Yii 2.0

std::optional::value_or

STD：：可选：：Value[医]或

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

返回包含的值，如果*this有一个值，否则返回default_value...

1%29相当于bool(*this)?**this :static_cast<T>(std::forward<U>(default_value))

2%29相当于bool(*this)? std::move(**this):static_cast<T>(std::forward<U>(default_value))

参数

default_value-the value to use in case *this is empty

类型要求

-T必须符合CopyConstrucable的要求，才能使用过载%281%29。

-T必须符合可移动建筑的要求，才能使用过载%282%29。

-U&&&必须转换为T。

返回值

当前值如果*this有价值，或default_value否则。

例外

返回值的选定构造函数引发的任何异常。T...

例

二次

#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';
}

二次

可能的产出：

二次

(none)

二次

另见

valuereturns the contained value (public member function)

 © cppreference.com

在CreativeCommonsAttribution下授权-ShareAlike未移植许可v3.0。

http：//en.cppreference.com/w/cpp/实用程序/可选/值[医]或

  • 黔ICP备2021005045号