Bootstrap 4
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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
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EXIT_FAILURE
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Fixed width integer types
FLT_EVAL_METHOD
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Low level memory management
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offsetof
operators (delete[])
operators (new[])
operators (std::bitset)
operators (std::bitset)
operators (std::bitset)
operators (std::chrono::duration)
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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
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SIGILL
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SIGSEGV
SIGTERM
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std::abort
std::addressof
std::add_const
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std::align
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std::alignment_of
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std::allocate_shared
std::allocator_arg
std::allocator_arg_t
std::any
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std::any::emplace
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std::any::reset
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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
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std::bad_alloc
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std::bad_typeid
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std::bitset::to_string
std::bitset::to_ullong
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std::bit_and
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std::bit_not
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std::bit_or
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std::bit_xor
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std::boyer_moore_horspool_searcher
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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
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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
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std::chrono::high_resolution_clock
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std::chrono::time_point::max
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std::hash
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std::is_abstract
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std::is_arithmetic
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std::is_class
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std::is_const
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std::is_empty
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std::is_final
std::is_floating_point
std::is_function
std::is_fundamental
std::is_integral
std::is_invocable
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std::is_literal_type
std::is_lvalue_reference
std::is_member_function_pointer
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std::launder
std::length_error
std::less
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std::less_equal
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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
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std::logical_not
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std::logical_or
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std::holds_alternative

STD：：搁置[医]替代

Defined in header ​​
template <class T, class... Types> constexpr bool holds_alternative(const std::variant<Types...>& v)​(since C++17)

检查变体是否v持有另一种选择T...打电话是不正确的，如果T不止一次出现在Types...

参数

v-variant to examine

返回值

true如果变体当前持有可供选择的T，，，false否则。

例外

noexcept规格：

noexcept

例

二次

#include <variant>
#include <string>
#include <iostream>
int main()
{
    std::variant<int, std::string> v = "abc";
    std::cout << std::boolalpha
              << "variant holds int? "
              << std::holds_alternative<int>(v) << '\n'
              << "variant holds string? "
              << std::holds_alternative<std::string>(v) << '\n';
}

二次

产出：

二次

variant holds int? false
variant holds string? true

二次

另见

indexreturns the zero-based index of the alternative held by the variant (public member function)
std::get(std::variant) (C++17)reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)

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