std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N

 

C++

语言

概念

实用工具库

字符串库

集装箱库

算法库

迭代器库

Numerics库

输入/输出库

本地化库

正则表达式库 （C++11）

原子操作库 （C++11）

线程的支持库 （C++11）

 

实用工具库

类型的支持 (basic types, RTTI, type traits)

动态内存管理

错误处理

程序实用工具

可变参数函数

日期和时间

函数对象

initializer_list （C++11）

bitset

hash （C++11）

关系运算符 Original: Relational operators The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

rel_ops::operator!= rel_ops::operator> rel_ops::operator<= rel_ops::operator>=

双和元组 Original: Pairs and tuples The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

pair

tuple （C++11）

piecewise_construct_t （C++11）

piecewise_construct （C++11）

掉期，远期和移动 Original: Swap, forward and move The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

swap

forward （C++11）

move （C++11）

move_if_noexcept （C++11）

declval （C++11）

 

函数对象

功能包装 Original: Function wrappers The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
function （C++11）
mem_fn （C++11）
bad_function_call （C++11）
绑定 Original: Bind The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
bind （C++11）
is_bind_expression （C++11）
is_placeholder （C++11）
_1, _2, _3, ... （C++11）
参考包装 Original: Reference wrappers The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
reference_wrapper （C++11）
ref cref （C++11） （C++11）
操作包装 Original: Operator wrappers The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
plus minus negate multiplies divides modulus logical_and logical_or logical_not	equal_to not_equal_to greater less greater_equal less_equal bit_and bit_or bit_xor
否定词 Original: Negators The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
unary_negate binary_negate	not1 not2
已过时粘合剂和适配器 Original: Deprecated binders and adaptors The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
unary_function （过时了）
binary_function （过时了）
ptr_fun （过时了）
pointer_to_unary_function （过时了）
pointer_to_binary_function （过时了）
mem_fun （过时了）
mem_fun_t mem_fun1_t const_mem_fun_t const_mem_fun1_t （过时了） （过时了） （过时了） （过时了）
mem_fun_ref （过时了）
mem_fun_ref_t mem_fun1_ref_t const_mem_fun_ref_t const_mem_fun1_ref_t （过时了） （过时了） （过时了） （过时了）
binder1st binder2nd （过时了） （过时了）
bind1st bind2nd （过时了） （过时了）

 

Defined in header <functional>
extern /*unspecified*/ _1; extern /*unspecified*/ _2; . . extern /*unspecified*/ _N;

std::placeholders命名空间包含了占位符对象[_1, . . . _N]N是实现定义的最大数量.

Original:

The std::placeholders namespace contains the placeholder objects [_1, . . . _N] where N is an implementation defined maximum number.

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使用时，作为参数在std::bind表达，占位符对象存储在生成的函数对象，未绑定的参数调用时，函数对象，每个占位符被替换为相应的第N个未绑定的参数_N.

Original:

When used as an argument in a std::bind expression, the placeholder objects are stored in the generated function object, and when that function object is invoked with unbound arguments, each placeholder _N is replaced by the corresponding Nth unbound argument.

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占位符对象的类型是DefaultConstructible和CopyConstructible，它们的默认复制/移动的构造函数不抛出异常，和任何占位符_N，std::is_placeholder<decltype(_N)>定义的类型，是来自std::integral_constant<int, N>.

Original:

The types of the placeholder objects are DefaultConstructible and CopyConstructible, their default copy/move constructors do not throw exceptions, and for any placeholder _N, the type std::is_placeholder<decltype(_N)> is defined and is derived from std::integral_constant<int, N>.

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为例

下面的代码显示了一个占位符参数的函数对象的创建.

Original:

The following code shows the creation of function objects with a placeholder argument.

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#include <functional>
#include <string>
#include <iostream>
 
void goodbye(const std::string& s)
{
    std::cout << "Goodbye " << s << '\n';
}
 
class Object {
public:
    void hello(const std::string& s)
    {
        std::cout << "Hello " << s << '\n';
    }
};
 
int main(int argc, char* argv[])
{
    typedef std::function<void(const std::string&)> ExampleFunction;
    Object instance;
    std::string str("World");
    ExampleFunction f = std::bind(&Object::hello, &instance,
                                  std::placeholders::_1);
 
    // equivalent to instance.hello(str)
    f(str);
    f = std::bind(&goodbye, std::placeholders::_1);
 
    // equivalent to goodbye(str)
    f(str);
    return 0;
}

Output:

Hello World
Goodbye World

另请参阅

bind （C++11）	绑定一个或多个参数的函数对象 Original: binds one or more arguments to a function object The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. （函数模板）
is_placeholder （C++11）	指示一个对象是一个标准的占位符或可以用作1 Original: indicates that an object is a standard placeholder or can be used as one The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. （类模板）