std::placeholders::_2
STD::占位符:[医]1,STD::占位符::[医]2、...,STD::占位符::[医]n
Defined in header | | |
---|---|---|
/*see below*/ _1; /*see below*/ _2; . . /*see below*/ _N; | | |
大std::placeholders
命名空间包含占位符对象。[_1, . . . _N]
何地N
是实现定义的最大数目。
当用作参数时,std::bind
表达式,占位符对象存储在生成的函数对象中,当使用未绑定参数调用该函数对象时,每个占位符。_N
被相应的Nth无界参数替换。
Each placeholder is declared as if by extern /*unspecified*/ _1; | (until C++17) |
---|---|
Implementations are encouraged to declare the placeholders as if by inline constexpr /*unspecified*/ _1;, although declaring them by extern /*unspecified*/ _1; is still allowed by the standard. | (since C++17) |
占位符对象的类型为DefaultConstructible和CopyConstructible,它们的默认复制/移动构造函数不会引发异常,并且对于任何占位符_N,类型std::is_placeholder<decltype(_N)>定义并派生自std::integral_constant<int, N>...
例
下面的代码显示了使用占位符参数创建函数对象的过程。
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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;
}
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产出:
二次
Hello World
Goodbye World
二次
另见
bind (C++11) | binds one or more arguments to a function object (function template) |
---|---|
is_placeholder (C++11) | indicates that an object is a standard placeholder or can be used as one (class template) |
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