Defined in header <functional> | ||
|---|---|---|
/*see below*/ _1; /*see below*/ _2; . . /*see below*/ _N; |
The std::placeholders namespace contains the placeholder objects [_1, . . . _N] where N is an implementation defined maximum number.
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.
| Each placeholder is declared as if by | (until C++17) |
| Implementations are encouraged to declare the placeholders as if by | (since C++17) |
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>.
The following code shows the creation of function objects with a placeholder argument.
#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()
{
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
|
(C++11) | binds one or more arguments to a function object (function template) |
|
(C++11) | indicates that an object is a standard placeholder or can be used as one (class template) |
placeholder to skip an element when unpacking a tuple using tie (constant) |
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