std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N
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, where std::is_placeholder<decltype(_N)> is derived from std::integral_constant<int, N>.
Example
The following code shows the creation of function objects with placeholder arguments.
#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()
{
using namespace std::placeholders;
using ExampleFunction = std::function<void(const std::string&)>;
Object instance;
std::string str("World");
ExampleFunction f = std::bind(&Object::hello, &instance, _1);
f(str); // equivalent to instance.hello(str)
f = std::bind(&goodbye, std::placeholders::_1);
f(str); // equivalent to goodbye(str)
auto lambda = [](std::string pre, char o, int rep, std::string post) {
std::cout << pre;
while (rep-- > 0) std::cout << o;
std::cout << post << '\n';
};
// binding the lambda:
std::function<void(std::string, char, int, std::string)> g =
std::bind(&decltype(lambda)::operator(), &lambda, _1, _2, _3, _4);
g("G", 'o', 'o'-'g', "gol");
}
Output:
Hello World Goodbye World Goooooooogol
See also
|
(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) |
|
(C++11)
|
placeholder to skip an element when unpacking a tuple using tie (constant) |
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