std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N
<functional>
/*see below*/ _2;
.
.
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 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)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>.
[edit] Example
The following code shows the creation of function objects with placeholder arguments.
#include <functional> #include <iostream> #include <string> 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
[edit] See also
(class template) [edit]