std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N
| Defined in header <functional>
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| extern /*unspecified*/ _1; extern /*unspecified*/ _2; |
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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.
The types of the placeholder objects are DefaultConstructible and CopyConstructible, their default copy/move constructors do not throw exceptions, and for for any placeholder _N, the type std::is_placeholder<decltype(_N)> is defined and is derived from std::numeric_constant<int, N>.
[edit] Example
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(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
[edit] 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) |
