c++ bind what are placeholders
std::bind
is a function template in C++ that allows you to create a callable object (a function or a function object) by binding arguments to a function or member function. Placeholders are special objects used in conjunction with std::bind
to represent future arguments that will be provided when the bound function is called.
std::bind
Function Template
The std::bind
function template is defined in the <functional>
header. It takes a callable object (a function or a member function) as the first argument, followed by a sequence of arguments that are used to bind values to the parameters of the callable object. The return value of std::bind
is a new callable object that can be invoked with the remaining arguments.
Placeholders
Placeholders are used in the arguments of std::bind
to represent future arguments that will be provided when the bound function is called. They are defined as special objects in the std::placeholders
namespace. The placeholders are named as _1
, _2
, _3
, and so on, up to _N
, where N
is an implementation-defined maximum number of arguments that can be bound.
When you use placeholders in the arguments of std::bind
, they act as a placeholder for an argument that will be passed when the bound function is called. For example, if you bind a function with std::bind
and use _1
as a placeholder in the binding, the first argument passed to the bound function will be used as the value for that placeholder.
Example
Here's an example to illustrate the usage of std::bind
and placeholders:
#include <iostream>
#include <functional>
void printValues(int a, int b, int c) {
std::cout << "a: " << a << ", b: " << b << ", c: " << c << std::endl;
}
int main() {
auto boundFunction = std::bind(printValues, std::placeholders::_1, 2, std::placeholders::_2);
boundFunction(1, 3);
return 0;
}
In this example, we define a function printValues
that takes three integer arguments and prints their values. We then use std::bind
to bind the printValues
function with arguments. We use _1
, _2
, and _3
as placeholders to represent the first, second, and third arguments, respectively. In the main
function, we create a bound function boundFunction
by calling std::bind
with the printValues
function and the placeholders _1
, 2
, and _2
. Finally, we call boundFunction
with arguments 1
and 3
, which will be passed as the values for the placeholders _1
and _2
, respectively.
When we run this program, it will output:
a: 1, b: 2, c: 3
This demonstrates how placeholders can be used to bind values to arguments in a callable object created with std::bind
.
I hope this explanation helps! Let me know if you have any further questions.