std::placeholders (C++)
The std::placeholders namespace contains the placeholder objects [_1, ..., _N] where N is an implementation defined maximum number.
Resources
std::placeholders are used in conjunction with std::bind to create partially applied functions.
Example
#include <iostream>
#include <functional>
void add(int a, int b) {
std::cout << "Sum: " << a + b << std::endl;
}
int main() {
// Bind the second argument as a placeholder (_1)
auto add_with_first_arg_fixed = std::bind(add, 5, std::placeholders::_1);
// Calling the bound function with the second argument
add_with_first_arg_fixed(10); // Output: Sum: 15
return 0;
}
#include <iostream>
#include <functional>
void multiply(int a, int b, int c) {
std::cout << "Result: " << a * b * c << std::endl;
}
int main() {
// Reordering arguments: here, _2 will be passed to the first argument of multiply,
// _1 to the second argument, and 3 is fixed as the third argument.
auto reorderedMultiply = std::bind(multiply, std::placeholders::_2, std::placeholders::_1, 3);
// Call the reordered function
reorderedMultiply(4, 5); // Output: Result: 60 (i.e., 5 * 4 * 3)
return 0;
}
Here, boundFunc is this function
void myFunction(int x, int y) {
return;
}
std::function<void(int)> boundFunc = std::bind(myFunction, 10, std::placeholders::_1);We see this a lot in ROS, when you bind the first value to this.