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Showing content from https://cplusplus.com/reference/functional/bind1st/ below:

function template

std::bind1st

template <class Operation, class T>  binder1st<Operation> bind1st (const Operation& op, const T& x);

Return function object with first parameter bound

This function constructs an unary function object from the binary function object

by binding its first parameter to the fixed value

The function object returned by bind1st has its operator() defined such that it takes only one argument. This argument is used to call binary function object op with x as the fixed value for the first argument.

It is defined with the same behavior as:

template <class Operation, class T>
  binder1st<Operation> bind1st (const Operation& op, const T& x)
{
  return binder1st<Operation>(op, typename Operation::first_argument_type(x));
}

To bind the second parameter to a specific value, see

bind2nd

Parameters

op: Binary function object derived from binary_function.
x: Fixed value for the first parameter of op.

Return value

An unary function object equivalent to

but with the first parameter always set to

binder1st

is a type derived from

unary_function

Example

// bind1st example
#include <iostream>
#include <functional>
#include <algorithm>
using namespace std;

int main () {
  int numbers[] = {10,20,30,40,50,10};
  int cx;
  cx = count_if (numbers, numbers+6, bind1st(equal_to<int>(),10) );
  cout << "There are " << cx << " elements that are equal to 10.\n";
  return 0;
}

There are 2 elements that are equal to 10.