binder1st Class

Article
08/03/2021

A class template providing a constructor that converts a binary function object into a unary function object by binding the first argument of the binary function to a specified value. Deprecated in C++11 in favor of bind, and removed in C++17.

Syntax

template <class Operation>
class binder1st
    : public unaryFunction <typename Operation::second_argument_type,
                             typename Operation::result_type>
{
public:
    typedef typename Operation::argument_type argument_type;
    typedef typename Operation::result_type result_type;
    binder1st(
        const Operation& binary_fn,
        const typename Operation::first_argument_type& left);

    result_type operator()(const argument_type& right) const;
    result_type operator()(const argument_type& right) const;

protected:
    Operation op;
    typename Operation::first_argument_type value;
};

Parameters

binary_fn
The binary function object to be converted to a unary function object.

left
The value to which the first argument of the binary function object is to be bound.

right
The value of the argument that the adapted binary object compares to the fixed value of the second argument.

Return Value

The unary function object that results from binding the first argument of the binary function object to the value left.

Remarks

The class template stores a copy of a binary function object binary_fn in op, and a copy of left in value. It defines its member function operator() as returning op(value, right).

If binary_fn is an object of type Operation and c is a constant, then bind1st(binary_fn, c) is a more convenient equivalent to binder1st<Operation>(binary_fn, c). For more information, see bind1st.

Example

// functional_binder1st.cpp
// compile with: /EHsc
#include <vector>
#include <functional>
#include <algorithm>
#include <iostream>

using namespace std;

int main()
{
    vector<int> v1;
    vector<int>::iterator Iter;

    int i;
    for (i = 0; i <= 5; i++)
    {
        v1.push_back(5 * i);
    }

    cout << "The vector v1 = ( ";
    for (Iter = v1.begin(); Iter != v1.end(); Iter++)
        cout << *Iter << " ";
    cout << ")" << endl;

    // Count the number of integers > 10 in the vector
    vector<int>::iterator::difference_type result1;
    result1 = count_if(v1.begin(), v1.end(),
        binder1st<less<int> >(less<int>(), 10));
    cout << "The number of elements in v1 greater than 10 is: "
         << result1 << "." << endl;

    // Compare use of binder2nd fixing 2nd argument:
    // count the number of integers < 10 in the vector
    vector<int>::iterator::difference_type result2;
    result2 = count_if(v1.begin(), v1.end(),
        binder2nd<less<int> >(less<int>(), 10));
    cout << "The number of elements in v1 less than 10 is: "
         << result2 << "." << endl;
}

The vector v1 = ( 0 5 10 15 20 25 )
The number of elements in v1 greater than 10 is: 3.
The number of elements in v1 less than 10 is: 2.

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