std::bind1st,std::bind2nd(3)

💖 NAME 💖

std::bind1st, std::bind2nd — bestie, this is the std::bind1st,std::bind2nd(3) page from the C++ Standard Libary (yes, that spelling is literally in the header, don't come for me 😭). It’s the old-school pair for binding one argument to a binary function object.

✨ Synopsis ✨

Okay so first thing: it’s defined in header <functional> 🫶

Defined in header <functional>

template< class F, class T >                         (1) (deprecated in C++11)
std::binder1st<F> bind1st( const F& f, const T& x );     (removed in C++17)

template< class F, class T >                         (2) (deprecated in C++11)
std::binder2nd<F> bind2nd( const F& f, const T& x );     (removed in C++17)

So the vibe is: it binds a given argument x to a first or second parameter of the given binary function object f. That is, stores x within the resulting wrapper, which, if called, passes x as the first or the second parameter of f. Basically it’s holding your little argument for you and handing it over at the right spot. Periodt 💅

1. Binds the first argument of f to x. Effectively calls std::binder1st<F>(f, typename F::first_argument_type(x)).
2. Binds the second argument of f to x. Effectively calls std::binder2nd<F>(f, typename F::second_argument_type(x)).

🎀 Parameters 🎀

• f — pointer to a function to bind an argument to
• x — argument to bind to f

🌸 Return value 🌸

A function object wrapping f and x. Simple as that, icon ✨

💅 Exceptions 💅

May throw implementation-defined exceptions. No cap.

🦋 Example 🦋

The original has a little // Run this code marker at the top, so I’m keeping it — here’s the whole thing, bestie:

// Run this code

#include <algorithm>
#include <cmath>
#include <cstddef>
#include <functional>
#include <iomanip>
#include <iostream>
#include <vector>

int main()
{
    std::vector<double> a = {0, 30, 45, 60, 90, 180};
    std::vector<double> r(a.size());
    const double pi = std::acos(-1); // since C++20 use std::numbers::pi

    std::transform(a.begin(), a.end(), r.begin(),
        std::bind1st(std::multiplies<double>(), pi / 180.0));
//  an equivalent lambda is: [pi](double a) { return a * pi / 180.0; });

    for (std::size_t n = 0; n < a.size(); ++n)
        std::cout << std::setw(3) << a[n] << "° = " << std::fixed << r[n]
                  << " rad\n" << std::defaultfloat;
}

⭐ Output ⭐

This is giving exactly:

  0° = 0.000000 rad
 30° = 0.523599 rad
 45° = 0.785398 rad
 60° = 1.047198 rad
 90° = 1.570796 rad
180° = 3.141593 rad

🌈 See also 🌈

• binder1st, binder2nd — function object holding a binary function and one of its arguments (class template) (deprecated in C++11)(removed in C++17)
• bind_front — bind a variable number of arguments, in order, to a function object (function template) (C++20)
• bind_back — bind a variable number of arguments, in order, to a function object (function template) (C++23)

source: http://cppreference.com · version/date: 2024.06.10 — mwah, bestie 💋💖