Hello, I need help with understanding & in C++ how does it work

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I was looking at simple c++ code and I saw something like int& b in a function.
I don't get it that what this & does here.
Any help would be appreciated.

What I have tried:

#include<iostream>

using namespace std;


int fun1(int a ,int& b);

int main(){
	int a=3,b=12;
	
	fun1(a,b);
	

}

int fun1(int a, int& b){
	a = a*b;
	b=a+7;
}

Posted 13-Jun-18 15:57pm

Nabeel Munir

Updated 13-Jun-18 22:11pm

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nv3 14-Jun-18 2:42am

Any textbook on C++ would have told you :-) But solution 1 will get you there also.

2 solutions

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User 7429338 · Answer 1 · 2018-06-13T19:48:00

Solution 1

In the function int fun1(int a ,int& b), a is an argument passed by value and b is an argument passed by reference.

The difference is explained here:
7.2 - Passing arguments by value | Learn C++[^]
7.3 - Passing arguments by reference | Learn C++[^]

Posted 13-Jun-18 19:48pm

User 7429338

Updated 13-Jun-18 19:49pm

v2

CPallini · Answer 2 · 2018-06-13T22:12:00

References are a way to implement function's out-parameters, like pointers did in plain C programming. C++17 offers destructuring in order to avoid them. Try for instance

#include <iostream>
#include <tuple>
#include <cmath>
using namespace std;

// rho and phi modification are visible to the caller
void polar( double x, double y, double & rho, double & phi)
{
  rho = sqrt(x*x + y*y);
  phi = atan2(x,y);
}

// prho, pphi provide access to actual caller variables
void c_style_polar( double x, double y, double * prho, double *pphi)
{
  *prho = sqrt(x*x +y*y);
  *pphi = atan2(x,y);
}

// the return value is an aggregate, the C++17 syntax allows to destructure it in plain doubles
tuple <double , double> tupled_polar(double x, double y)
{
  return make_tuple<double, double> ( sqrt(x*x+y*y), atan2(x,y));
}



int main()
{
  double x = 100, y = 100;

  // C++ out parameters
  {
    double rho, phi;
    rho = 0.0; phi = 0.0;
    polar(x, y, rho, phi);
    cout << "rho = " << rho << ", phi = " << phi << '\n';
  }

  // C-style out parameters
  {
    double rho,  phi;
    c_style_polar(x, y, &rho, &phi);
    cout << "rho = " << rho << ", phi = " << phi << '\n';
  }

  // C++17 destructuring
  {
    auto  [rho, phi] = tupled_polar(x, y);
    cout << "rho = " << rho << ", phi = " << phi << '\n';
  }
}