C++: how to use pass by reference to return multiple values in a method?

Question

I need to return two doubles. I know a little about pass by reference, but I'm not sure how to use it to return multiple. Here's what I've tried so far. I made two reference parameters and added it to the method I hoped would return two values.

#include "stdafx.h" 
#include <iostream>
using namespace std;
int main() {
  double min;
  double max;
  getDistance(766, 981, 328, 609, min, max);
  getDistance(899, 171, 1009, 282, min, max);
  cout << "Minimum distance is "+minDistance+"inches";
  cout << "Maximum distance is "+maxDistance+"inches";
  return 0;
}

void getDistance(int left, int right, int top, int bottom, double& minDistance, double& maxDistance) {
  int height=bottom - top;
  int width=right - left;
  bool isCNN=width !=height;
  if (isCNN) {
    if (width >=215 && height >=281) minDistance=0;
    maxDistance=14;
    else if (width >=124 && height >=167) minDistance=14;
    maxDistance=33.75;
    else if (width >=76 && height >=111) minDistance=33.75;
    maxDistance=53.5;
    else if (width >=56 && height >=94) minDistance=53.5;
    maxDistance=73.25;
    else if (width >=49 && height >=84) minDistance=73.25;
    maxDistance=93;
    else if (width >=41 && height >=71) minDistance=93;
    maxDistance=112.75;
    else if (width >=28 && height >=57) minDistance=132.5;
    maxDistance=172;
    else if (width >=23 && height >=49) minDistance=191.75;
    maxDistance=270.75;
    else minDistance=270.75;
    maxDistance=480;
  }
  else {
    if (width >=330) minDistance=0;
    maxDistance=6.375;
    else if (width >=238) minDistance=6.375;
    maxDistance=16.25;
    else if (width >=168) minDistance=16.25;
    maxDistance=26.125;
    else if (width >=122) minDistance=26.125;
    maxDistance=36;
    else if (width >=108) minDistance=36;
    maxDistance=55.75;
    else if (width >=91) minDistance=55.75;
    maxDistance=75.5;
    else minDistance=75.5;
    maxDistance=144;
  }
  return;
}

values.

Just pass two references. One for each of your return values. — john, Jul 25 '18 at 20:59
You should show what you've tried so far. If you don't know how to pass by reference, I suggest studying from a [good c++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — Ben Jones, Jul 25 '18 at 21:01
The use of the `if` and `else` statements is a little wonky . `if (width >=215 && height >=281) minDistance=0;` is the whole `if`. `maxDistance=14;` is outside the `if`, so the `else` is disconnected. This results in a cascade of error messages. The braces are required: `if (width >= 215 && height >= 281) { minDistance = 0; maxDistance = 14; }` — user4581301, Jul 25 '18 at 22:20

score 2 · Answer 1 · answered Jul 25 '18 at 21:05

2

You can use std::pair<double, double> as the return type. E.g.

std::pair<double, double> foo()
{
   return {10, 20};
}

answered Jul 25 '18 at 21:05

R Sahu

204,454
14
159
270

everybody0523 · Answer 2 · 2018-07-25T21:54:34.583

If you really want to 'return' two values with pass-by-reference:

void foo(int& x, int& y)
{
    x = 55;
    y = 66;
}

int main()
{
    int x = 0;
    int y = 0;
    foo(x, y);
    if (x == 55 && y == 66)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}

Something like this should do. As others have noted, there are many other ways to actually return two values, but here you go.

In the code you submitted, (syntax issues aside), you need to refer to min and max in your main method in the cout statement. In getDistance() with minDistance and maxDistance, you have created aliases to the doubles passed to it, but those aliases do not exist outside the function!

score 0 · Answer 3 · answered Jul 26 '18 at 07:55

As others have pointed out, there are other ways of returning 2 values than passing by reference. If you do in fact want to return via reference, what you have written is almost correct. Rather than use the variables 'minDistance' and 'maxDistance' in the main method (where they have not been declared), you should use the 'min' and 'max' variables that you have declared. In addition, you should use '<<' rather than '+' to append the output in the cout call. Also, if you want the output to appear on separate lines (rather than run together), you should end the line with '<< endl'. Finally, as user4581301 has said, the bodies of your 'if' statements do not include both minDistance and maxDistance. You need braces around the statements in order to include both.

Rewriting your code to take these changes into account (but without ensuring the correctness of your 'calculation' in any way) gives:

#include <iostream>

int main(int, char**) {
  double min;
  double max;
  getDistance(766, 981, 328, 609, min, max);
  getDistance(899, 171, 1009, 282, min, max);
  std::cout << "Minimum distance is " << min << "inches" << std::endl;
  std::cout << "Maximum distance is " << max << "inches" << std::endl;
  return 0;
}

void getDistance(int left, int right, int top, int bottom, double& minDistance, double& maxDistance) {
  int height = bottom - top;
  int width = right - left;
  bool isCNN = width !=height;

  if (isCNN) {
    if (width >=215 && height >=281) {
      minDistance=0;
      maxDistance=14;
    } else if (width >=124 && height >=167) {
      minDistance=14;
      maxDistance=33.75;
    } else if (width >=76 && height >=111) {
      minDistance=33.75;
     maxDistance=53.5;
    } else if (width >=56 && height >=94) {
      minDistance=53.5;
      maxDistance=73.25;
    } else if (width >=49 && height >=84) {
      minDistance=73.25;
      maxDistance=93;
    } else if (width >=41 && height >=71) {
      minDistance=93;
      maxDistance=112.75;
    } else if (width >=28 && height >=57) {
      minDistance=132.5;
      maxDistance=172;
    } else if (width >=23 && height >=49) {
      minDistance=191.75;
      maxDistance=270.75;
    } else {
      minDistance=270.75;
      maxDistance=480;
    }
  } else {
    if (width >=330) {
      minDistance=0;
      maxDistance=6.375;
    } else if (width >=238) {
      minDistance=6.375;
      maxDistance=16.25;
    } else if (width >=168) {
      minDistance=16.25;
      maxDistance=26.125;
    } else if (width >=122) {
      minDistance=26.125;
      maxDistance=36;
    } else if (width >=108) {
      minDistance=36;
      maxDistance=55.75;
    } else if (width >=91) {
      minDistance=55.75;
      maxDistance=75.5;
    } else {
      minDistance=75.5;
      maxDistance=144;
    }
  }
  return;
}

C++: how to use pass by reference to return multiple values in a method?

3 Answers3