Returning two variables in a C++ function

Question

I would like to return two double variables: when calling a function that I have created. According to some tutorials (which deal with the basics of the C++), I would be unable to do that.

Is there a way to do so?

Answer 1

You could write a simple struct that holds the variables and return it, or use an std::pair or std::tuple:

#include <utility>

std::pair<double, double> foo()
{
  return std::make_pair(42., 3.14);
}

#include <iostream>
#include <tuple> // C++11, for std::tie
int main()
{
  std::pair<double, double> p = foo();
  std::cout << p.first << ", " << p.second << std::endl;

  // C++11: use std::tie to unpack into pre-existing variables
  double x, y;
  std::tie(x,y) = foo();
  std::cout << x << ", " << y << std::endl;

  // C++17: structured bindings
  auto [xx, yy] = foo(); // xx, yy are double
}

Answer 2

If you're using C++11, I'd say the ideal way is to use std::tuple and std::tie.

Example taken from the std::tuple page I linked to:

#include <tuple>
#include <iostream>
#include <string>
#include <stdexcept>

std::tuple<double, char, std::string> get_student(int id)
{
    if (id == 0) return std::make_tuple(3.8, 'A', "Lisa Simpson");
    if (id == 1) return std::make_tuple(2.9, 'C', "Milhouse Van Houten");
    if (id == 2) return std::make_tuple(1.7, 'D', "Ralph Wiggum");
    throw std::invalid_argument("id");
}

int main()
{
    auto student0 = get_student(0);
    std::cout << "ID: 0, "
              << "GPA: " << std::get<0>(student0) << ", "
              << "grade: " << std::get<1>(student0) << ", "
              << "name: " << std::get<2>(student0) << '\n';

    double gpa1;
    char grade1;
    std::string name1;
    std::tie(gpa1, grade1, name1) = get_student(1);
    std::cout << "ID: 1, "
              << "GPA: " << gpa1 << ", "
              << "grade: " << grade1 << ", "
              << "name: " << name1 << '\n';
}

Answer 3

You can pass references to two doubles into a function, setting their values inside the function

void setTwoDoubles(double& d1, double& d2)
{
    d1 = 1.0;
    d2 = 2.0;
}

double d1, d2;
setTwoDoubles(d1, d2);
std::cout << "d1=" << d1 << ", d2=" << d2 << std::endl

Answer 4

You can use a std::pair, for instance.

Answer 5

Technically, no you are not able to return two variables in the way you would normally return a variable. You can, however, use references. That way, you can pass multiple variables to a function, and the function will assign them, rather than returning anything:

void function(double & param1, double & param2) {
    param1 = 6.28;
    param2 = 3.14;
}

And you would call it like this:

double var1, var2;
function(var1, var2);

Answer 6

You can't do it directly (because a return value is singular).
But, you could put a few values in a structure, and return that (like a pair<>).

A common pattern is to return output variables by reference:

ReturnVal Myfunction(/*in*/ BlahType _someParameters, /*out*/ ReturnType& _firstReturn, /*out*/ OtherReturnType& _secondReturn)
{
   _firstReturn = //someStuff
   _secondReturn = //someOtherStuff


return SUCCESS;
}

Answer 7

No you can not return two variables you need to use by reference method as

    #include <iostream>
using namespace std;

// function declaration
void swap(int &x, int &y);

int main ()
{
   // local variable declaration:
   int a = 100;
   int b = 200;

   cout << "Before swap, value of a :" << a << endl;
   cout << "Before swap, value of b :" << b << endl;

   /* calling a function to swap the values using variable reference.*/
   swap(a, b);

   cout << "After swap, value of a :" << a << endl;
   cout << "After swap, value of b :" << b << endl;

   return 0;
}


// function definition to swap the values.
void swap(int &x, int &y)
{
   int temp;
   temp = x; /* save the value at address x */
   x = y;    /* put y into x */
   y = temp; /* put x into y */

   return;
}

the output will be 100 // x before calling swap function 200 // y before calling swap function 200 // x after calling swap function 100 // y after calling swap function

this as return two values

this link help you

Answer 8

You cannot return two values from one function, but you could return a pointer to an array or some other structure which contains the two doubles.