How to Return a Vector From a Function in C++?

In C++, by returning vectors, users to return multiple values at once from a function as the elements of vector. In this article, we will learn how to return a vector from a function in C++.

The recommended way to return a vector from a function is by using return vector by value method. Let’s take a look at a simple example:

#include <bits/stdc++.h>
using namespace std;

// Function that returns a vector
vector<int> f() {
    vector<int> v = {1, 2, 3, 4, 5};

    // Return the vector 
    return v;
}

int main() {

    // Store the returned vector
    vector<int> v = f();

    for (int i : v)
        cout << i << " ";
    return 0;
}

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Explanation: We simply have to specify the vector of the given type as the return type of the function return it using return statement.

Note: In this method, the copy of the vector that is returned is created and passed to the caller function.

There are also some other methods in C++ to return a vector from a function. Some of them are as follows:

By Reference

In addition to returning a vector by value, we can also return a vector by reference. The return type of the function is defined as the reference to the vector of given type.

#include <bits/stdc++.h>
using namespace std;

// Function that returns a reference to a vector
vector<int>& f() {
	
  	// Creating a static vector
  	static vector<int> v = { 1, 2, 3, 4, 5};
	
  	// Returning a vector
    return v;
}

int main() {
	vector<int> v = f();

    for (int i : v)
        cout << i << " ";
    return 0;
}

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In this method, the reference of the original vector is returned, so it must be ensure that the vector being returned is not destroyed when the function ends. This can be done by either dynamically creating vector using new or declaring vector as static.

By Pointer

Returning a vector as a pointer is similar to returning vector as reference. The only difference will be that the syntax of pointers will be used instead of reference.

#include <bits/stdc++.h>
using namespace std;

// Function that returns a pointer to a vector
vector<int>* f() {
  
     // Creating the dynamic vector
  	vector<int>* v = new vector<int>
      ({1, 2, 3, 4, 5});
	
  	// Returning pointer to the vector
    return v;
}

int main() {
    
    // Create a vector pointer
    vector<int>* v = f();

    for (int i : *v) 
        cout << i << " ";
    return 0;
}

1 2 3 4 5 

Again, it must be ensured that the vector is not destroyed after it is returned from the function.

By r-value Reference

In this method, we return the r-value reference of the vector which transfers ownership of the vector's resources to the caller without creating a temporary copy. The move() function is used to provide the r-value reference to the vector.

#include <bits/stdc++.h>
using namespace std;

// Class to test the construction/destruction
class A {
  public:
  A() {
    cout << "Default Constructor" << endl;
  }
  A(const A& obj) {
    cout << "Copy Constructor" << endl;
  }
  ~A() {
    cout << "Destructor Called" << endl;
  }
};

// Function that returns the r-value reference
vector<A>&& f() {
  
   // Creating the dynamic vector
    vector<A>* v = new vector<A>(1);
    return move(*v);
}

int main() {
    vector<A> v = f();
    return 0;
}

Default Constructor
Destructor Called

Explanation: As we can see, the vector elements are created and destroyed only once and no copy is created.