Tutorialspoint.dev

Passing vector to a function in C++

When we pass an array to a function, a pointer is actually passed.

When a vector is passed to a function, a copy of the vector is created. For example, we can see below program, changes made inside the function are not reflected outside because function has a copy.

// C++ program to demonstrate that when vectors
// are passed to functions without &, a copy is
// created.
#include<bits/stdc++.h>
using namespace std;
   
// The vect here is a copy of vect in main()
void func(vector<int> vect)
{
   vect.push_back(30);
}
   
int main()
{
    vector<int> vect;
    vect.push_back(10);
    vect.push_back(20);
   
    func(vect);
   
    // vect remains unchanged after function
    // call
    for (int i=0; i<vect.size(); i++)
       cout << vect[i] << " ";
   
    return 0;
}

Output:

10 20

The above style of passing might also take a lot of time in cases of large vectors. So it is a good idea to pass by reference.

// C++ program to demonstrate how vectors
// can be passed by reference.
#include<bits/stdc++.h>
using namespace std;
   
// The vect is passed by reference and changes
// made here reflect in main()
void func(vector<int> &vect)
{
   vect.push_back(30);
}
   
int main()
{
    vector<int> vect;
    vect.push_back(10);
    vect.push_back(20);
   
    func(vect);
  
    for (int i=0; i<vect.size(); i++)
       cout << vect[i] << " ";
   
    return 0;
}

Output:

br>
10 20 30

If we do not want a function to modify a vector, we can pass it as a const reference.

// C++ program to demonstrate how vectors
// can be passed by reference with modifications
// restricted.
#include<bits/stdc++.h>
using namespace std;
   
// The vect is passed by constant reference
// and cannot be changed by this function.
void func(const vector<int> &vect)
{
    // vect.push_back(30);  // Uncommenting this line would
                             // below error
    // "prog.cpp: In function 'void func(const std::vector<int>&)':
    // prog.cpp:9:18: error: passing 'const std::vector<int>' 
    // as 'this' argument discards qualifiers [-fpermissive]"   
    
    for (int i=0; i<vect.size(); i++)
       cout << vect[i] << " ";
}
   
int main()
{
    vector<int> vect;
    vect.push_back(10);
    vect.push_back(20);
   
    func(vect);
    
    return 0;
}

Output:

10 20 

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



This article is attributed to GeeksforGeeks.org

You Might Also Like

leave a comment

code

0 Comments

load comments

Subscribe to Our Newsletter