Passing vector to a function in C++

2.5

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:

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 

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