Passing Vector to a Function in C++

To perform operations on vector belonging to one function inside other function, we pass this vector to the function as arguments during the function call.

C++ provides three methods to pass a vector to a function in C++. Let's look at each of them one by one.

Pass Vector by Value

The pass by value method is the simplest method to pass any vector to the function. This method will create a copy of the original vector for passing. Therefore, modifications done inside the function will not affect the original vector.

Syntax

In the function declaration, we define the argument name and type as shown:

rtype foo(vector<type> para_name);

In the function call, we just pass the vector name.

foo(vec_name);

where,

• para_name is the name that will be used in the function for referring to the copy of the vector.
• vec_name is the name of the original vector.

Example

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

// Pass by value
void foo(vector<int> vec) {
  
  	// Modifying vector inside function
    vec.push_back(8);
  	for (auto i: vec) {
      	cout << i << " ";
    }
  	cout << endl;
}

int main() {
    vector<int> v = {1, 3, 7, 9};

    // Passing the vector to foo()
    foo(v);

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

1 3 7 9 8 
1 3 7 9 

Explanation: The vector inside the function got one more element while the original vector remained unchanged even after the function execution.

This method is only preferred if we don't want to modify the vector, and the vector size is small because making a copy of a large vector is expensive

Pass Vector by Reference

The pass by reference method passes the reference of the vector to the function. It does not create the copy of vector therefore, all the modifications done in the function will be reflected in the original array.

Syntax

In the function declaration,

rtype foo(vector<type>& para_name);

The & symbol indicates that the vector is passed by reference.

In the function call, we just pass the vector name.

foo(vec_name);

Example

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

// Pass by reference
void foo(vector<int> &vec) {

    // Modifying the vector
    vec.push_back(8);
    for (auto i : vec)
        cout << i << " ";
    cout << endl;
}

int main() {

    vector<int> v = {1, 3, 7, 9};

    // Pass the vector to foo()
    foo(v);

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

1 3 7 9 8 
1 3 7 9 8 

Explanation: The function modified the original vector as we can see in the output.

Constant Vector

If we want to pass the vector to a function without modifying it, we can pass it as a constant reference using const. This avoids unnecessary copying while ensuring the original vector remains unchanged.

Pass Vector by Pointer

The pass by pointer method is similar to the pass by reference method, but instead of a reference, we pass the memory address of the vector to the function.

Syntax

In the function declaration,

rtype foo(vector<type>* para_name);

The * symbol indicates that the pointer to the vector.

In the function call, we just pass address of the vector.

foo(vec_name);

Example

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

// Pass by pointer
void foo(vector<int> *ptr) {

    // Modify the original vector
    ptr->push_back(8);
  	for (int i = 0; i < ptr->size(); i++) {
      cout << ptr->at(i) << " ";
    }
  	cout << endl;
}

int main() {
    vector<int> v = {1, 3, 7, 9};

    // Passing address of the vector to foo()
    foo(&v);

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

1 3 7 9 8 
1 3 7 9 8 

Explanation: The function modified the original vector as we can see in the output.

This method is not recommended as it brings the errors and risks associated with pointers.

Pass by Value Vs Pass by Reference Vs Pass by Pointer

Following are the differences between the pass by value, pass by reference and pass by pointer method: