Passing a vector to constructor in C++

When class member is a vector object (not a reference).
We can simply assign in constructor. 

CPP

// Passing vector object to a constructor.
#include <iostream>
#include <vector>
using namespace std;

class MyClass {
    vector<int> vec;

public:
    MyClass(vector<int> v)
    {
       vec = v;
    }
    void print()
    {
        /// print the value of vector
        for (int i = 0; i < vec.size(); i++)
            cout << vec[i] << " ";
    }
};

int main()
{
    vector<int> vec;
    for (int i = 1; i <= 5; i++)
        vec.push_back(i);
    MyClass obj(vec);
    obj.print();
    return 0;
}

Output:

1 2 3 4 5

Time complexity : O(n)

Space complexity : O(n)

We can also initialize using the initializer list.

CPP

// Initializing vector object using initializer
// list.
#include <iostream>
#include <vector>
using namespace std;

class MyClass {
    vector<int> vec;

public:
    MyClass(vector<int> v) : vec(v)
    {
    }
    void print()
    {
        /// print the value of vector
        for (int i = 0; i < vec.size(); i++)
            cout << vec[i] << " ";
    }
};

int main()
{
    vector<int> vec;
    for (int i = 1; i <= 5; i++)
        vec.push_back(i);
    MyClass obj(vec);
    obj.print();
    return 0;
}

Output:

1 2 3 4 5

Time complexity : O(n)

Space complexity : O(n)

When class member is a vector a reference. 
In C++, references must be initialized using initializer list.

CPP

// CPP program to initialize a vector reference.
#include <iostream>
#include <vector>
using namespace std;

class MyClass {
    vector<int>& vec;

public:
    // this is the right way to assign
    // the reference of stl container
    MyClass(vector<int>& arr)
        : vec(arr)
    {
    }
    void print()
    {
        /// print the value of vector
        for (int i = 0; i < vec.size(); i++)
            cout << vec[i] << " ";
    }
};

int main()
{
    vector<int> vec;
    for (int i = 1; i <= 5; i++)
        vec.push_back(i);
    MyClass obj(vec);
    obj.print();
    return 0;
}

Output:

1 2 3 4 5

Time complexity : O(n)

Space complexity : O(n)