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When is a Copy Constructor Called in C++?

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A copy constructor is a member function that initializes an object using another object of the same class. The Copy constructor is called mainly when a new object is created from an existing object, as a copy of the existing object. 

In C++, a Copy Constructor may be called for the following cases: 

1) When an object of the class is returned by value. 
2) When an object of the class is passed (to a function) by value as an argument. 
3) When an object is constructed based on another object of the same class. 
4) When the compiler generates a temporary object. 

Example:

C++

// CPP Program to demonstrate the use of copy constructor
#include <iostream>
#include <stdio.h>
using namespace std;
  
class storeVal {
public:
    // Constructor
    storeVal() {}
    // Copy Constructor
    storeVal(const storeVal& s)
    {
        cout << "Copy constructor has been called " << endl;
    }
};
  
// Driver code
int main()
{
    storeVal obj1;
    storeVal obj2 = obj1;
    getchar();
    return 0;
}

                    

Output
Copy constructor has been called 


It is, however, not guaranteed that a copy constructor will be called in all these cases, because the C++ Standard allows the compiler to optimize the copy away in certain cases, one example being the Return Value Optimization (sometimes referred to as RVO). 

Note: C++ compiler implicitly provides a copy constructor, if no copy constructor is defined in the class.


Last Updated : 29 Nov, 2021
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