When is it best to define a copy constructor

Question

I'm new to C++ and I was asked a question that I really can't answer: when is the best time to overload or define your own copy constructor in C++?

The purpose of the copy-constructor is to create a copy of your object. If the default copy-constructor does not correctly create a copy then you should either disable it, or write your own one that does correctly create a copy. If you never need to copy your object, you can disable it. — M.M, Jul 16 '14 at 09:16
It is also necessary when you manage resources that require special handling when copying/sharing. Some background information: http://en.wikipedia.org/wiki/Rule_of_three_(C%2B%2B_programming) and http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization — Enigma, Jul 16 '14 at 09:17

Nayana Adassuriya · Answer 1 · 2014-07-16T10:11:47.407

If your Instance need deep copy, then your have to define the copy constructor. Copy constructor does only the shallow copy. As example if your class has a char pointer.

See this code example.

#include <iostream>
using namespace std;

class Student
{
public:
    char * m_name;
    int m_age;
    Student(char* name, int age){
        int name_len = strlen(name)+1;
        m_name= new char[name_len];
        strncpy(m_name, name, name_len);
        m_age = age;
    }
};

int main()
{
    char* firstStudent = "Kevin stefan";
    Student kevin(firstStudent,1);

    Student jhon = kevin;
    char* secondStudent = "Jhon Mac";
    strncpy(jhon.m_name,secondStudent,strlen(secondStudent));

    cout << kevin.m_name << endl;

    system("pause");
    return 0;
}

The result of this program prints Jhon Macefan. because both Student instance pointing to same memory location because of the shallow copy.

In this kind of situation you have to define a copy constructor for deep copy like this

Student::Student(const Student& stu)

    {
        int name_len = strlen(stu.m_name);    
        m_name = new char[name_len+1];
        strncpy(m_name, stu.m_name, name_len);

        m_age = stu.m_age;  
    }

I think this answer will only end up confusing people, even though your example is valid. As a counterexample, if the instance contains a `vector`, then I (and I suspect, most) would call what the default copy constructor does a deep copy. — , Jul 16 '14 at 09:18
C++ does not really have "deep copy" and "shallow copy". Either the copy does what you want or it doesn't. So-called "shallow copy" could be correct if the resource being pointed to is not meant to be deleted by the class. — M.M, Jul 16 '14 at 09:19
@MattMcNabb That's one thing that this answer does cover correctly: "If your Instance need deep copy" -- it already acknowledges that there are classes that don't need it. — , Jul 16 '14 at 09:21

When is it best to define a copy constructor

1 Answers1