CODEWITHSUNDEEP
c++objectconstructorcopyimplicit
jaayy
jaayy

Reputation: 11

Weird behavior of default copy constructor in C++

I've encountered a weird behavior during learning about implicit copy constructors in C++. This is about this bunch of code:

#include <iostream>

class Person
{
public:
    char* name;
    int age;

    Person( const char* the_name, int the_age )
    {
        name = new char[strlen( the_name ) + 1];
        strcpy( name, the_name );
        age = the_age;
    }

    ~Person()
    {
        delete[] name;
    }
};

int main(){
    Person p1( "Tom", 22 );

    Person p2( p1 );
    p2.name = "Bill";

    // the output is: "Tom"
    std::cout << p1.name << std::endl;

    std::cin.get();
}

By default, copying an object means copying its members, therefore while creating p2 object using default copy constructor it should copy merely a pointer name, not the character array it points to. Therefore changing name member of object p2 should change the name member of object p1. Thereupon p1.name should be "Bill" not "Tom".

What's wrong in my reasoning? Why it's printing "Tom"?

Upvotes: 0

Views: 442

Answers (1)

Rakete1111
Rakete1111

Reputation: 48938

By default, copying an object means copying its members

Yes.

therefore while creating p2 object using default copy constructor it should copy merely a pointer name, not the character array it points to.

Correct.

Therefore changing name member of object p2 should change the name member of object p1.

Nope. Yes, p2.name and p1.name point to the same memory location, but that doesn't mean that changing the pointer value of p1.name will change the value of p2.name. If that were ints, would you be surprised if changing p1.name has no effect p2.name? Because it's the same thing here. You have two different variables, and changing one's value doesn't change the value of the other.

int a = 0;
int b = 1;

int* ptrA = &a;
int *ptrA2 = &a;

*ptrA = 1; // now a == 1
*ptrA2 = 4; // now a == 4

ptrA2 = &b;

*ptrA2 = 10; // now b == 10
*ptrA = 3; // now a == 3, a != 10

Upvotes: 2

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