CODEWITHSUNDEEP
c++
Stefan Palnica
Stefan Palnica

Reputation: 3

Why can I use memcpy to copy one object variable to another

I have this class

class TestClass
{
public:
    int* tab;
};

int main()
{
    TestClass a;
    a.tab[0]=8;
    a.tab[1]=5;
    TestClass b;
    memcpy(b.tab,a.tab,sizeof(int)*2);
    cout << b.tab[0] << endl;
    return 0;
}

and I want to copy tab from a to b tab. But this code doesn't work. I tried to copy tab to dynamic array and it worked.

TestClass a;
a.tab[0]=8;
a.tab[1]=5;
int* b=new int[2];
memcpy(b,a.tab,sizeof(int)*2);
cout << b[0] << endl;

Can someone explain why first code doesn't work but second works?

Upvotes: 0

Views: 356

Answers (1)

Aykhan Hagverdili
Aykhan Hagverdili

Reputation: 29985

You're causing undefined behavior by trying to use the subscript operator with an uninitialized pointer. Instead of doing any of these, use std::vector or std::array, depending on what you are trying to achieve:

#include <iostream>
#include <vector>

class TestClass
{
public:
    std::vector<int> tab;
};

int main()
{
    TestClass a{ { 8, 5 } };
    TestClass b{ a };
    std::cout << b.tab[0] << '\n';
}

This keeps it simple and maintainable. STL containers have more friendly value semantics and they are in general easier to use, harder to abuse.

Upvotes: 2

Related Questions