Why delete char array cause crash after assign a value by operator=?

Question

This is my code. I create a char array and assign a string literal to it by operator=. After which, I free it by delete.However, it cause segmentation. But it works fine with strcpy. Besides, Is char array always assigned by strcpy?

I got to this problem from Implementaion of STL std::string, I wonder why private char* _data is always assigned by strcpy, can't it assign by operator=? since char* A= "HELLO WORLD" works as well.

int main()
{
    char* a=new char[3];
    a="12";
    //strcpy(a,"aa");
    delete[] a;

}

Answer 1

You can only delete[] something that you allocate with new[].

So this code is OK, because a points to memory you allocated with new[]

int main()
{
    char* a = new char[3];
    delete[] a;
}

This code is also OK, because a is still pointing at memory allocated with new[]

int main()
{
    char* a=new char[3];
    strcpy(a, "aa"); // this doesn't change a
    delete[] a;
}

But this code is different. In this code a starts pointing at memory allocated with new[] but then you change the pointer. You make a point at "aa". That is not memory allocated with new[] and so you get a crash.

int main()
{
    char* a=new char[3];
    a = "aa"; // this does change a
    delete[] a;
}

The difference between the second and third version, is that the second version changes the characters that a is pointing at, but the third version changes the pointer a itself. That's the crucial difference.

When you work with pointers you must understand the difference between changing the pointer and changing whatever the pointer is pointing at. These are two different things.