CODEWITHSUNDEEP
c++deep-copyshallow-copy
rx432
rx432

Reputation: 93

Copy constructor c++ weird behavoir?

Hi I have a class which includes an array, I'm not passing this through my class constuctor (could be going wrong here?) The array is just defined in the class then initialized in a read method.

But in the main I make an instance of the class like:

myClass myClass1;

Then do:

myClass1 = myClass2;

Now I use a bubble sort method on the first class instance like:

MyClass1.sort();

Now when I write both of these instances out to text after the sort they are both sorted?

I have had a read into shallow and deep copys, but I can't understand why a method called after the shallow copy, would result in both of them being sorted? Is this expected behavoir?

a little more information:

In my class I just define the array as:

static string myArray[5];

then in a write method I write the 5 elements to it, is there a better way of doing this, I have to use a method to intialize it though.

Upvotes: 2

Views: 153

Answers (2)

Mario
Mario

Reputation: 36487

How is your array defined/created? If you create it with new and reference it with a pointer, you'll need a custom copy constructor creating a deep copy (i.e. creating a new instance of the array and copying its contents). The default copy constructor will only do a shallow copy, so both instances would use the same memory (i.e. both pointers' point to the same array on the heap).

This will work with the default copy constructor:

class myclass {
    char text[256];
}

This won't work with the default copy constructor (at least not as expected):

class myclass {
    char *text;
    myclass() : text(new char[256]) {}
    ~myclass() { delete [] text; }
}

A member defined as static will never be copied. All instances share it without further code required:

class myclass {
    static char text[256];
}

A copy constructor for the second case could look like this (note this is simplified and not checking for valid values and such):

myclass(const myclass &base) : text(new char[256]) { strcpy(text, base.text); }

General rule of thumb: Do I use any pointer member that is assigned the return value of some new? If so, write a copy constructor. If not, hen use he default one (unless you've got other reasons).

Upvotes: 3

R. Martinho Fernandes
R. Martinho Fernandes

Reputation: 234474

In my class I just define the array as:

static string myArray[5];

Should I declare it in the header, as its only declared in the cpp file? As its still copying both.

If you declare that outside of a class definition it will be a global variable. There will only be one instance of it. You need to declare it inside the class, and not static:

class MyClass1 {
    // other things...
    string myArray[5];
};

In any case, this kind of basic question is better answered by consulting a good C++ introductory book.

Upvotes: 2

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