Copy constructor c++ weird behavoir?

Question

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.

*Are you sure there is an array involved*? Are you not [confusing arrays with pointers](http://stackoverflow.com/a/1641963/46642)? It would help if you could post a small example that demonstrates this behaviour. — R. Martinho Fernandes, Mar 17 '12 at 10:43
give me more code. How do I know you didn't mess up somewhere else? — ApprenticeHacker, Mar 17 '12 at 10:45
I'll update my post with a little more code but I dont want to post too much as its for an assignment I just want to get my head around why this is happening! Although I know that means I have to post more code! — rx432, Mar 17 '12 at 10:50
If you declare something as `static`, it is shared between *all* instances of the class. — Philipp, Mar 17 '12 at 11:08
“is there a better way of doing this”—yes, use `std::vector` or `std::array` instead of a C array. The class templates in the standard library provide more functionality and are more type safe (e.g., they don't decay to pointers implicitly). — Philipp, Mar 17 '12 at 11:10

Mario · Answer 1 · 2012-03-17T11:10:46.020

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).

A static member is shared between all instances of the class. If you want each instance to have its own copy, remove the `static` keyword. — Mario, Mar 17 '12 at 11:04

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

2

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.

edited Jun 20 '20 at 09:12

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answered Mar 17 '12 at 11:04

R. Martinho Fernandes

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Should I declare it in the header, as its only declared in the cpp file? As its still copying both. – rx432 Mar 17 '12 at 11:06
What? You should declare it *inside* the class! – R. Martinho Fernandes Mar 17 '12 at 11:07

Copy constructor c++ weird behavoir?

2 Answers2