Copy constructor not deleted on c++ class

Question

I am trying to create a class that never duplicates as it has a LOT of data inside of its fields. To do this I defined the class like this:

class Foo {
public:
   Foo(int i);
   Foo();
   Foo(const Foo&) = delete;
}

This causes a compiler error if I tried to do something like:

Foo a(2);
Foo b = a;

However for whatever reason this does not cover this scenario where data is copied.

Foo* array;
array = new Foo[10000];
Foo a(2);
array[1] = a;

Why is this?

`Foo a();` - compiler error resulting from here would have nothing to do with copy constructor - that does not define a `Foo` object, it declares a function. — , Sep 13 '18 at 17:47
In this example, `Foo` has no constructors at all. Declaring a constructor, even to delete it, prevents the default constructor from being generated. You would need to declare the default constructor. — François Andrieux, Sep 13 '18 at 17:48
**−1** Especially the last example is clearly **not real code**. Also voted to close as lacking a reproducible example. — Cheers and hth. - Alf, Sep 13 '18 at 17:49
Updated the question with a constructor and some small edits. Hopefully this is less psuedocodey for y'all. — J.Doe, Sep 13 '18 at 17:51
@J.Doe, unfortunately, you can't use `array = new Foo[10000];` unless you have a default constructor. — R Sahu, Sep 13 '18 at 17:52
Gotcha. Updated question again. I didnt feel the need to include more than necessary code for Foo but I can see the confusion it causes so added a default constructor. Believe it or not the class I am writing is not named Foo. — J.Doe, Sep 13 '18 at 17:53
Time to review the basics of constructors including definition & when they are called. — philipxy, Sep 13 '18 at 18:10

score 7 · Answer 1 · answered Sep 13 '18 at 17:48

7

First of all,

Foo a();

does not create an object of type Foo. It declares a function that takes no arguments and returns a Foo.

Assuming that you have an object of type Foo,

Foo b = a;

calls the copy constructor to initialize b. However, when you use

array[1] = a;

it does not use the copy constructor. It is an assignment operation. deleteing the copy constructor does not delete the copy assignment operator. Hence, the last line is not an error.

answered Sep 13 '18 at 17:48

R Sahu

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There is still the problem that `Foo` is not default constructible. – François Andrieux Sep 13 '18 at 17:49
Ah so assignment does not cause any new memory to be used? Simple two locations pointing at the same object? – J.Doe Sep 13 '18 at 17:52
@J.Doe, there are two different objects. The value of one object is assgined to the other object. – R Sahu Sep 13 '18 at 17:53
That makes sense! – J.Doe Sep 13 '18 at 17:54
@J.Doe Assignment won't involve the copy constructor, but rather the assignment operator overload. – πάντα ῥεῖ Sep 13 '18 at 17:54
That said, one of the simplest ways to implement the assignment operator uses the copy constructor. [See the Copy and Swap Idiom.](https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom) Note that while it is simple and bulletproof, Copy and Swap can be expensive. – user4581301 Sep 13 '18 at 18:08

Copy constructor not deleted on c++ class

1 Answers1