c++ Using const in a copy constructor?

Question

I have never written copy constructor, so in order to avoid pain i wanted to know if what i have coded is legit. It compiles but i am not sure that it works as a copy constructor should.

Also do i have to use const in the copy constructor or i can simply drop it. (What i dont like about const is that the compiler cries if i use some non const functions).

//EditNode.h
class EditNode
{
      explicit EditNode(QString elementName);
      EditNode(const EditNode &src);
}

//EditNodeContainer.h
class EditNodeContainer : public EditNode
{
      explicit EditNodeContainer(QString elementName);
      EditNodeContainer(const EditNodeContainer &src);
}

//EditNodeContainer.cpp
EditNodeContainer::EditNodeContainer(QString elementName):EditNode(elementName)
{
}       

//This seems to compile but not sure if it works
EditNodeContainer::EditNodeContainer(const EditNodeContainer &src):EditNode(src)
{

}


//the idea whould be to do something like this
EditNodeContainer *container1 = new EditNodeContainer("c1");
EditNodeContainer *copyContainer = new EditNodeContainer(container1);

Your copy constructor doesn't do anything that the compiler-generated one doesn't do. — Oliver Charlesworth
– Oliver Charlesworth, Commented Mar 23, 2012 at 23:24
Is there some reason why you need to write a custom copy constructor? In many cases, the default one should be fine. — Philipp
– Philipp, Commented Mar 23, 2012 at 23:25
i.e. when you want to copy data pointed from a member and not the pointer itself — kappa
– kappa, Commented Mar 23, 2012 at 23:27

Luchian Grigore · Accepted Answer · 2012-03-23 23:30:39Z

A copy constructor is a constructor that has one of the following signatures:

class A
{
    A(A& other);
    //or
    A(const A& other);
    //or
    A(volatile A& other);
    //or
    A(const volatile A& other);
    //or any of the above + other parameters that have default arguments
    //example:
    A(const A& other, int x = 0) //this is also a copy constructor
};

The above is specified in 12.8.2 of the standard - C++03. so you are implementing correctly a copy constructor.

The reason it should receive a const parameter is that you're not changing the object you're copying from. If you call non-const functions on it, you're doing something wrong.

Also, in your snippet

EditNodeContainer *container1 = new EditNodeContainer("c1");
EditNodeContainer *copyContainer = new EditNodeContainer(container1);

you're not calling a copy constructor, because you're passing an EditNodeContainer* as parameter, not a EditNodeContainer.

Dialecticus · Accepted Answer · 2012-03-23 23:25:45Z

1

You're missing one * symbol. Copy constructor expects reference to object, but is given pointer to object. Just replace container1 with *container1 as parameter of copy constructor.

answered Mar 23, 2012 at 23:25

Dialecticus

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1 Comment

kappa Over a year ago

you are right, since container1 is a pointer so he need to de reference it

Philipp · Accepted Answer · 2012-03-23 23:28:57Z

0

The parameter of a copy constructor can be an lvalue reference to non-const or an lvalue reference to const, but in practice it is always a reference to const (the deprecated auto_ptr is an exception).

You should not write a copy constructor unless you have to and you fully understand the consequences. If you are consequent in using RAII classes everywhere, you rarely need a custom copy constructor (unless you are writing a RAII class).

Also, please avoid raw pointers and new wherever possible.

answered Mar 23, 2012 at 23:28

Philipp

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