Calling assignment operator in copy constructor

Question

Are there some drawbacks of such implementation of copy-constructor?

Foo::Foo(const Foo& i_foo)
{
   *this = i_foo;
}

As I remember, it was recommend in some book to call copy constructor from assignment operator and use well-known swap trick, but I don't remember, why...

Answer 1

Yes, that's a bad idea. All member variables of user-defined types will be initialized first, and then immediately overwritten.

That swap trick is this:

Foo& operator=(Foo rhs) // note the copying
{
   rhs.swap(*this); //swap our internals with the copy of rhs
   return *this;
} // rhs, now containing our old internals, will be deleted 

Answer 2

There are both potential drawbacks and potential gains from calling operator=() in your constructor.

Drawbacks:

• Your constructor will initialize all the member variables whether you specify values or not, and then operator= will initialize them again. This increases execution complexity. You will need to make smart decisions about when this will create unacceptable behavior in your code.

• Your constructor and operator= become tightly coupled. Everything you need to do when instantiating your object will also be done when copying your object. Again, you have to be smart about determining if this is a problem.

Gains:

• The codebase becomes less complex and easier to maintain. Once again, be smart about evaluating this gain. If you have a struct with 2 string members, it's probably not worth it. On the other hand if you have a class with 50 data members (you probably shouldn't but that's a story for another post) or data members that have a complex relationship to one another, there could be a lot of benefit by having just one init function instead of two or more.

Answer 3

You're looking for Scott Meyers' Effective C++ Item 12: Copy all parts of an object.