Under what circumstances must I provide, assignment operator, copy constructor and destructor for my C++ class?

Question

Say I've got a class where the sole data member is something like std::string or std::vector. Do I need to provide a Copy Constructor, Destructor and Assignment Operator?

Answer 1

In case your class contains only vector/string objects as its data members, you don't need to implement these. The C++ STL classes (like vector, string) have their own copy ctor, overloaded assignment operator and destructor.

But in case if your class allocates memory dynamically in the constructor then a naive shallow copy will lead to trouble. In that case you'll have to implement copy ctor, overloaded assignment operator and destructor.

Answer 2

The usual rule of thumb says: if you need one of them, then you need them all.

Not all classes need them, though. If you class holds no resources (memory, most notably), you'll be fine without them. For example, a class with a single string or vector constituent doesn't really need them - unless you need some special copying behavior (the default will just copy over the members).

Answer 3

The default copy constructor will copy the vector if it is declared by value. Beware if you stored pointers in your vector, in such a case, you need to provide specific behaviour for copy/assignement/destruction to avoid memory leaks or multiple delete.

Answer 4

I can think of a few cases when you need to write your own Big Three. All standard containers know how to copy and destroy themselves, so you don't necessarily need to write them. Here's how to know when you do:

Does my class own any resources?

The default copy semantics for pointers is to copy the value of the pointer, not what it points to. If you need to deep copy something, even if it's stored inside a standard container, you need to write your own copy constructor and assignment operator. You also need to write your own destructor to properly free those resources.

Might someone inherit from my class?

Base classes need a destructor. Herb Sutter recommends making them either public and virtual (most common case) or protected and non-virtual, depending on what you want to do with them. The compiler-generated destructor is public and non-virtual, so you'll have to write your own, even if it doesn't have any code in it. (Note: this doesn't imply you have to write a copy constructor or assignment operator.)

Should I prevent a user from copying objects of my class?

If you don't want the user to copy your objects (maybe that's too expensive), you need to declare the copy constructor and assignment operators either protected or private. You don't have to implement them unless you need them. (Note: this doesn't imply you have to write a destructor.)

Bottom line:

The most important thing is to understand what the compiler-generated copy constructor, assignment operator, and destructor will do. You don't need to be afraid of them, but you need to think about them and decide if their behavior is appropriate for your class.

Answer 5

No but there are a number of reasons why you shouldn't allow the compiler to auto generate these functions.

In my experience it is always best to define them yourself, and to get into the habit of making sure that they are maintained when you change the class. Firstly you may well want to put a breakpoint on when a particular ctor or dtor is called. Also not defining them can result in code bloat as the compiler will generate inline calls to member ctor and dtor (Scott Meyers has a section on this).

Also you sometimes want to disallow the default copy ctors and assignments. For example I have an application that stores and manipulates very large blocks of data. We routinely have the equivalent of an STL vector holding millions of 3D points and it would be a disaster if we allowed those containers to be copy constructed. So the ctor and assignment operators are declared private and not defined. That way if anyone writes

class myClass {
  void doSomething(const bigDataContainer data); // not should be passed by reference
}

then they'll get a compiler error. Our experience is that an explicit become() or clone() method is far less error prone.

So all in all there are many reason to avoid auto generated compiler functions.

Answer 6

those container will need a "copy constructible" element, and if you don't supply the copy constructor, it will call default copy constructor of your class by deducing from your class members (shallow copy).

easy explanation about default copy constructor is here : http://www.fredosaurus.com/notes-cpp/oop-condestructors/copyconstructors.html

it is so with destructor, the container need to have access to your destructor or your default class destructor if you don't provides one (ie. it will not work if you declare your destructor as private )

Answer 7

you need to provide them if you need them. or possible users of your classes. destructor is always a must, and copy constructors and assignment operator are automatically created by compiler. (MSVC at least)

Answer 8

When ever you have a class that requires deep copies, you should define them.

Specifically, any class which contains pointers or references should contain them such as:

class foo {
private:
    int a,b;
    bar *c;
}

Subjectively, I would say always define them, as the default behavior provided by the compiler generated version may not be what you expect / want.

Answer 9

Not for strings or vectors, since the trivial constructors / destructors etc, will do fine.

If your class has pointers to other data and need deep copies, or if your class holds a resource that has to be deallocated or has to be copied in a special way.