C++ default destructor

c++ constructor destructor default-constructor
Simplicity picture Simplicity · Jan 29, 2011 · Viewed 74.4k times · Source

When I don't declare a constructor for example, the compiler will provide me with a default constructor that will have no arguments and no definition (body), and thus, will take no action.

If I now don't declare a destructor, the compiler will provide me with a default destructor with no defintion (body), and thus, I think no action.

So, if I'm finished with an object for example, wouldn't the default destructor reallocate (free) memory used by the object? If it doesn't, why are we getting it?

And, maybe the same question applies to the default constructor. If it doesn nothing, why is it created for us by default?

Thanks.

Answer

Sergei Tachenov picture Sergei Tachenov · Jan 29, 2011

It's wrong to say that a compiler-generated default constructor takes no action. It is equivalent to a user-defined constructor with an empty body and an empty initializer list, but that doesn't mean it takes no action. Here is what it does:

  1. It calls the base class'es default constructor.
  2. It initializes the vtable pointer, if the class is polymorphic.
  3. It calls the default constructors of all members that have them. If there is a member with some constructors, but without a default one, then it's a compile-time error.

And only if a class is not polymorphic, has no base class and has no members that require construction, then a compiler-generated default constructor does nothing. But even then a default constructor is sometimes necessary for the reasons explained in other answers.

The same goes for the destructor - it calls base class'es destructor and destructors of all members which have them, so it isn't true in general case that a compiler-generated destructor does nothing.

But memory allocation really has nothing to do with this. The memory is allocated before the constructor is called, and it is freed only after the last destructor has finished.

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