How do I *not* delete a member in a destructor?

Question

I'd like the destructor of my class to delete the entire object except for one of the members, which is deleted elsewhere. First of all, is this totally unreasonable? Assuming it's not, how do I do this? I thought that created an destructor with an empty body would prevent all the members from being deleted (because the destructor wouldn't do anything), but that doesn't seem to be the case.

Answer 1

Short answer: You don't.

Longer answer: If the "member" is actually a pointer to some other allocation, you can arrange to not delete the other allocation.

But usually, if you allocated the other block in the constructor, you want to delete it in the destructor. Anything else will require careful handling of the "ownership" of the block in question. It will be a lot like memory management in plain c. Possible, but fraught with danger.

Good luck.

Answer 2

Depends on what you mean by "deleted". If they aren't in a smart pointer, and aren't explicitly deleted, then they aren't deleted. Members that are just part of the class:

class Bar {
//...
private: 
  Foo foo;
};

Aren't deleted by the destructor (because they weren't dynamically allocated), they are just destroyed. They "live" inside the class, so once it is destroyed, it's gone.

If you are looking the share "ownership" between two locations, what you want is a dynamically allocated shared_ptr:

#include <memory>
class Bar {
// ...
private:
  std::tr1::shared_ptr<Foo> foo;
};

Answer 3

I think that in most cases you're asking for trouble if you don't destruct the entire object in the same action. It sounds like your class should have a clean up method for that member, which is called within the destructor. If for some reason the member has to be destroyed sooner, the method can return early.

Answer 4

If the member is contained by value (not by pointer or by reference) then you can't prevent it from being deleted and you shouldn't want to.

If you want to delete it elsewhere instead, then make it contained by pointer or by reference.

class House
{
  Door door; //contained by value, will be destroyed when the House is
}

class House
{
  Door& door; //contained by reference, will not be destroyed when the House is
}

Answer 5

First of all, is this totally unreasonable?

I wouldn't say unreasonable, perhaps questionable.

It's perfectly valid for one class to own and therefore should take care of clean up, while at the same time having a reference or a pointer to that object in another class.

However, it might be questionable if the second class reall should have that pointer or not, I'd prefer to always use a get-method to retrieve that pointer whenever I need it, e.g. by calling a parent class or some resource manager.

Answer 6

The code in the destructor is only to delete members that are dynamically allocated. The destruction of members is not optional, you can only control the deallocation of what you explicitly allocated before (with operator new).

What you want to do can be obtained using a shared_ptr, in which both your class and the external code share a pointer to the same external object. This way, only when all the pointers to that object go out of scope it will be deleted. But beware not to do circular references, shared_ptr has no "garbage collector" wisdom.

Of course you could use a regular pointer shared by those places, but this is in most cases a bad idea, prone to give you headaches about proper resource deallocation later.

Answer 7

If you have dynamically allocated memory for this member it is possible once you have shared the reference to this member before destroying the object and if you ensure the member is not destroyed in the object's destructor. However I think this practice isn't so reasonable.

Answer 8

How come no one mentioned weak and strong pointers?
A strong pointer is a smart pointer that acts normally.
A weak pointer is a smart pointer that cannot delete itself unless all of the strong pointers are out of scope.
Strong pointer indicates ownership, a weak pointer indicates sharing.
Look at boost.shared_ptr and boost.weak_ptr and Loki's StrongPtr for implementations.
Also take a look at RAII. If you knew RAII you would have known the answer to this question yourself.

Answer 9

First of all, if the member object is contained by value, it simply goes out of scope when the container object is destroyed, and you cannot prevent it from being deallocated automatically.

If, instead, it is indirectly referenced by your container object (for example with a pointer), you don't have to do anything in particular to not delete it. The destructor doesn't delete anything unless you explicitly write the code to do so.

As for the question whether this is unreasonable, I think it is not, in general, but you have to make clear (usually in the documentation, since C++ has no language support for this concept) what is the object that owns the member in question.

Answer 10

It is not unreasonable, but care should be taken to ensure that cleanup of any managed resources is handled implicitly.

(The first managed resource that people generally worry about is memory, but anything that can leak - memory, file handles, IDispatch pointers - should have code which handles the cleanup implicitly).

For managed resources shared by multiple objects (almost certainly the case if "this object" is supposed to have a pointer to something that gets cleaned up by "that object"), you are normally needing either a "reference counted pointer" to manage the object or a "weak pointer", depending on your lifetime requirements.

For managed resources which are not shared (and in particular those that need to be managed properly when exceptions can be thrown), then an auto_ptr or other variant may be more suitable.

The Scott Meyers Effective C++ books were a reasonable starting point for learning about smart pointers, but in practice you should probably just grab a vetted library like Boost and let somebody else worry about getting the obscure corner cases (like what happens if a constructor throws an exception?) right.