allocator implementation

Hello everyone,


I have debugged into STL implementation for allocator in file xmemory (Visual Studio 2008). I found it has not data members, the same as its base class _Allocator_base. Is that correct?

http://msdn2.microsoft.com/en-us/library/6s8wyf7c.aspx

My questions is what is the purpose of copy? It always returns true. Here is the code,

you can see do nothing for copy constructor as well.

Code:

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allocator(const allocator<_Ty>&) _THROW0()
{
    // construct by copying (do nothing)
}

                // allocator TEMPLATE OPERATORS
template<class _Ty,
        class _Other> inline
        bool operator==(const allocator<_Ty>&, const allocator<_Other>&) _THROW0()
        {        // test for allocator equality (always true)
        return (true);
        }

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thanks in advance,
George

>I found it has not data members, the same as its base class _Allocator_base. Is that correct?
Yes.

>My questions is what is the purpose of copy?
What copy? There's no copy member function in the allocator class. Do you mean the assignment operator? That and the copy constructor are for the allocator objects, and because the default allocator doesn't maintain any internal state (it just uses new/delete), they don't need to do anything beyond the default behavior.

Why do you think this is wrong?

Sorry, Prelude. I do not think it is wrong. You reply is helpful and I agree. :-)


My question is, for example, the operator==, it always returns true, is it useful?

Quote:

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Originally Posted by Prelude

>I found it has not data members, the same as its base class _Allocator_base. Is that correct?
Yes.

>My questions is what is the purpose of copy?
What copy? There's no copy member function in the allocator class. Do you mean the assignment operator? That and the copy constructor are for the allocator objects, and because the default allocator doesn't maintain any internal state (it just uses new/delete), they don't need to do anything beyond the default behavior.

Why do you think this is wrong?

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regards,
George

Quote:

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Originally Posted by George2

My question is, for example, the operator==, it always returns true, is it useful?

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Since all standard allocators work the same way, it makes sense they compare equal.

If you create your own custom allocator class then you will (probably) not want it to test as equal to a standard allocator. So you would supply versions of operator==() that behave accordingly.

You would create your own custom allocator class if you needed to manage dynamic memory using a different means than operators new/delete. For example, if you needed to allocate memory in some memory pool that is shared between processes.

Thanks grumpy,


Question answered.

Quote:

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Originally Posted by grumpy

Since all standard allocators work the same way, it makes sense they compare equal.

If you create your own custom allocator class then you will (probably) not want it to test as equal to a standard allocator. So you would supply versions of operator==() that behave accordingly.

You would create your own custom allocator class if you needed to manage dynamic memory using a different means than operators new/delete. For example, if you needed to allocate memory in some memory pool that is shared between processes.

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regards,
George