How do I explicitly call the destructor of std iterators?

Question

Given std::vector<A>::iterator and std::map<A, B>::iterator, how do I explicitly call their destructors? I do not know the actual names of these types, because the ::iterator member types are typedefs/aliases to implementation-specific classes.

I ask this question, because I am storing these iterators in an unrestricted c++ union and Visual Studio asks me to manually handle destruction. I could simply not call the destructor of the active element and assume iterators do not need clean-up, but that reeks of bad practice.

This smells like you are doing it wrong. But I don't know what you're doing, so I can't help you do it another, perhaps better way. — rubenvb, Mar 07 '17 at 11:49

eerorika · Accepted Answer · 2017-03-07T12:00:51.540

4

Like this:

// let iter be vector<int>::iterator to be destroyed
iter.std::vector<int>::iterator::~iterator();

I do not know the actual names of these types, because the ::iterator member types are typedefs/aliases to implementation-specific classes.

Doesn't matter. Type aliases are type names as well.

LLVM appears to have a bug with this: https://bugs.llvm.org//show_bug.cgi?id=12350

Until it is fixed, as a workaround, introduce a non-nested alias:

using iterator = std::vector<int>::iterator;
iterator it;
it.iterator::~iterator();

or refer to the type as template argument (this code is from the bug report):

template <typename T>
inline void placement_delete(T *x) 
{
   // C++ lacks placement delete, so fake it with a function
   x->~T();
}

edited Mar 07 '17 at 12:00

answered Mar 07 '17 at 11:28

eerorika

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@RichardCritten appears to not be a problem according to standard: `The notation for explicit call of a destructor can be used for any scalar type name` – eerorika Mar 07 '17 at 11:34
@StijnFrishert I think this is a clang bug. It appears to affect qualified nested type names - not type aliases. You can work around it with non-nested alias: https://godbolt.org/g/0TsBi7 – eerorika Mar 07 '17 at 11:41
@user2079303 thanks it does, going to ask a follow up question as it looks really strange. ... found a duplicate: http://stackoverflow.com/questions/24000710/pseudo-destructor-call-does-not-destroy-an-object – Richard Critten Mar 07 '17 at 11:43
@StijnFrishert I updated the answer regarding the bug. – eerorika Mar 07 '17 at 11:47
@user2079303 Accepted this answer, because it follows the standard and the clang bug doesn't change that. Was about to file a report, good you found one already. – Stijn Frishert Mar 07 '17 at 11:51

score 3 · Answer 2 · answered Mar 07 '17 at 11:44

3

std::destroy_at(&iter);

std::destroy and friends are a C+17 feature. If your compiler lacks support it is trivial to implement yourself:

template <typename T>
void destroy_at(T* p) {
    p->~T();
}

Source.

answered Mar 07 '17 at 11:44

n. m. could be an AI

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Very nice! Good to know this exists. So looking forward to widespread c++17 support! – Stijn Frishert Mar 07 '17 at 11:51

score 0 · Answer 3 · answered Mar 07 '17 at 11:27

0

Most often iterator are nothing but pointer to already allocated memory. Unless the iterators are created using new

 typedef std::vector<A>::iterator VecIter;
 VecIter* iter= new VecIter;

there is no need to delete. If you are using new, then you can call delete explicitly for the iterator variable.

delete iter;

answered Mar 07 '17 at 11:27

Mrnell

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4

I am not talking about calling delete on an iterator, but explicitly calling its destructor. https://msdn.microsoft.com/en-us/library/6t4fe76c.aspx#Anchor_3 – Stijn Frishert Mar 07 '17 at 11:32

How do I explicitly call the destructor of std iterators?

3 Answers3