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C++ how to avoid calling destructor


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#1 Aroidzap   Members   -  Reputation: 664

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Posted 29 January 2013 - 12:43 PM

Hi, I've got code like this :

 

 

std::vector<some_class> array;

void pushback_function()
{
    some_class temp;
    array.push_back(temp);
}

 

Is it somehow possible to avoid calling destructor of temp



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#2 Álvaro   Crossbones+   -  Reputation: 13912

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Posted 29 January 2013 - 12:47 PM

Not really. Why don't you want the destructor called?

If you are trying to save the copy, you can look into C++11's move semantics.

#3 frob   Moderators   -  Reputation: 22732

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Posted 29 January 2013 - 12:47 PM

Every object that is created needs to be destroyed.  There is a very tiny exception to the rule and that is if you are basically writing your own memory manager.  But I doubt that is the case here.

 

 

 

What are you trying to accomplish?  Why don't you want your destructor to get called?


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#4 Aroidzap   Members   -  Reputation: 664

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Posted 29 January 2013 - 12:51 PM

ok... so... is it possible to push_back pointer to "temp" (create temp using some_class *temp = new some_class;) in array?

Edited by Aroidzap, 29 January 2013 - 12:52 PM.


#5 Karsten_   Members   -  Reputation: 1655

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Posted 29 January 2013 - 12:53 PM

Perhaps push the memory contained within an std::tr1::shared_ptr onto the vector but set it's deleter function to a function that does nothing.

array.push(std::tr1::shared_ptr<some_class>(new temp(), std::tr1::bind(std::tr1::placeholders::_1, nullFunc));

...

void nullFunc(some_class* unused)
{
  // do nothing...
}


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#6 Aroidzap   Members   -  Reputation: 664

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Posted 29 January 2013 - 01:01 PM

I think I will try to solve my problem another way... I don't want to make it messy. But thanks for help ;)



#7 Karsten_   Members   -  Reputation: 1655

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Posted 29 January 2013 - 01:07 PM

Whoever decreased my rep, I am hoping you did so for the nullFunc stuff rather than the std::tr1::shared_ptr because although that looks messy, it is pretty standard C++.

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#8 Álvaro   Crossbones+   -  Reputation: 13912

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Posted 29 January 2013 - 01:07 PM

There are many things you could do, like having a container of pointers, a container of smart pointers or a Boost pointer container. But unless we know what you are trying to do and why it is a problem that the destructor is being called, we can't really give you much advice.

#9 Álvaro   Crossbones+   -  Reputation: 13912

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Posted 29 January 2013 - 01:08 PM

Whoever decreased my rep, I am hoping you did so for the nullFunc stuff rather than the std::tr1::shared_ptr because although that looks messy, it is pretty standard C++.

It's probably because of the misspelled "its". smile.png

#10 swiftcoder   Senior Moderators   -  Reputation: 10367

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Posted 29 January 2013 - 01:10 PM

ok... so... is it possible to push_back pointer to "temp" (create temp using some_class *temp = new some_class;) in array?

Sure, you can create a std::vector<some_class *>, and then push pointers into it instead. Just remember to delete each one when you are actually done with it.

You can also use std::shared_ptr, or (from C++11 onwards) std::unique_ptr, to avoid having to remember to delete the objects in the end.

But I'm curious: how much work is this destructor doing, that you are so adamant to avoid calling it? In general, a copyable class should do very little work in its destructor (and if your class isn't meant to be copyable, well, then you need to take the pointer approach to start with).


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#11 rip-off   Moderators   -  Reputation: 8727

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Posted 29 January 2013 - 01:13 PM

You could emplace_back(), if your compiler(s) support it (C++11 once again).



#12 Eric Lengyel   Crossbones+   -  Reputation: 2492

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Posted 29 January 2013 - 01:44 PM

You could do this, but I don't recommend it:

void pushback_function()
{
    char storage[sizeof(some_class)];
    some_class *temp = new(storage) some_class;
    array.push_back(*temp);
}


#13 ApochPiQ   Moderators   -  Reputation: 16401

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Posted 29 January 2013 - 01:51 PM

You could do this, but I don't recommend it:

void pushback_function()
{
    char storage[sizeof(some_class)];
    some_class *temp = new(storage) some_class;
    array.push_back(*temp);
}


That won't suppress the some_class destructor being called when array is deallocated or otherwise shuffles elements.

#14 Ravyne   GDNet+   -  Reputation: 8160

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Posted 29 January 2013 - 02:11 PM

If your element is default constructed, you can just resize the vector to its size plus one. vector::resize() creates default constructed objects, you can them modify the member through accessing it from the vector if necessary.

In most cases, this will not cause a re-allocation of memory backing the vector, and in cases where it does it was unavoidable anyhow. However, if you will be constructing several of these objects at once, you can hit them all at once by resizing to the size of the array plus the required number of objects.

Also, be wary of any pointers you're holding, they become invalid if the memory backing the vector is re-allocated -- but again, this would be true of the standard solution. Just something to be aware of.

[Edit] -- don't you hate it when you read responses, and in your head that changes the nature of the question? Anyhow, if your ends are to eliminate the redundant initialization, copy of, and deconstruction of temp just to put it in vector, then what I suggested is useful. If your ends are different, well, you should probably explain or find different ends, because they're opaque to me.

Edited by Ravyne, 29 January 2013 - 02:24 PM.


#15 SiCrane   Moderators   -  Reputation: 9670

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Posted 29 January 2013 - 02:13 PM

That won't suppress the some_class destructor being called when array is deallocated or otherwise shuffles elements.

I'm not sure that's an issue in this case, as the OP only specified suppressing the destructor of the temp object. One problem with the locally allocated char array is that it may not have the correct alignment for some_class. A similar example from the C++98 standard had to be changed in the C++03 standard to use dynamic allocation of the char array so that alignment requirements were met.

Edited by SiCrane, 29 January 2013 - 02:14 PM.


#16 ApochPiQ   Moderators   -  Reputation: 16401

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Posted 29 January 2013 - 02:19 PM

Ah, I see.

It's still a bad idea of some_class does any allocations or controls any resources, obviously, as they will leak. I suspect if some_class has a nontrivial destructor this is likely to be the case.

#17 Cornstalks   Crossbones+   -  Reputation: 6991

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Posted 29 January 2013 - 02:20 PM

Perhaps push the memory contained within an std::tr1::shared_ptr onto the vector but set it's deleter function to a function that does nothing.

array.push(std::tr1::shared_ptr<some_class>(new temp(), std::tr1::bind(std::tr1::placeholders::_1, nullFunc));

...

void nullFunc(some_class* unused)
{
  // do nothing...
}


That's just... horrible (plus it doesn't even compile). Why would you even recommend that? I mean, what's the point of even using shared_ptr here? It accomplishes nothing. Might as well use a raw pointer and never call delete on it... You're leaking memory here.
 
I'd just recommend (properly) using smart pointers (that is, just a plain old std::shared_ptr<some_class> temp(new some_class())).

Edit: I actually like Ravyne's idea. You could do:
array.resize(array.size() + 1);
some_class& back = array.back(); // You only need to do this if you want to edit the last element

Edited by Cornstalks, 29 January 2013 - 02:23 PM.

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#18 Karsten_   Members   -  Reputation: 1655

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Posted 29 January 2013 - 02:51 PM

It should compile, I would be interested in knowing why it doesnt for you.
On g++ I need tr1/memory and tr1/functional headers included.

As for the OP, he is the one that didn't want to call the constructor. I didn't question it lol (ironically in fear of losing rep haha). The "some_class" might add itself to a cache pool on creation and so not want to be deleted once the std::vector goes out of scope (i.e how Irrlicht handles meshes etc...)

As for (properly) using smart pointers, once you get this to compile, it works really nicely for C libraries (with the appropriate deleter func added) without needing to wrap loads of stuff in C++ classes. Though the problem in this thread isnt perhaps the best use-case for it.

Edited by Karsten_, 29 January 2013 - 03:04 PM.

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#19 swiftcoder   Senior Moderators   -  Reputation: 10367

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Posted 29 January 2013 - 02:56 PM

As for (properly) using smart pointers, once you get this to compile, it works really nicely for C libraries without needing to wrap loads of stuff in C++ classes.

What about it "works nicely"? From where I'm standing, you just tried to spin guaranteed memory leaks as a "feature"...

 

The OP said he wanted to avoid calling the destructor on the temp object, not altogether.


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#20 Karsten_   Members   -  Reputation: 1655

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Posted 29 January 2013 - 03:03 PM

Yeah you read it whilst I was still fixing my post.<br /><br />Internet is too slow and these forums are a tad broken but I finally got my post submitted.<br /><br />I am off for a beer now lol.

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