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Manual construction and destruction

Jamin Grey · 2012-11-08T05:15:39

For the first time in 7 years of C++ programming, I think I need to use explicit constructor and destructor calls. I am finding myself needing (more out of convenience and code cleanliness than necessity) a specific type of container class that isn't already a part of the standard library. Like vector, since my container is resizable, I want to have more memory available than is actually currently being used. std::vector has the current 'size' and also the 'capacity' to grow without reallocations. When vector has the memory reserved, the reserved memory isn't automatically constructed as that could have undesired side-effects, or simply just not be possible if the templated type didn't have a default constructor. So I basically have everything already written out, except I'd like some clarity about explicitly calling the constructors and destructors. Question 1: For allocating memory, without calling the constructors in the memory, which of these are considered better practice/more-common: unsigned char *memory = new unsigned char[num * sizeof(MyType)]; void *memory = ::operator new(num * sizeof(MyType)); Is this the time and place to use malloc() and free()? Should I store the pointer as void* or unsigned char*? Why unsigned char* over char*? If I access a void* using the subscript operator[], will [n] access (n * sizeof(unsigned char)) or (n * sizeof(void*)) into that block of memory? (I would assume sizeof(void*), but if void* is used for raw data access, I'd suspect people would want to increment a byte at a time) Question 2: Is this the correct way to manually call the constructor on raw memory? new (&memory[atByte]) MyType(constructorArguments); Is placement-new the only way? Is this the correct way to destruct the object? memory[atByte].~MyType(); And I can turn around and reconstruct it (with placement new again) to reuse the memory location? Question 3: How am I supposed to free the raw memory? Just delete[]? But I have to run through it and destructor all the objects that haven't been destructed yet, right? But I can't destruct any objects that haven't been constructed yet, right? [hr] Any other tips or traps to watch out for?

General and Gameplay Programming Programming

Started by Servant of the Lord November 01, 2012 03:26 AM

35 comments, last by Servant of the Lord 11 years, 5 months ago

Servant of the Lord

33,739

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November 06, 2012 06:39 AM

I think this covers it - any exception should roll back the Resize(), properly deconstructing any elements constructed.

[the entire code]
[just the relevant parts]

Did I make a mistake anywhere?

The code seems less straight-forward than before, but I guess that's what I get when I mess with manual allocation and deallocation.

SiCrane

11,840

November 06, 2012 01:57 PM

Your moveElements() function doesn't look right. When an exception occurs in the constructors and destroying elements, only the last row show go up to lastColumn, all the previous rows should go to columns. The way you've coded it looks like it misses destroying elements at the end of previous rows.

Servant of the Lord

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Author

November 06, 2012 04:22 PM

Great catch - fixed.

SiCrane

11,840

November 06, 2012 04:36 PM

Also if reallocateMemory() is called with an empty newCapacity, it frees memory without destroying objects first.

Servant of the Lord

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November 07, 2012 05:17 PM

Excellent, fixed that, and simplified the for-loops.
[latest version] ([size=2]If anyone wants to use the code, for whatever reason, it's public domain - slap your own name on it and use it in your code)

Thank you for all the help, everyone!
Any other mistakes in it I'll find through practical usage in my code.

Here's some example usage code.

SiCrane

11,840

November 07, 2012 06:43 PM

If that's your latest code then you also seem to have a rule of three violation: I don't see either a copy constructor or an assignment operator. And since you're using C++11's rvalue references you might also want to add a move constructor and move assignment operator. A nothrow swap function might also be a good idea.

Servant of the Lord

33,739

Author

November 08, 2012 05:15 AM

Thanks, I usually don't manually handle memory, so I often forget the rule of three.

Here's how I'm implementing it:
[relevant parts] [entire code]

Doing a few tests, I counted the number of destructions, constructions, moves, and copies, and the numbers balance out.

Manual construction and destruction

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