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double free or corruption (out) when using allocator.deallocate() (lemmy.world)

submitted 11 months ago by prettydarknwild@lemmy.world to c/cpp@programming.dev

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i was trying to create a deque, and when the function that resizes the array is excuted, it crashes with the error from the title, and when i delete the deallocate() it stops happening, what im doing wrong? code: https://pastebin.com/0yHHcLnj

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[–] prettydarknwild@lemmy.world 0 points 11 months ago* (last edited 11 months ago) (1 children)

so, that practice of calling .destroy() and then .deallocate() is redundant and error-prone

[–] ratatosk@feddit.de 0 points 11 months ago* (last edited 11 months ago) (1 children)

If you develop or debug a container, it is useful to have a special test class for the elements that covers potential container specific errors.

struct ContainerTester {
	static int instances;
	int counter{};
	int val;
	
	ContainerTester() : ContainerTester(0) {}
	ContainerTester(int val) : val(val)
	{
		++counter;
		++instances;
	}
	~ContainerTester() {
		--counter;
		--instances;
		if (counter &lt; 0) std::cout &lt;&lt; "multiple destructor calls on same element" &lt;&lt; std::endl;
		if (instances &lt; 0) std::cout &lt;&lt; "negative number of instances" &lt;&lt; std::endl;
	}
};

int ContainerTester::instances{};

std::ostream&amp; operator&lt;&lt;(std::ostream&amp; o, const ContainerTester&amp; c) {return o &lt;&lt; c.val;}

If I run your code with ContainerTester instead of int, i get:

negative number of instances
negative number of instances
negative number of instances
negative number of instances
negative number of instances
negative number of instances
multiple destructor calls on same element
negative number of instances
double free or corruption (out)
segmentation fault

So it's more obvious that very bad things do happen :)

Oh and note, that allocator::destroy is deprecated in C++17 and was removed with C++20.

[–] prettydarknwild@lemmy.world 0 points 11 months ago (1 children)

i used tthe tester class with my code removing the .deallocate(), and although it doesnt crash, it still runs the destructor multiple times on the same element, i think its because im just pushing i into the container, and because that constructor creates an implicit conversion between int and ContainerTester, it creates a temporary object that gets destroyed once it is pushed into the deque, am i right?

[–] ratatosk@feddit.de 0 points 11 months ago* (last edited 11 months ago) (1 children)

sorry that I let you down yesterday
as an excuse, I have fixed and commented your code. (remember to compile it with -std=c++20)

[–] prettydarknwild@lemmy.world 0 points 11 months ago (1 children)

dont worry, i have one more question, i should initialize variables with = or with {}?

[–] ratatosk@feddit.de 0 points 11 months ago* (last edited 11 months ago)

Direct initialization is recommended over copy initialization (=).

Okay, to be fair: it's some sort of holy war, the answer depends on who you ask.