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Dynamic class array: memory leak


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#1 Retsu90   Members   -  Reputation: 208

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Posted 20 February 2012 - 06:41 PM

Hi!
I'm trying to create an array of classed dynamically, where constructs and destructs are called automatically. This code that I wrote test this thing, but I get memory leaks. How it's possible? I'm deleting every class that I created previously and I delete also the array that holds the classes pointers. In C# it works without problems.
There is the code:

int count;
class Test
{
public:
    int var;
    Test()
    {
	    var = count++;
	    printf("Test() %i\n", var);
    }
    ~Test()
    {
	    printf("~Test() %i\n", var);
    }
};
int main()
{
    count = 0;
    Test *test;
    while(1)
    {
	    test = new Test[10];
	    for(int i=0; i<10; i++)
		    delete &test[i];
	    delete test;
    }
}


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#2 TheUnbeliever   Members   -  Reputation: 961

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Posted 20 February 2012 - 07:00 PM

You make a single allocation, using new[] (not plain new). You want to make a single use of delete[] - not several of delete.

T* single = new T;
T* array = new T[10];

delete single;
delete[] array;

[TheUnbeliever]

#3 Retsu90   Members   -  Reputation: 208

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Posted 20 February 2012 - 07:17 PM

You make a single allocation, using new[] (not plain new). You want to make a single use of delete[] - not several of delete.

T* single = new T;
T* array = new T[10];

delete single;
delete[] array;


delete[] ? It was so easy? Now my code works without memory leaks. Thank you so much for your help and sorry for the n00b question.

#4 e‍dd   Members   -  Reputation: 2105

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Posted 22 February 2012 - 01:54 PM

Someone has to say it: You 'should' be using std::vector<> in C++ for dynamic arrays.

Here's your code converted to do so:

#include <vector>


int count = 0; // initialising absolutely everything is a good habit to get in to, IMO.
class Test
{
public:
    int var;
    Test()
    {
            var = count++;
            printf("Test() %i\n", var);
    }
    ~Test()
    {
            printf("~Test() %i\n", var);
    }
};

int main()
{
    while(1) { std::vector<Test> tests(10); }
}

It should also be noted that the use of 'delete' inside your loop ("delete &test[i]") is very misguided. Make sure you really understand why that's bad.

#5 Washu   Senior Moderators   -  Reputation: 4850

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Posted 22 February 2012 - 03:02 PM

Someone has to say it: You 'should' be using std::vector<> in C++ for dynamic arrays.

Here's your code converted to do so:

#include <vector>


int count = 0; // initialising absolutely everything is a good habit to get in to, IMO.
class Test
{
public:
	int var;
	Test()
	{
			var = count++;
			printf("Test() %i\n", var);
	}
	~Test()
	{
			printf("~Test() %i\n", var);
	}
};

int main()
{
	while(1) { std::vector<Test> tests(10); }
}

It should also be noted that the use of 'delete' inside your loop ("delete &test[i]") is very misguided. Make sure you really understand why that's bad.

*Cough*
int main()
{
    std::vector<Test> tests(10);
}

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#6 e‍dd   Members   -  Reputation: 2105

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Posted 22 February 2012 - 05:22 PM

*Cough*

int main()
{
	std::vector<Test> tests(10);
}

That's not a faithful representation of the O.P's code(?). Though I agree it's far more sensible as a C++-behaviour-discovery mechanism :)

#7 slicer4ever   Crossbones+   -  Reputation: 3453

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Posted 22 February 2012 - 11:23 PM

Someone has to say it: You 'should' be using std::vector<> in C++ for dynamic arrays.

i have to disagree with you in this regard, simply because c++ provides the vector container's for array management, in no way means that you should force yourself to use them.

for what he is demonstrating, understanding the underlying idea's of data management in this regard is much more important, than relying on optional systems to do it for you.
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#8 kunos   Crossbones+   -  Reputation: 2205

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Posted 23 February 2012 - 03:02 AM

simply because c++ provides the vector container's for array management, in no way means that you should force yourself to use them.


Are you suggesting he should rewrite the entire Standard Library? Or that he should use C-like reallocs to do dynamic structures?

The Standard Library IS part of the language, you should come up with a quite big reason not to use it.

As for the OP.. C# != C++ .. it's not the same language with a couple of syntax differences.. it's a different language with different rules. You simply don't "new" things inside a function like that, it's quite slow and messy and, most of the time, useless (ie. there is a better way to do it). And actually, in modern C++ you aren't supposed to use delete at all, ever.
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#9 slicer4ever   Crossbones+   -  Reputation: 3453

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Posted 23 February 2012 - 03:22 AM


simply because c++ provides the vector container's for array management, in no way means that you should force yourself to use them.


Are you suggesting he should rewrite the entire Standard Library? Or that he should use C-like reallocs to do dynamic structures?

The Standard Library IS part of the language, you should come up with a quite big reason not to use it.


Not at all, i never made this claim. All I'm saying is that the standard library isn't required for every situation. and particularly for this situation, it appeared that he was simply trying to learn basic constructor/destructor workings, and basic memory management. telling him instead to use standard library methods to do all his memory management is just creating a programmer whom doesn't understand the basics in terms of memory management.

in modern C++ you aren't supposed to use delete at all, ever.


I have never heard this before.
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#10 kauna   Crossbones+   -  Reputation: 2336

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Posted 23 February 2012 - 05:42 AM

Nobody wrote this down so just to clear up things:

you should use delete with new and delete[] with new[], otherwise you'll end up in some undefined behaviour zone.

Otherwise std::vector and other standard library templates will make your life easier in many cases.

Cheers!

#11 TheUnbeliever   Members   -  Reputation: 961

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Posted 23 February 2012 - 08:26 AM

in modern C++ you aren't supposed to use delete at all, ever.


I have never heard this before.


Obviously 'at all, ever' is an exaggeration, but the idea is that you should be making use of smart pointers like std::unique_ptr or std::shared_ptr.
[TheUnbeliever]

#12 iMalc   Crossbones+   -  Reputation: 2299

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Posted 23 February 2012 - 11:56 PM

All I'm saying is that the standard library isn't required for every situation. and particularly for this situation, it appeared that he was simply trying to learn basic constructor/destructor workings, and basic memory management. telling him instead to use standard library methods to do all his memory management is just creating a programmer whom doesn't understand the basics in terms of memory management.

in modern C++ you aren't supposed to use delete at all, ever.


I have never heard this before.

You've got the old way of thinking there. The whole, "you should use this lower level thing so that you understand it" doesn't apply as much to modern C++. It is like saying that you should understand what happens at the assembly level before using a while loop. In the new modern C++ way of doing things, a good programmer is expected to go for years without using delete.
I highly recommend Herb Sutter's presentation on modern C++.

I wouldn't even suggest std::vector here any more. The most appropriate thing to use for a fixed-size array is now a std::array. Posted Image
"In order to understand recursion, you must first understand recursion."
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