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Xeeynamo

Dynamic class array: memory leak

11 posts in this topic

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:

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

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

delete single;
delete[] array;[/code]
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[quote name='TheUnbeliever' timestamp='1329786004' post='4915013']
You make a single allocation, using new[] (not plain new). You want to make a single use of delete[] - not several of delete.

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

delete single;
delete[] array;[/code]
[/quote]

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

Here's your code converted to do so:

[code]
#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); }
}
[/code]

It should also be noted that the use of 'delete' [i]inside[/i] your loop ("delete &test[i]") is very misguided. Make sure you really understand why that's bad.
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[quote name='edd²' timestamp='1329940442' post='4915622']
Someone has to say it: You 'should' be using std::vector<> in C++ for dynamic arrays.

Here's your code converted to do so:

[code]
#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); }
}
[/code]

It should also be noted that the use of 'delete' [i]inside[/i] your loop ("delete &test[i]") is very misguided. Make sure you really understand why that's bad.
[/quote]
*Cough*
[code]
int main()
{
std::vector<Test> tests(10);
}
[/code]
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[quote name='Washu' timestamp='1329944564' post='4915649']
*Cough*
[code]
int main()
{
std::vector<Test> tests(10);
}
[/code]
[/quote]
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 :)
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[quote name='edd²' timestamp='1329940442' post='4915622']
Someone has to say it: You 'should' be using std::vector<> in C++ for dynamic arrays.
[/quote]
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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[quote name='slicer4ever' timestamp='1329974601' post='4915776']
simply because c++ provides the vector container's for array management, in no way means that you should force yourself to use them.
[/quote]

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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[quote name='kunos' timestamp='1329987740' post='4915814']
[quote name='slicer4ever' timestamp='1329974601' post='4915776']
simply because c++ provides the vector container's for array management, in no way means that you should force yourself to use them.
[/quote]

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.
[/quote]

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.

[quote name='kunos' timestamp='1329987740' post='4915814'] in modern C++ you aren't supposed to use delete at all, ever.[/quote]

I have never heard this before.
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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!
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[quote name='slicer4ever' timestamp='1329988925' post='4915819'][quote name='kunos' timestamp='1329987740' post='4915814'] in modern C++ you aren't supposed to use delete at all, ever.[/quote]

I have never heard this before.[/quote]

Obviously 'at all, ever' is [url="http://www.parashift.com/c++-faq-lite/big-picture.html#faq-6.15"]an exaggeration[/url], but the idea is that you should be making use of smart pointers like std::unique_ptr or std::shared_ptr.
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[quote name='slicer4ever' timestamp='1329988925' post='4915819']
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.

[quote name='kunos' timestamp='1329987740' post='4915814'] in modern C++ you aren't supposed to use delete at all, ever.[/quote]

I have never heard this before.
[/quote]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. [img]http://public.gamedev.net//public/style_emoticons/default/wink.png[/img]
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