C++ dynamic allocation

Question

I'm very confused with regard to the following instructions:

#include <iostream>
#define MAX_IT 100
using namespace std;

class Integer{

private :
    int a;
public:
    Integer(int valoare){a=valoare;}
    int getA(){return a;}
    void setA(int valoare){a=valoare;}
};

int main(){
    Integer* a=new Integer(0);

    //cout<<a[0].getA();
    for(int i=1;i<=MAX_IT;i++)
    {
        a[i]=*(new Integer(i));
    }

    for(int i=0;i<=MAX_IT;i++)
        cout<<a[i].getA()<<endl;

    return 13;
}

It works for small values of MAX_IT, but when I try to set MAX_IT to 1000 it doesn't work anymore. Initially, I thought "new" operator was supposed to do the job, but after some reading documentation I understood it is not supposed to work at all like this (out of bound array).

So my question is: why is it working for small values of MAX_IT and not for bigger ones?

EDIT:

I am experimenting with this code for a larger program, where I am not allowed to use STL. You have not understood my concern: if I have Integer *var=new Integer[10]; for(int k=1;K<10;k++) *(var+k)=k; //this is perfectly fine, but if I try var[10]=new Integer; //this should not be working and should generate a memory problem //My concern is that it is working if I do it only 100 times or so...The question if why is it working everytime for small number of iterations?

Answer 1

Because by allocating space for one Integer then using it as an array of multiple Integers, your code invokes undefined behavior, meaning that it can do anything, including crashing, working seemingly fine, or pulling demons out of your nose.

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And anyways it's leaking memory. If you don't need dynamic memory allocation, then don't use it.

a[i]=*(new Integer(i));

And kaboom, you lost the pointer to the Integer, no chance to delete it later. Leaks.

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If you don't need raw arrays, don't use them. Prefer std::vector. Or switch to C if C++ is too hard.

std::vector<Integer> vec;
vec.push_back(Integer(1337));

Answer 2

The reason that things tend to work nicely when you overflow your buffer by just a little bit is... memory fragmentation! Who would have guessed?

To avoid memory fragmentation, allocators won't return you a block of just sizeof (Integer). They'll give you a somewhat larger block, to ensure that if the block is later freed before the adjacent blocks, it's at least big enough to be useful.

Exactly how big this is can vary by architecture, OS, compiler version, or even how much memory is physically present in the machine. You should consider it to be completely unpredictable. Also, some libraries designed to help catch this sort of bug force any small object to be placed at the end of the block instead of the beginning, so the extra bytes could be negative array indices instead of positive.

Therefore, don't ever rely on having spare area given to you for free after (or before) an object.

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Guru note: Occasionally someone comes up with a valid use for the extra memory, and asks for a way to discover how large it is. One good example is that the capacity (not size!) of a std::vector could be adjusted to match the actual allocated space instead of the requested space, and therefore reduce (on average) the number of reallocations needed. Such requests usually come paired with other guru allocator APIs, such as the ability to expand an allocation in-place if there happen to be free blocks adjacent.

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Note that in your particular case you do still have undefined behavior, because you're calling operator= on a non-POD object which hasn't first been constructed. If you gave class Integer a trivial default constructor that would change.

Answer 3

you actually need

Integer* a=new Integer[MAX_IT];

//cout<<a[0].getA();
for(int i=1;i<MAX_IT;i++) << note < not <=
{
    a[i]=i;
}

better would be to use std::vector though