Segmentation fault in memory allocation when making a dynamically resized array C++

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I get a segmentation fault when the append(int val) function is run, and called polymorphically, but I can't see where the memalloc error is coming from. I'm still new to C++, and run into this problem a lot, but when I do fix it on my own, it's always by happenchance. Any pointer? (No pun intended, or is it :))

IntegerCombination.h

#ifndef INTEGERCOMBINATION_H
#define INTEGERCOMBINATION_H


using namespace std;

class IntegerCombination
{
public:
    IntegerCombination();
    void append(int Val);
    virtual int combine() = 0;
protected:
    int* _collection;
    int _length;
};

#endif

IntegerCombination.cpp

#include "IntegerCombination.h"

IntegerCombination::IntegerCombination()
{
    _length = 0;
}

void IntegerCombination::append(int val)
{
    int newValPos = _length;            // Stores current length as new position for new 
                                        // value
    int* temp = _collection;            //Stores current array
    delete _collection;                 // Deletes current array
    _length++;                          // Increases the length for the new array
    _collection = new int[_length];     // Creates a new array with the new length
    for(int i = 0; i < newValPos; i++)
    {
        _collection[i] = temp[i];       // Allocates values from old array into new array
    }
    _collection[newValPos] = val;       // Appends new value onto the end of the new array
}

Main.cpp


#include "IntegerCombination.h"
#include "ProductCombination.h"

using namespace std;

int main()
{

    ProductCombination objProd;

    for(int i = 1; i <= 10; i++)
    {
        objProd.append(i);
    }

    return 0;
}

Note: The code in ProductCombination.h and ProductCombination.cpp is not entirely relevant, as in the .cpp file, append(int val) is only delegating the append call to the base class in IntegerCombination.h

segmentation-fault polymorphism inheritance c++
1 Answers

For starters the constructor does not initializes the data member _collection

IntegerCombination::IntegerCombination()
{
    _length = 0;
}

So this data member can have an indeterminate value and using the operator delete with such a pointer invokes undefined bejavior.

Moreover as you are trying to allocate an array then you need to use the operator delete [] instead of delete.

And the class has to define explicitly at least a virtual destructor. Also either declare the copy constructor and the copy assignment operator as deleted or also define them explicitly.

The function append has several bugs.

As it was mentioned you need to use the operator delete [] in this statement

delete _collection;

instead of the operator delete.

But this operator must be called after the new array is allocated. Otherwise the pointer temp will have an invalid value

int* temp = _collection;            //Stores current array
delete [] _collection;                 // Deletes current array

That is you need to delete the previous array after you copied elements of it to the new allocated array.

user_28772410
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