CODEWITHSUNDEEP
c++dynamiccorruptionheap-corruption
user1233963
user1233963

Reputation: 1490

Dynamic array heap corruption

I have the following class:

template <typename T>
class matrix
{
private:
    int _n;
    T* array;
public:
    matrix(): _n(0) 
    {
        array = new T[_n * _n];
    }
    matrix(int n): _n(n)
    {
        if( n < 0 )
            throw "Invalid array size!";
        array = new T[_n * _n];
    }
    ~matrix()
    {
        delete[] array;
    }
    void Set(const int x, const int y,const T val)
    {
        if( ( x<0 || x>_n ) && ( y<0 || y>_n) )
            throw "Invalid index";
        array[x*_n + y] = val;
    }
    T& Get(const int x, const int y)
    {
        if( ( x<0 || x>_n ) && ( y<0 || y>_n) )
            throw "Invalid index";
        return array[x*_n + y];
    }
};

and using it this way:

matrix<int> k(5);
k.Set(5,5,6);
cout<<k.Get(5,5);

The problem is I get a heap corruption error when calling Set. What am I doing wrong? (my guess is it's the way I access they array elements)

Upvotes: 2

Views: 711

Answers (4)

Glenn
Glenn

Reputation: 1167

You are indexing beyond the array into memory which is why you are getting the error.

The issue is with indexing. The indexing of arrays in C++ starts at 0, so for your declaration of k(5), it produces an array of 5 which is indexed 0-4, so an index of 5 is not a valid valid index. You should change your check in set to be x>=_n and y>=_n, as and index of 5 is not valid.

Upvotes: 0

Haatschii
Haatschii

Reputation: 9319

What is worng is that you have to start counting from 0. If you create a array with size n*n you can acces the element up to (n-1)*(n-1). Your example create a arry of size 5*5=25 and trys to acces element 25. However 24 is the highes element you array contains.

Upvotes: 0

Luchian Grigore
Luchian Grigore

Reputation: 258618

Arrays in C++ are 0-base, that means if you have an array int x[5], x[5] is invalid.

Your conditions should be if( ( x<0 || x>=_n ) && ( y<0 || y>=_n) ) and if( ( x<0 || x>=_n ) && ( y<0 || y>=_n) )

To set the last element, you'd need 

k.Set(4,4,6);

and to print it:

cout<<k.Get(4,4);

Upvotes: 0

Dark Falcon
Dark Falcon

Reputation: 44181

A 5 element array can be accessed at the indexes 0-4. You're passing 5 for x and y, which results in an invalid index when accessing array.

Upvotes: 1

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