C++ - Array value becomes 1 after being read

Question

So, I'm running a pretty trivial and bare-bones piece of code:

template <typename T>
matrix<T>::matrix(int r, int c, int a)
{
   int data2[6*4] = {-1, 12, 23, 34, 45, 56,
                     11, 22, 33, 44, 55, 66,
                     21, 32, 43, 54, 65, -1,
                     31, 42, 53, 64, -1, -1};

    mData = new T(6*4);

    for(int p=0; p < 6*4; p++){
        mData[p] = T(data2[p]);
        std::cout << " " << mData[p] << ":";
        std::cout << " " << mData[p] << ":" << std::endl; //second time is read
    };

    for(int p=0; p < 4; p++){
        for(int p2=0; p2 < 6; p2++){
            if(mData[p2 + p * 6] == 1)std::cout << "FOUND ONE" << std::endl;
        }
    }
}

int main(){

    int i = 6;
    matrix<int> b = matrix<int>(i,i, 0);
}

However, at random intervals there are times where a value of a data CHANGES to 1 the second time it's being read. It's never the first time, always the second one. Here you have an example:

-1: -1:                                                                                                                                 
12: 12:                                                                                                                     
23: 23:                                                                                                                 
34: 34:                                                                                                                 
45: 45:                                                                                                                         
56: 56:                                                                                                                 
11: 11:                                                                                                                 
22: 22:                                                                                                                 
33: 1:                                                                                                                  
44: 44:                                                                                                                 
55: 55:                                                                                                                 
66: 66:                                                                                                                 
21: 21:                                                                                                                 
32: 32:                                                                                                                 
43: 43:                                                                                                                    
54: 54:                                                                                                                 
65: 65:                                                                                                                 
-1: -1:                                                                                                                 
31: 31:                                                                                                                 
42: 42:                                                                                                                 
53: 53:                                                                                                                 
64: 64:                                                                                                                 
-1: -1:                                                                                                                 
-1: -1:                                                                                                                
FOUND ONE                                                                                                                                                                                                                                       
Process returned 0 (0x0)   execution time : 0.085 s                                                                     
Press any key to continue.  

There is nothing else touching the structure, so I have no idea what could be going on. The number is always saved properly (since the first print is ALWAYS correct), but afterwards it gets corrupted... I got no idea what's going any. Any help?

EDIT:

Solution found, apparently I simply mistook a "[]" for a "()", and despite looking the code over and over never noticed. (it's obvious at hindsight).

Thank you guys.

Answer 1

In this line of code:

mData = new T(6*4);

a single instance of T is allocated. Afterwards, the code tries to access that as an array, leading to out of bounds access.

It was probably intended like this:

mData = new T[6*4];

---

Edit and another tip: In order to reduce error like these and to improve the overall code style, I almost always go for std::vector<T> instead of T* when creating arrays of elements. The access pattern rarely changes in code but you don't need to worry about memory management and gain additional bonus methods for almost no costs.