CODEWITHSUNDEEP
c++arrays
Keshav Rajasekaran
Keshav Rajasekaran

Reputation: 33

Accessing array memory in c++ throws error

int** z= new int *[5];
for (int j = 0; j < 8; j ++)
{
    z[j] = new int[5];        
}

for (int n=0; n<8; ++n)
{
    for(int m=0;m<5;++m)
    {
        int  x=n%4;
        int  y=x*wB;
        int p=(*(B+(y+m)));

        z[n][m]=p;            
    }                
}    
return z;

throws a Bad_Acess_error at n=6, but 

int** z= new int *[5];
for (int j = 0; j < 8; j ++)
{
    z[j] = new int[5];        
}

for (int n=0; n<8; ++n)
{
    for(int m=0;m<5;++m)
    {
        int  x=n%4;
        int  y=x*5;
        int p=(*(B+(y+m)));

        z[6][m]=p;            
    }            
return z;               
}

throws no error. Why? This is really weird and I can't seem to understand why this is happening. I am just typing in more text so it allows me to publish this question.

Edit: replaced variables with numbers. The numbers are just limits of the array in question. I know the second code works because the output is exactly what I expect it to be.

Upvotes: 1

Views: 71

Answers (1)

Thomas Matthews
Thomas Matthews

Reputation: 57749

Simply put: Buffer Overrun.

int** z= new int *[5];  // Allocates space for 5 slots.

for (int j = 0;
     j < 8;   // <---- *** Assumes space for 8 slots!!!!!
     j ++)
{
    z[j] = new int[5];  // At j==6, access is outside the array.  
}

Upvotes: 1

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