c++ memset cause segment fault of int** pointer

Question

    int **dpTable = new int* [iMatrixHeight + 1];
    for (int i = 0; i < iMatrixHeight + 1; i++)
    {
        dpTable[i] = new int [iMatrixWidth + 1];
    }

    memset(dpTable, 0, (sizeof(int)) * (iMatrixHeight + 1)*(iMatrixWidth + 1));

I'm using operator new to allocate a two-dimensional array, but if I use memset to initialize the array, I got a segmentfault when I access the array later. Without the memset, it's ok.

Am I doing anything wrong? THX!

Answer 1

The arrays dpTable[i] do not point to contiguous memory. You have to initialize then one by one

for (int i = 0; i < iMatrixHeight + 1; i++)
{
    dpTable[i] = new int [iMatrixWidth + 1];
    memset(dpTable[i], 0, (iMatrixWidth + 1) * sizeof(int)) ;
}

Answer 2

Instead of this code:

int **dpTable = new int* [iMatrixHeight + 1];
for (int i = 0; i < iMatrixHeight + 1; i++)
{
    dpTable[i] = new int [iMatrixWidth + 1];
}

memset(dpTable, 0, (sizeof(int)) * (iMatrixHeight + 1)*(iMatrixWidth + 1));

… you can do this:

int **dpTable = new int* [iMatrixHeight + 1];
for (int i = 0; i < iMatrixHeight + 1; i++)
{
    dpTable[i] = new int [iMatrixWidth + 1]();
}

Look ma, no memset – instead asking for zeroing of the memory.

Still that's very ugly in C++.

So, do this:

vector<vector<int>> table( iMatrixHeight + 1, vector<int>( iMatrixWidth + 1 ) );

where vector is std::vector .

Or, consider defining a matrix class with just a single vector as backing store.

Or just use an existing matrix class, eg from Boost.