c++中的归并排序函数

归并排序 MergeSort函数里if（pSrc！=R）语句删了运行结果出错，但我不太理解这个条件的作用是什么

template<class T>
void Merge(T* pSrc, T* pDst, int low, int mid, int high)合并
{
    int i = low, j = mid + 1, k = low;
    while (i <= mid && j <= high)
    {
        if (pSrc[i] <= pSrc[j])
        {
            pDst[k] = pSrc[i];
            i++;
        }
        else
        {
            pDst[k] = pSrc[j];
            j++;
        }
        k++;
     }
    while (i <= mid)
    {
        pDst[k] = pSrc[i];
        i++;
        k++;
    }
    while (j <= high)
    {
        pDst[k] = pSrc[j];
        j++;
        k++;
    }
}
template<class T>
void MergePass(T* pSrc, T* pDst, int nsize, int nLength)
{
    int i, j;
    for (i = 0; (i + 2) * nLength < nsize; i+=2)
        Merge(pSrc, pDst, i * nLength , (i + 1) * nLength-1, (i + 2) * nLength-1);
    if ((i + 1) * nLength < nsize)
        Merge(pSrc, pDst, i * nLength , (i + 1) * nLength-1, nsize-1);
    else
    {
        for (j = i * nLength ; j <=nsize; j++)
            pDst[j] = pSrc[j];
    }
}
template<class T>
void MergeSort(T R[], int nSize)
{
    int nLength;
    T* MR = new T[nSize + 1];
    T* pSrc = R, * pDst = MR, * p;
    int i;
    if (MR == NULL)
        exit(1);
    for (nLength = 1; nLength < nSize; nLength *= 2)
    {
        MergePass(pSrc, pDst, nSize, nLength);
        p = pSrc;
        pSrc =pDst;
        pDst = p;
    }
    if (pSrc != R)
    {
        for (i = 0; i < nSize; i++)
            R[i] = MR[i];
    }
    delete[]MR;
}
int main()
{
    int A[] = {43,56,37,28,17,37,22,30,2 };
    MergeSort(A, 9);
    for(int i=0;i<9;i++)
        cout<<A[i]<<',';
    return 0;
}

删除if后运行会显示MR为未初始化的内存，麻烦能帮我看看

主要是和A[]的长度有关