help on arrays

[faybesp]

i am getting difficulties understanding the following piece of code:
(assuming T being any variable type)
*M = new T* [xdim]; **M = new T [xdim*ydim];
if (*M==NULL || **M==NULL) return false;
for(sint i=1; i < xdim; i++)
(*M)[i] = (*M)[i-1] + ydim;

Thanks in advance

[weaknessforcats]

*M = new T* [xdim];

This allocates an array of T* that has xdim elements. The new operator returns
a pointer to element 0 of the array so *M has to be a T** making
M itself a T***.

 **M = new T [xdim*ydim];

Again the new operator returns the address of an array of T that has
xdim*ydim elements. Since M is a ***T from the first example, then **M
is a T* and the new operator is returning a T*.
Also, the variable M is used for two arrays resulting in a horrible memory corruption.

if (*M==NULL || **M==NULL) return false;

This code is baloney:
a) there is no NULL in C++. NULL is a C macro for zero.
b) the new operator never returns zero when it runs out of memory. Instead, it throws a bad_alloc exception.
c) you can't have two arrays at the same address.

for(sint i=1; i < xdim; i++)
(*M)[i] = (*M)[i-1] + ydim;

Here sint has to be a typedef of int. Why??
The loop is garbage since the *M array has only xdim elements.