Re: Can't get custom allocation function to work

["kathrin_69@..." <kathrin_69@...>]

Ralph Hempel wrote:
> kathrin_69@gmx.de wrote:
> > Hi,
> >
> > I'm trying to replace the Lua default l_alloc function with my own 
> > one. My aim is to have all memory Lua allocates inside a single array 
> > of pre-allocated memory.
>
> <snip>
>
> > void* alloc_impl_manual( void* ptr, size_t osize, size_t nsize )
> > {
> >   if (nsize)
> >   {
> >       std::vector<unsigned char>* mem = new std::vector<unsigned 
> > char>(nsize);
> >                //yes, it will leak. don't care for now
> >
> >       return &(*mem)[0]; //returned memory position should be aligned 
> > by new
> >   }
> >   return NULL;
> > }
>
> Ummm, besides the leak, should you not be returning:
>
>   return( mem )
>
> since mem is already a pointer to the memory you allocated?
>
> And this expression  &(*mem)[0] is very strange. I parse that as:
std::vector<int> v;
int* c_array = &v[0]; //not just &v !

v.push_back(4);
v.push_back(2);

printf( "%d%d", c_array[0], c_array[1] ); //"42"

The expression gives the position of the continuous memory regions hold 
by std::vector. mem would just be the this-pointer of the vector object 
and therefore the wrong thing.
This is an stl feature to get backward compatibility to c-style arrays 
as shown in the little example above.

> (the adress of ( (the contents of mem) ???? )
>
> my compiler warns that the subscripted value is neither an array nor a
> pointer...
>
> Ralph

John had the right solution. I forgot to move the old mem to the new 
range in case of a re-allocation. Was my first allocator :)
Thank you all!