realloc without moving?

[chris jefferson]

>To Chris: What exactly are your requirements?  You are aware that
>glibc on Linux also has efficient realloc for very large chunks thanks
>to the mremap() system call?
>
>  
>
Hmm.. I may have to think about this harder :)

My exact area of interest is std::vector, that is the vector class 
provided in the C++ standard library (although I may expand my interest 
later).

When someone tries to add a new element (via .push_back) to an already 
full vector, then we must allocate a new larger block of memory and copy 
the existing vector into it.

Now if the class is sufficently simple (and we can check for that) then 
we can simply copy the old memory bit-for-bit into the new block. In 
this case a call to realloc is exactly what we want.

If however the elements of the vector are complex classes then this is 
forbidden, and this is where this reallocing business comes in. If we 
can expand the current allocation of memory that the vector is in, all 
is well and good. If however this is not possible, we cannot simply copy 
the elements of the vector bit-for-bit, we must allocate a new block of 
memory to put them in and use the class's copy constructors.

Chris