Question

This is very similar to this question, but the answers don’t really answer this, so I thought I’d ask again:

Sometimes I interact with functions that return variable-length structures; for example, FSCTL_GET_RETRIEVAL_POINTERS in Windows returns a variably-sized RETRIEVAL_POINTERS_BUFFER structure.

Using malloc/free is discouraged in C++, and so I was wondering:
What is the “proper” way to allocate variable-length buffers in standard C++ (i.e. no Boost, etc.)?

vector<char> is type-unsafe (and doesn’t guarantee anything about alignment, if I understand correctly), new doesn’t work with custom-sized allocations, and I can’t think of a good substitute. Any ideas?

Answer 1

If you like malloc()/free(), you can use

RETRIEVAL_POINTERS_BUFFER* ptr=new char [...appropriate size...];

... do stuff ...

delete[] ptr;

Quotation from the standard regarding alignment (expr.new/10):

For arrays of char and unsigned char, the difference between the
result of the new-expression and the address returned by the
allocation function shall be an integral multiple of the strictest
fundamental alignment requirement (3.11) of any object type whose size
is no greater than the size of the array being created. [ Note:
Because allocation functions are assumed to return pointers to storage
that is appropriately aligned for objects of any type with fundamental
alignment, this constraint on array allocation overhead permits the
common idiom of allocating character arrays into which objects of
other types will later be placed. — end note ]