[MOAB-dev] [Fathom] variable-length arrays

[James Porter]

On Tue, 2009-11-17 at 13:48 -0600, Jason Kraftcheck wrote:
> It is possible to get the same functionality in c++ using std::vector
> (except for arrays of bool, and assuming the time to zero the memory isn't
> significant).  However, std::vector allocates storage on the heap while a
> automatic array will (typically) be allocated on the stack.  For a few cases
> involving tight loops where the performance with heap allocation is
> unacceptable, it would be nice to be able to use var-len auto arrays.

Is alloca() unacceptable? That would explicitly require stack
allocation, whereas VLAs generally only do so because it's easier in the
context of a C compiler (no need to try to shoehorn in C++
stack-unwinding semantics to call free()). For implementations that do
put VLAs on the stack, alloca() is (in effect) what they'll use.

Both stack-allocated VLAs and alloca() obviously have the same concerns
about stack overflows and all that, but for arrays whose size is known
to be small, they should be safe.

> We have not this feature anywhere in MOAB or CGM so far because of possible
> portability issues.  Does anyone have any experience with using
> variable-length automatic arrays in C++ code that is built on diverse systems?

This would pretty much irrevocably break compilation in MSVC, which
probably isn't a *major* problem, but it's something to consider.
Microsoft has explicitly stated that they have no real interest in C99
support.

- Jim