On Tue, Nov 4, 2008 at 11:41 AM, Saaa <[EMAIL PROTECTED]> wrote: >> >> You can have a dynamic array that _points_ to the stack, but there is >> no general mechanism for allocating a dynamic array directly on the >> stack. You might be able to do something with alloca, but stack space >> is usually limited and you wouldn't be able to have very large arrays. >> >> Performance of dynamic arrays is the same no matter where their data >> is. Fixed-size 2D arrays are not faster _because_ they are on the >> stack, they just happen to be allocated on the stack. They are faster >> (usually) because they don't need two pointer dereferences. You can >> allocated a fixed-size 2D array on the heap (well.. inside a struct or >> class anyway) and it will be just as fast. > > Ok, so when I want a multimillion array with speed I should use a static > array on the heap. That would be using new, right :)
You.. can't actually allocate a static array on the heap using new. At least not directly. It's kind of an embarrassing hole in the syntax. In fact, I'm not sure if you can even get a static array to point onto the heap, since a static array "reference" is treated like a value type when assigned to, so if you do something like: int[3][4] b = (new int[3][4][](1))[0]; The weirdness on the right-hand-side is needed to get around the compiler "helpfully" rewriting "new int[3][4]" as "new int[][](3, 4)". But this code will simply allocate a static array on the heap, and then copy its contents onto the stack. Dumb. Other than using a struct/class that contains a static array and allocating _that_ on the heap, I don't know of any way of getting it on the heap. But then you'd be double-dereferencing anyway since you'd have to go through the struct pointer or class reference! Sigh.
