On Sun, Jan 11, 2009 at 10:16 AM, bearophile <bearophileh...@lycos.com> wrote:
> Bill Baxter:
>> I've never found a use for typedef myself. I don't think it's used much,<
>
> In Pascal (and its variant and children, like ObjectPascals, etc) there is a
> section named Type where you define your typedefs. People used to program in
> Pascal-like languages (ObjectPascals, Ada, Oberon, etc) may appreciate the
> typedef of D and they may use it.
> I use typedef now and then, I presume it's mostly useful for imperative style
> of programming and not much in OOP.
>
> A simple usage example: you have a procedural/functional program that has
> several functions that process a matrix, the same matrix, for example a
> float[12][7]. In such situation you may want to define:
>
> typedef float[12][7] FPfield;
>
> Then if you use FPField in function signatures like this:
> void foo(FPfield mat, ...) { ... }
> you gain some things:
> - The type name may be shorter, saving you some typing. And you don't need to
> remember each time the size of the dimensions.
> - If you later want to FPfield into a matrix of doubles or the matrix you
> have to change only one line. If your code uses a dynamic array this is less
> important. But from coding a lot of programs in D I have seen programs up to
> 2-5-10 times faster when I use 3D/4D static arrays, mostly because lot of
> address computations are done at compile time or partially optimized away
> instead of run time, and because of higher cache coherence. I can show an
> extreme example, if you want. D dynamic arrays can't replace all static
> arrays where speed matters.
> - It's a way to document the code, because you aren't just giving a generic
> matrix to foo, you are giving it a FPfield, this often has an important
> semantic meaning.
> - It's type safe, so you don't risk giving the wrong matrix to foo. You can't
> do this well with an alias. This is more useful in largish programs.
>
> If you use OOP or lot generic programming this becomes less important.
Actually it might be useful to me. The first time I tried to use it
my thought was to do
typedef Exception MyException;
To create a different exception type. That doesn't work so I kinda
just said, eh whatever, this typedef stuff doesn't work yet.
But probably that's just because that's not the use case they were intended for.
Something I might be able to use it for is for index types. Like this:
typedef size_t VertexHandle;
typedef size_t FaceHandle;
Now I can't accidentally assign a vertex handle to a face handle.
That could be useful.
--bb
