Am Sun, 14 May 2017 20:18:24 +0000
schrieb Kevin Brogan <ke...@brogan.ca>:
> I have a piece of code that takes a callback function.
>
> The callback has the signature void callback(void* state, void*
> data)
>
> There are several of these functions. All of them use state and
> data as differing types.
>
> As an example, let's look at one that uses both of them as int*.
>
> addInt(void* state, void* data)
> {
> *cast(int*)state += *cast(int*)data;
> }
>
> Is it not possible to specify the cast as an alias so that I can
> declare the cast once at the beginning of the function?
>
> Something like this?
>
> addInt(void* state, void* data)
> {
> alias _state = cast(int*)state; // Error: basic type
> expected, not cast
> alias _data = cast(int*)data; // Error: basic type expected,
> not cast
>
> *_state += *_data;
> }
No, that is not possible. An alias can only be assigned a
symbol.
> I can always do this:
>
> addInt(void* state, void* data)
> {
> int* _state = cast(int*)state;
> int* _data = cast(int*)data;
>
> *_state += *_data;
> }
>
> But I don't want to create a new variable and assign it everytime
> I call the function. The examples I'm using are contrived, but in
> the c code I am porting this from, the callback gets called
> thousands of times a second, every optimization matters, and the
> variables are used many times per function. I don't want to
> riddle the code with casts if i can avoid it and I don't want to
> create and destroy useless proxy variables every time the
> function is called.
Let the compiler optimize the assignment away and don't worry
much about it. Inlining also works well within the same module.
In this case here I would probably use "consume" functions as
I dub them:
import std.traits;
pragma(inline, true) /* Not really needed I hope ;) */
ref T consume(T)(ref void* data) if (!hasIndirections!T)
{
T* ptr = cast(T*)data;
data += T.sizeof;
return *ptr;
}
You can then rewrite your addInt function like this:
void add(T)(void* state, void* data) if (isNumeric!T)
{
state.consume!T += data.consume!T;
}
--
Marco
