Hi, D.learn!

I encountered a problem, while i was designing a fast, performance-critical
image storage mechanism.
Here's what i want to do:
Have a FragmentTraits struct, which contains fields, like number of
components, size in bytes of the components, padding of the components in
bytes, etc.
Have a Fragment struct, which takes a FragmentTraits object as a template
parameter and defines the fragment components as it's fields according to
given traits.
Have a BufferTraits struct, which contains fields, like row padding in bytes,
etc.
Have a Buffer class, which takes a Fragment type and a BufferTraits object and
defines appropriate storage and indexing routines.

I know, structs are CTFE-able, so there should be no problem reading the
fields of those compile-time defined struct objects.
However, D does not allow me to have structs as template parameters and if I
define them as alias parameters, it refuses to read the fields at compile-time.

Having the user pass all those fields separately will lead to a very ugly API and having the user define a template, containing enum-constants as fields will lead to unnecessarily complicated and difficult interface and additional templates to test, whether the templates are defined correctly or not.

How do i work around this limitation and are there any plans to include structs in the list of allowed template parameters?

On Sun, Sep 25, 2011 at 16:49, Gor F. Gyolchanyan <gor.f.gyolchanyan@gmail.com> wrote:

> Have a FragmentTraits struct, which contains fields, like number of
> components, size in bytes of the components, padding of the components in
> bytes, etc.
> Have a Fragment struct, which takes a FragmentTraits object as a template
> parameter and defines the fragment components as it's fields according to
> given traits.

> How do i work around this limitation and are there any plans to include structs in the list of allowed template parameters?

I'm not sure I understand what you'd like your code to look like. This
works for me (DMD 2.055):


struct FragmentTraits
{
    uint componentsNumber;
    uint componentsSize;
}

struct Fragment(alias trait)
if (is(typeof(trait) == FragmentTraits))
{
    mixin(getTraits!trait);
}

string getTraits(alias trait)()
if (is(typeof(trait) == FragmentTraits))
{
    return selectSize(trait.componentsSize) ~ "[" ~
to!string(trait.componentsNumber) ~ "] components;";
}

string selectSize(uint size)
{
    if (size == 1)
        return "byte";
    if (size <= 2)
        return "short";
    if (size <= 4)
        return "int";
    if (size <= 8)
        return "long";

    assert(0, "Bad component size: " ~ to!string(size) ~ ". Size must
be between 1 and 8.");
}

void main(string[] args)
{
    enum f = FragmentTraits(100,2);

    Fragment!f ff; // ff will contain a "short[100] components" member.

    writeln(typeof(ff.components).stringof);
}

Thanks! This is exactly what i wanted!
Seems like the problem of not being able to pass struct template parameters can be
worked around by putting the struct attribute access in a mixin.

On Mon, Sep 26, 2011 at 11:19, Gor F. Gyolchanyan <gor.f.gyolchanyan@gmail.com> wrote:
> Thanks! This is exactly what i wanted!
> Seems like the problem of not being able to pass struct template parameters can be
> worked around by putting the struct attribute access in a mixin.

Keep us informed on your progress, because since I read "Modern C++ Design", I wondered how to do that in D (and if that is indeed idiomatic in D).