Re: Templates, D way

[John Colvin via Digitalmars-d]

On Tuesday, 5 September 2017 at 11:08:57 UTC, Void-995 wrote:
Hi, everyone. I'm pretty new to D and trying my first steps in 
it. Currently I'm trying to port some code from C/C++ with 
pretty weird data structures and don't like idea of making 
boilerplate functions for accessing sub-lists in main binary 
structure (lets not talk about it's design, it's from legacy 
thing I want to deal with and I can't change the format itself 
or writing a lot of additional parsing code). With hour I spent 
on trying I've ended with what you may see below, but I wonder 
if there is more pleasant variant of re-writing that template 
as I haven't found anything better in either books or online 
documentation yet:

template DataList(const char[] listName, T, alias dataOffset, 
alias listLength)
{
        const char[] DataList = format(q{
                %s[] %s()
                {
                        return (cast(%s *)(cast(byte *)(&this) + %s))[0 .. %s];
                }
	}, T.stringof, listName, T.stringof, dataOffset.stringof, 
listLength.stringof);
}

struct MyBinarySubStructAForA
{
        int someIntegerFieldA;
        float someFloatFieldA;
}

struct MyBinarySubStructBForA
{
        int someIntegerFieldB;
        float someFloatFieldB;
}

struct MyBinaryStructA
{
        int firstSublistMembersCount;
        int firstSublistMembersOffset;

        int secondSublistMembersCount;
        int secondSublistMembersOffset;
        
	@property mixin(DataList!("firstSublist", 
MyBinarySubStructAForA, firstSublistMembersCount, 
firstSublistMembersOffset));
	@property mixin(DataList!("secondSublist", 
MyBinarySubStructBForA, secondSublistMembersCount, 
secondSublistMembersOffset));
}

...

MyBinaryStructA *binaryData = cast(MyBinaryStructA 
*)fileData.ptr;

mixin template DataList(string listName, T, alias dataOffset, 
alias listLength)
{
        mixin(
                q{T[] } ~ listName ~ q{() @property
                {
			return (cast(T*)(cast(ubyte*)&this + dataOffset))[0 .. 
listLength];
                }}
        );
}

struct MyBinarySubStructAForA
{
        int someIntegerFieldA;
        float someFloatFieldA;
}

struct MyBinarySubStructBForA
{
        int someIntegerFieldB;
        float someFloatFieldB;
}

struct MyBinaryStructA
{
        int firstSublistMembersCount;
        int firstSublistMembersOffset;

        int secondSublistMembersCount;
        int secondSublistMembersOffset;
        
	mixin DataList!("firstSublist", MyBinarySubStructAForA, 
firstSublistMembersCount, firstSublistMembersOffset);
	mixin DataList!("secondSublist", MyBinarySubStructBForA, 
secondSublistMembersCount, secondSublistMembersOffset);
}