struct vs class for a simple token in my d lexer

[Dmitry Olshansky]

On 14.05.2012 21:20, Roman D. Boiko wrote:
> On Monday, 14 May 2012 at 16:41:39 UTC, Jonathan M Davis wrote:
>> On Monday, May 14, 2012 17:10:23 Roman D. Boiko wrote:
>>> (Subj.) I'm in doubt which to choose for my case, but this is a
>>> generic question.
>>>
>>> http://forum.dlang.org/post/odcrgqxoldrktdtarskf@forum.dlang.org
>>>
>>> Cross-posting here. I would appreciate any feedback. (Whether to
>>> reply in this or that thread is up to you.) Thanks
>>
>> For the question in general, there are primarily 2 questions to consider:
>>
>> 1. Do you need inheritance/polymorphism?
>> don't have inheritance.
>>
>> 2. Do you need a value type or a reference type?
>>
>> If you need inheritance/polymorphism, you have to use classes, because
>> structs
>>
>> If you want a value type, you have to use structs, because classes are
>> reference types.
>>
>> If you want a reference type, then you can use either a class or a
>> struct, but
>> it's easier with classes, because classes are always reference types,
>> whereas
>> structs are naturally value types, so it can take more work to make them
>> reference types depending on what its member variables are.
>>
>> One other thing to consider is deterministic destruction. The only way
>> to get
>> deterministic destruction is to have a struct on the stack. Value
>> types will
>> naturally get that, but if you want a reference type with determinstic
>> destruction, then you're probably going to have to use a ref-counted
>> struct.
>>
>> In the vast majority of cases, that should be enough to decide whether
>> you
>> need a class or a struct. The main case that it doesn't is the case
>> where you
>> want a reference type and don't necessarily want a class, and that
>> decision
>> can get complicated.
>>
>> In the case of a token in a lexer, I would definitely expect that to
>> be a value
>> type, which means using a struct.
>>
>> - Jonathan M Davis
> Semantics is clearly of value type. But it would be nice to
> minimize size of instance to be copied. (I could simply split it
> into two, if some part is not used in most scenarios.)
>
> Just curious: how to implement a struct as ref type? I could add
> indirection (but then why not just use a class?), or simply pass
> by ref into all methods. Are there any other options?
>

Use the damn pointer ;)

In a few words (lines) overwrite struct contents (first 4-8 bytes) with 
pointer to next free token:

//setup:
void get_some_memory()
{
	Token pool = new Token[block_size];
	for(size_t i=0;i<pool.length-1; i++)
		*(void**)(tok.ptr+i) =   tok.ptr+i+1;
	*(void**)&tok[$-1]  = null; //end of list
}
Token* head = pool.ptr;//this guy in TLS

//usage:
Token* new_tok(){
	return head ? get_some_memory(), head : head;
}

void free_tok(Token* tok){
	*(void**)tok =  head;
	head = tok;
}

Untested but trustworthy ;)

> Thanks to everybody for your feedback, it looks like I'll stay
> with struct.


-- 
Dmitry Olshansky