Re: Use of GUID constants

[KlausO via Digitalmars-d-learn]

For GUIDs you often have to take the address (e.g. for calls to 
QueryInterface), so I think phobos does not correctly implement this.

In the meantime I took a look at the VisualD project which accesses the 
COM interfaces of visual studio. They solve the problem by using the 
following idiom (see 
https://github.com/D-Programming-Language/visuald/blob/master/sdk/port/servprov.d 
for an example):

const GUID IID_IServiceProvider = IServiceProvider.iid;

interface IServiceProvider : IUnknown
{
	static const GUID iid = { 0x6d5140c1, 0x7436, 0x11ce, [ 0x80, 0x34, 
0x00, 0xaa, 0x00, 0x60, 0x09, 0xfa ] };
public:
        /* [local] */ HRESULT QueryService(
                /* [in] */ in GUID* guidService,
                /* [in] */ in IID* riid,
                /* [out] */ void **ppvObject);
}

If every interface declaration contains the static iid member this 
enables you to provide something similar to the the __uuidof operator in 
VisualC (see https://msdn.microsoft.com/de-de/library/zaah6a61.aspx).

In VisualD something along that line is implemented in the qi_cast 
template (see 
https://github.com/D-Programming-Language/visuald/blob/master/stdext/com.d).


Is the above pair (const GUID and static member) the right way to 
declare this idiom or would the following be better ?
Both compile and seem to be equivalent (in terms of achieving the same 
goal):

enum IID IID_IServiceProvider = { 0x6d5140c1, 0x7436, 0x11ce, [ 0x80, 
0x34, 0x00, 0xaa, 0x00, 0x60, 0x09, 0xfa ] };

interface IServiceProvider : IUnknown
{
        static immutable GUID iid = IID_IServiceProvider;
public:
/* [local] */ HRESULT QueryService(
/* [in] */ in GUID* guidService,
/* [in] */ in IID* riid,
/* [out] */ void **ppvObject);
}

Thanks for your insights

-- KlausO


Am 10.03.2016 um 14:49 schrieb Mike Parker:
On Thursday, 10 March 2016 at 10:16:30 UTC, KlausO wrote:
Ok, but what's the intention behind defining GUIDs as enums in the
first place ?

Probably just an implementation error, i.e. someone not fully
appreciating how GUIDs are intended to be used.

Is there a recommended way to declare/define constants (e.g. as enums
or consts) ?

Generally, you should use a manifest constant, e.g.

enum myConst = 10;

Unless you need to take the address, then you should use immutable:

immutable myConst = 10;

The value of a manifest constant is substituted for the symbol at the
point of use and is not stored in the data segment (so has no memory
address), but an immutable (or const) variable is stored in the data
segment.