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digitalmars.D.learn - Syntax for Pointer to Class

reply Q. Schroll <qs.il.paperinik gmail.com> writes:
Say I have a class C and I want a pointer to a C handle.

I tried the following pieces of syntax:

   C* obj = new C(); // gives me a C
   C* obj = new C*(); // gives me a C**
   C* obj = C*(); // refuses to compile

Is it even possible? This sounds so newbie...
I know it's fairly simple with structs: S() gives me a struct 
instance, new S() gives me a ptr to a struct instance.
Jan 25 2019
parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Jan 25, 2019 at 08:12:33PM +0000, Q. Schroll via Digitalmars-d-learn
wrote:

 Say I have a class C and I want a pointer to a C handle.
 
 I tried the following pieces of syntax:
 
   C* obj = new C(); // gives me a C
   C* obj = new C*(); // gives me a C**
   C* obj = C*(); // refuses to compile
 
 Is it even possible? This sounds so newbie...
 I know it's fairly simple with structs: S() gives me a struct
 instance, new S() gives me a ptr to a struct instance.


Classes in D are inherently by-reference, meaning that if you have a
class:

	class C { ... }

then the type `C` is implicitly a pointer to the class instance. Of
course, this is hidden by the language syntax so most of the time you
don't have to care that it's a pointer, just that it has reference
semantics.

If you want the raw pointer to the class instance, just cast the
reference to void*, for example:

	C c = new C(...);
	void* ptr = cast(void*) c;

Note that taking the address of `C` will actually give you a pointer to
the reference, not the pointer to the class instance itself. For that to
be valid, you'll need to store your class reference somewhere first,
since it's invalid to take the address of an rvalue:

	C c = new C(...);
	C* ptrToRef = &c;

	// And you'll need to explicitly dereference the pointer to get
	// to the class instance:
	(*ptrToRef).method(...);

Be warned that the pointer will become invalid when the reference `c`
goes out of scope, even if the object itself is still live (via another
reference), because the pointer is pointing to the class reference
rather than the actual object.


T

-- 
Once the bikeshed is up for painting, the rainbow won't suffice. -- Andrei
Alexandrescu
Jan 25 2019
parent reply Q. Schroll <qs.il.paperinik gmail.com> writes:
On Friday, 25 January 2019 at 20:31:29 UTC, H. S. Teoh wrote:

 On Fri, Jan 25, 2019 at 08:12:33PM +0000, Q. Schroll via 
 Digitalmars-d-learn wrote:

 Say I have a class C and I want a pointer to a C handle.


 Note that taking the address of `C` will actually give you a 
 pointer to the reference, not the pointer to the class instance 
 itself.


I know. This is precisely the thing I asked for: A pointer to a 
handle. I called it handle for exactly this reason.


 For that to be valid, you'll need to store your class reference 
 somewhere first, since it's invalid to take the address of an 
 rvalue


Yes. `&new C(...)` looked to wrong for me to even consider trying.


 	C c = new C(...);
 	C* ptrToRef = &c;


So the answer is no. At least not that simple.


 Be warned that the pointer will become invalid when the 
 reference `c` goes out of scope, even if the object itself is 
 still live (via another reference), because the pointer is 
 pointing to the class reference rather than the actual object.


That's kind of obvious if you get remembered, but thanks. I 
wasn't considering this as I wanted the handle to live on the 
heap anyway --- the same way for a struct `S`, the pointer 
pointed to by `S** ptr = new S*(new S(...));` lies on the heap.

I'll use

     C* ptr = &[ new C(...) ][0];

for now. It looks ugly, but it does the job.
Jan 25 2019
parent rikki cattermole <rikki cattermole.co.nz> writes:
On 26/01/2019 9:52 AM, Q. Schroll wrote:

      C* ptr = &[ new C(...) ][0];


C* ptr = [ new C(...) ].ptr;

Should work.
Jan 25 2019