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digitalmars.D.learn - Noob question about structs allocation

reply IM <3di gm.com> writes:
I probably used to know the answer to this question, but it's 
been a long time since I last used D, and I don't remember. 
Suppose we have:

struct S {
   int num;
}

Would allocating an instance on the heap using:

S* s = new S;

use the GC, or do we have to call destroy() or delete on s 
ourselves?
Oct 14 2018
parent reply Basile B. <b2.temp gmx.com> writes:
On Monday, 15 October 2018 at 03:19:07 UTC, IM wrote:

 I probably used to know the answer to this question, but it's 
 been a long time since I last used D, and I don't remember. 
 Suppose we have:

 struct S {
   int num;
 }

 Would allocating an instance on the heap using:

 S* s = new S;

 use the GC, or do we have to call destroy() or delete on s 
 ourselves?


new is an operator that always allocates using the GC,
calling destroy yourself won't hurt either but is not necessary.
Oct 14 2018
parent reply IM <3di gm.com> writes:
On Monday, 15 October 2018 at 03:33:04 UTC, Basile B. wrote:

 On Monday, 15 October 2018 at 03:19:07 UTC, IM wrote:

 I probably used to know the answer to this question, but it's 
 been a long time since I last used D, and I don't remember. 
 Suppose we have:

 struct S {
   int num;
 }

 Would allocating an instance on the heap using:

 S* s = new S;

 use the GC, or do we have to call destroy() or delete on s 
 ourselves?


 new is an operator that always allocates using the GC,
 calling destroy yourself won't hurt either but is not necessary.


What is the effect of calling destroy?
- calling the destructor?
- deallocating the memory?
- both?
Oct 14 2018
next sibling parent Laurent =?UTF-8?B?VHLDqWd1aWVy?= <laurent.treguier.sink gmail.com> writes:
On Monday, 15 October 2018 at 04:14:24 UTC, IM wrote:

 What is the effect of calling destroy?
 - calling the destructor?
 - deallocating the memory?
 - both?


IIRC, it only calls the destructor, the GC will decide when to 
deallocate the memory.
Oct 15 2018
prev sibling parent Mike Parker <aldacron gmail.com> writes:
On Monday, 15 October 2018 at 04:14:24 UTC, IM wrote:


 What is the effect of calling destroy?
 - calling the destructor?
 - deallocating the memory?
 - both?


It calls the destructor. The GC will deallocate the object's 
memory later. However, you need to be careful about how you use 
GC-allocated objects that have destructors. There's currently no 
built-in way to know when a destructor is called as part of the 
normal destruction process (a stack-based instance leaving its 
scope or a manual call to destroy) or when it's called as part of 
finalization, i.e. when the GC calls it. In D, we use the same 
destructor for both.

I recommend you read these links:

http://p0nce.github.io/d-idioms/#The-trouble-with-class-destructors
http://p0nce.github.io/d-idioms/#GC-proof-resource-class

The focus is on classes, but I think it's more important for 
structs. Classes are generally going to be used with default GC 
allocation, meaning their destructors are always finalzers. With 
a struct, it's not unrealistic to anticipate instances will be 
allocated on the stack, the non-GC heap, or the GC heap. In that 
case, the conflation of finalization and destruction is something 
you have to pay special attention to. If you don't, you can dig 
yourself an architectural hole that might not even be apparent 
until you're well and truly into it.

Unfortunately, there's currently no direct means in the public 
API of distinguishing the two cases. The second link above 
demonstrates a workaround that makes it possible. While less than 
ideal, it's all we've got at the moment.

I've got an unpublished blog post, the next in the GC series at 
the D blog, on hold right now because of this. I'm hoping we can 
get something done about it:

https://issues.dlang.org/show_bug.cgi?id=17563
Oct 15 2018