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digitalmars.D.learn - Scope statement

reply Lars V <nospamplease nospam.com> writes:
Hi, 

I have a question: 

void foo() 
{ 
  Obj o = new Obj(); 
}

In this code, the destructor of the object will be called ALWAYS at the end of
the function? 

In D, the objects are not destroyed when they go out of their scopes??? Is
necessary to use the "scope" statement? 

I really don't understand the necessity of the scope statement... could anybody
explain it?

Thanks, 
Regards
Feb 05 2008
parent reply Bill Baxter <dnewsgroup billbaxter.com> writes:
Lars V wrote:

 Hi, 
 
 I have a question: 
 
 void foo() 
 { 
   Obj o = new Obj(); 
 }
 
 In this code, the destructor of the object will be called ALWAYS at the end of
the function? 
 
 In D, the objects are not destroyed when they go out of their scopes??? Is
necessary to use the "scope" statement? 
 
 I really don't understand the necessity of the scope statement... could
anybody explain it?


Yes you need to use scope if you want deterministic destruction.

Allocation and deallocation are expensive, and one of the nice things 
about a garbage collection system is that it lets you do clean-up more 
lazily.  That way you can handle a bunch of de-allocations at once. 
Think of it as waiting a week to vacuum up the room instead of pulling 
out the vacuum cleaner every time a piece of dust hits the floor.

--bb
Feb 05 2008
parent reply Lars V <nospamplease nospam.com> writes:
Bill Baxter Wrote:


 Lars V wrote:

 Hi, 
 
 I have a question: 
 
 void foo() 
 { 
   Obj o = new Obj(); 
 }
 
 In this code, the destructor of the object will be called ALWAYS at the end of
the function? 
 
 In D, the objects are not destroyed when they go out of their scopes??? Is
necessary to use the "scope" statement? 
 
 I really don't understand the necessity of the scope statement... could
anybody explain it?


 
 Yes you need to use scope if you want deterministic destruction.
 
 Allocation and deallocation are expensive, and one of the nice things 
 about a garbage collection system is that it lets you do clean-up more 
 lazily.  That way you can handle a bunch of de-allocations at once. 
 Think of it as waiting a week to vacuum up the room instead of pulling 
 out the vacuum cleaner every time a piece of dust hits the floor.
 
 --bb



But, when the function ends the object should be collected by the GC (there are
no more references to it) and the GC will call the destructor, no? I don't
understand it.
Feb 05 2008
next sibling parent "Simen Kjaeraas" <simen.kjaras gmail.com> writes:
Lars V <nospamplease nospam.com> wrote:


 Bill Baxter Wrote:


 Lars V wrote:

 Hi,

 I have a question:

 void foo()
 {
   Obj o =3D new Obj();
 }

 In this code, the destructor of the object will be called ALWAYS at=






  =


 the end of the function?

 In D, the objects are not destroyed when they go out of their  =







 scopes??? Is necessary to use the "scope" statement?

 I really don't understand the necessity of the scope statement...  =







 could anybody explain it?

 Yes you need to use scope if you want deterministic destruction.

 Allocation and deallocation are expensive, and one of the nice things=





 about a garbage collection system is that it lets you do clean-up mor=




e

 lazily.  That way you can handle a bunch of de-allocations at once.
 Think of it as waiting a week to vacuum up the room instead of pullin=




g

 out the vacuum cleaner every time a piece of dust hits the floor.

 --bb



 But, when the function ends the object should be collected by the GC  =



 (there are no more references to it) and the GC will call the  =



 destructor, no? I don't understand it.


When the object goes out of scope, there are no more pointers to it,  =

correct. This only means however, that the next time the GC runs a  =

collection cycle (which might be right after you've exited the scope, bu=
t  =

you can't know that), the object's destructor will be called.

If you instead use
void foo()
{
	scope Obj o =3D new Obj();
}

You are guaranteed that the moment the object goes out of scope, its  =

destructor will be called.

-Simen
Feb 05 2008
prev sibling parent reply Michel Fortin <michel.fortin michelf.com> writes:
On 2008-02-05 18:26:50 -0500, Lars V <nospamplease nospam.com> said:


 But, when the function ends the object should be collected by the GC 
 (there are no more references to it) and the GC will call the 
 destructor, no? I don't understand it.


The garbage collector will generally destruct and deallocate the object 
when you ask for more memory, if there are no more reference to it (it 
doesn't scan memory for pointers each time a function returns). This 
could happen the next time you call new, or in a few more calls to new.

Basically, when to dispose of the object is left to the discretion of 
the garbage collector which may optimize things in various ways you 
shouldn't have to care much about. If you need deterministic 
destruction for your object, either use scope or call the destructor 
yourself (using delete).

-- 
Michel Fortin
michel.fortin michelf.com
http://michelf.com/
Feb 05 2008
parent reply Lars V <nospamplease nospam.com> writes:
Michel Fortin Wrote:


 On 2008-02-05 18:26:50 -0500, Lars V <nospamplease nospam.com> said:
 

 But, when the function ends the object should be collected by the GC 
 (there are no more references to it) and the GC will call the 
 destructor, no? I don't understand it.


 
 The garbage collector will generally destruct and deallocate the object 
 when you ask for more memory, if there are no more reference to it (it 
 doesn't scan memory for pointers each time a function returns). This 
 could happen the next time you call new, or in a few more calls to new.
 
 Basically, when to dispose of the object is left to the discretion of 
 the garbage collector which may optimize things in various ways you 
 shouldn't have to care much about. If you need deterministic 
 destruction for your object, either use scope or call the destructor 
 yourself (using delete).
 
 -- 
 Michel Fortin
 michel.fortin michelf.com
 http://michelf.com/
 


OK, thanks for all the replies. 

Only one more question; 
the deterministic destruction of the object -using scope or delete- will force
the GC to collect it? or it'll only call the destructor?
Feb 05 2008
parent Michel Fortin <michel.fortin michelf.com> writes:
On 2008-02-05 20:00:38 -0500, Lars V <nospamplease nospam.com> said:


 Only one more question;
 the deterministic destruction of the object -using scope or delete- 
 will force the GC to collect it? or it'll only call the destructor?


It'll call the destructor and free the memory, but I wouldn't say it's 
the garbage collector that does it in this case.

Note that when you use scope, the compiler may optimise things and 
allocate the object directly on the stack. In this case, the destructor 
is (implicitly) called but there is no memory that needs to be freed 
after the function call.

-- 
Michel Fortin
michel.fortin michelf.com
http://michelf.com/
Feb 05 2008