www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - scope(exit) and destructor prioity

reply Sasszem <mch atw.hu> writes:
I'm currently working on a project and for that I've created a 
thin OO-wrapper on top of derelict-sdl. However, when I close my 
app, the program terminates with a segfault. I've managed to 
track down the source, and found that the destructors of my 
objects are called AFTER the scope(exit) statements. This causes 
my program to call TTF_CloseFont() after TTF_Quit(), resulting in 
a segfault.
My questions:
  - Can I force D to call the destructors BEFORE the scope(exit) 
statements?
  - If not, is there a way around?
Sep 18 2017
parent reply Jerry <hurricane hereiam.com> writes:
On Monday, 18 September 2017 at 20:26:05 UTC, Sasszem wrote:

 I'm currently working on a project and for that I've created a 
 thin OO-wrapper on top of derelict-sdl. However, when I close 
 my app, the program terminates with a segfault. I've managed to 
 track down the source, and found that the destructors of my 
 objects are called AFTER the scope(exit) statements. This 
 causes my program to call TTF_CloseFont() after TTF_Quit(), 
 resulting in a segfault.
 My questions:
  - Can I force D to call the destructors BEFORE the scope(exit) 
 statements?
  - If not, is there a way around?


It's called inbetween the destructors of wherever you put the 
scope(exit).

import std.stdio;

struct De
{
	~this() { writeln("De"); }
}

void main()
{
	De a;
	scope(exit) writeln("scope exit");
	De b;
}


Output:
De
scope exit
De
Sep 18 2017
parent reply Sasszem <mch atw.hu> writes:
On Monday, 18 September 2017 at 20:30:20 UTC, Jerry wrote:

 On Monday, 18 September 2017 at 20:26:05 UTC, Sasszem wrote:

  [...]


 It's called inbetween the destructors of wherever you put the 
 scope(exit).

 import std.stdio;

 struct De
 {
 	~this() { writeln("De"); }
 }

 void main()
 {
 	De a;
 	scope(exit) writeln("scope exit");
 	De b;
 }


 Output:
 De
 scope exit
 De


If I write "auto a = new De()", then it calls the scope first, no 
matter where I place it.
Sep 18 2017
next sibling parent Eugene Wissner <belka caraus.de> writes:
On Monday, 18 September 2017 at 20:55:21 UTC, Sasszem wrote:

 On Monday, 18 September 2017 at 20:30:20 UTC, Jerry wrote:

 On Monday, 18 September 2017 at 20:26:05 UTC, Sasszem wrote:

  [...]


 It's called inbetween the destructors of wherever you put the 
 scope(exit).

 import std.stdio;

 struct De
 {
 	~this() { writeln("De"); }
 }

 void main()
 {
 	De a;
 	scope(exit) writeln("scope exit");
 	De b;
 }


 Output:
 De
 scope exit
 De


 If I write "auto a = new De()", then it calls the scope first, 
 no matter where I place it.


If I write "auto a = new De()" I have the same behaviour. If I 
have "auto b = new De()" aswell, then yes, destructors are called 
after scope exit.
Because you allocate on the heap with new, the destructor isn't 
called at the end of the scope at all. It is called later by the 
GC.

Try to put variable declarations with destructor after "scope 
exit" or destroy them manually with "destroy(a)".

See https://dlang.org/spec/statement.html#scope-guard-statement 
for order of calling destructors at the end of scope.
Sep 18 2017
prev sibling parent Moritz Maxeiner <moritz ucworks.org> writes:
On Monday, 18 September 2017 at 20:55:21 UTC, Sasszem wrote:

 If I write "auto a = new De()", then it calls the scope first, 
 no matter where I place it.


Because with `new`
a) your struct object is located on the heap (and referred to by 
pointer - `De*`) instead of the stack (which means no destructors 
for it are called at function scope end), and
b) the lifetime of your struct object is determined by D's 
garbage collector, which may or may not eventually collect it, 
finalizing it in the process (calling the destructor, as D 
doesn't separate finalizers and destructors a.t.m.).
In your case, it sounds like the GC collection cycle that (in the 
current implementation) occurs just before druntime shutdown 
collects it.
I highly recommend reading The GC Series on the D blog [1].

[1] https://dlang.org/blog/the-gc-series/
Sep 18 2017