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digitalmars.D.learn - Strange behavior when concatenating array

reply Jose Armando Garcia <jsancio gmail.com> writes:
It looks like the rt is not calling the postblit constructor when
concatenating arrays. For example, the following code:

import std.stdio;

struct Test
{
   this(this) { writeln("copy done"); }
   void opAssign(Test rhs) { writeln("assignment done"); }
   ~this() { writeln("destructor called"); }
}

void main()
{
   Test[] tests = new Test[1];
   {
      Test test;
      tests ~= test;
   }
   writeln("done");
}

Gives the following output:

destructor called
done

The dtr for 'Test test;' is getting call after the scope exits but the
postblit ctr for 'tests[0]' is never called. I believe the output of
this code should either be:

done

or:

copy done
destructor called
done

Thanks!
-Jose
Jun 16 2011
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 17 Jun 2011 00:05:56 -0400, Jose Armando Garcia  
<jsancio gmail.com> wrote:


 It looks like the rt is not calling the postblit constructor when
 concatenating arrays. For example, the following code:

 import std.stdio;

 struct Test
 {
    this(this) { writeln("copy done"); }
    void opAssign(Test rhs) { writeln("assignment done"); }
    ~this() { writeln("destructor called"); }
 }

 void main()
 {
    Test[] tests = new Test[1];
    {
       Test test;
       tests ~= test;
    }
    writeln("done");
 }

 Gives the following output:

 destructor called
 done

 The dtr for 'Test test;' is getting call after the scope exits but the
 postblit ctr for 'tests[0]' is never called. I believe the output of
 this code should either be:

 done

 or:

 copy done
 destructor called
 done


You are right, postblit should be called.   
http://d.puremagic.com/issues/show_bug.cgi?id=5272

I'll look at doing a fix.

-Steve
Jun 17 2011
parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 17 Jun 2011 06:42:51 -0400, Steven Schveighoffer  
<schveiguy yahoo.com> wrote:


 On Fri, 17 Jun 2011 00:05:56 -0400, Jose Armando Garcia  
 <jsancio gmail.com> wrote:


 It looks like the rt is not calling the postblit constructor when
 concatenating arrays. For example, the following code:

 import std.stdio;

 struct Test
 {
    this(this) { writeln("copy done"); }
    void opAssign(Test rhs) { writeln("assignment done"); }
    ~this() { writeln("destructor called"); }
 }

 void main()
 {
    Test[] tests = new Test[1];
    {
       Test test;
       tests ~= test;
    }
    writeln("done");
 }

 Gives the following output:

 destructor called
 done

 The dtr for 'Test test;' is getting call after the scope exits but the
 postblit ctr for 'tests[0]' is never called. I believe the output of
 this code should either be:

 done

 or:

 copy done
 destructor called
 done


 You are right, postblit should be called.   
 http://d.puremagic.com/issues/show_bug.cgi?id=5272

 I'll look at doing a fix.


https://github.com/D-Programming-Language/druntime/pull/29

See if that helps.  On my system, your code now results in your expected  
output.

-Steve
Jun 17 2011
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
Steven Schveighoffer wrote:

 https://github.com/D-Programming-Language/druntime/pull/29

 See if that helps.  On my system, your code now results in your expected
 output.

 -Steve


If the compiler optimizes well enough the output given could be valid (it might
construct the struct directly where it belongs). This needs some investigation.
Anyways, is druntime the correct place to fix this? If it _is_ a bug, isn't it a
compiler bug and not a bug in druntime? After a quick glimpse over your patch I
think the output will never be just "done" now.


Timon
Jun 17 2011
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Fri, 17 Jun 2011 18:52:05 -0400, Timon Gehr <timon.gehr gmx.ch> wrote:


 Steven Schveighoffer wrote:

 https://github.com/D-Programming-Language/druntime/pull/29

 See if that helps.  On my system, your code now results in your expected
 output.

 -Steve


 If the compiler optimizes well enough the output given could be valid  
 (it might
 construct the struct directly where it belongs). This needs some  
 investigation.
 Anyways, is druntime the correct place to fix this? If it _is_ a bug,  
 isn't it a
 compiler bug and not a bug in druntime? After a quick glimpse over your  
 patch I
 think the output will never be just "done" now.


The patch is still necessary in cases where the value being appended is  
not a temporary.  For example, if in the OP's code the test variable was  
used after appending it to the array.

Michel Fortin also brought up the point in a comment on the pull request.   
But the reality is, druntime cannot do any better without help from the  
compiler (the runtime needs to be told when to refrain from running a  
postblit).  It's a separate bug.

However, I think there is nothing invalid with doing both the postblit and  
the destructor on the temporary.  So at least the code is correctly  
calling the postblit.

-Steve
Jun 17 2011