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digitalmars.D.learn - question on [Issue 7853]

reply "Dan" <dbdavidson yahoo.com> writes:
This bug has no comments. The original says: "The fix is obvious:
redefine postblit as this(const this); but it isn't always obvious
when looking at hundreds of lines of code...."

Is this accepted/correct? In the following if I include the 
this(const
this) postblit it does not get called. It makes me question if 
the fix
is obvious?

Thanks
Dan


-------
import std.stdio;

struct S {
   static if(false) {
     this (const this) { writeln("const postblit");}
   } else {
     this (this) { writeln("non-const postblit");}
   }
}
void main () {
   const(S) s;
   S s2 = s;
}
Nov 04 2012
next sibling parent Tobias Pankrath <lists pankrath.net> writes:
On 04.11.2012 21:49, Dan wrote:
 This bug has no comments. The original says: "The fix is obvious:
 redefine postblit as this(const this); but it isn't always obvious
 when looking at hundreds of lines of code...."

 Is this accepted/correct? In the following if I include the this(const
 this) postblit it does not get called. It makes me question if the fix
 is obvious?

I don't think that currently qualifiers work with the postblit constructor. See here for a related discussion. http://forum.dlang.org/thread/CAFDvkcvvL8GxHQB=Rw9pTm-uxOKzNGVQNDv9w5Os3SkQCc=DLQ mail.gmail.com
Nov 04 2012
prev sibling next sibling parent "Dan" <dbdavidson yahoo.com> writes:
On Sunday, 4 November 2012 at 21:44:15 UTC, Tobias Pankrath wrote:

 I don't think that currently qualifiers work with the postblit 
 constructor. See here for a related discussion.

 http://forum.dlang.org/thread/CAFDvkcvvL8GxHQB=Rw9pTm-uxOKzNGVQNDv9w5Os3SkQCc=DLQ mail.gmail.com

Thanks. The reason I'm down this path is something like below. I really want to keep const ref for parms on a method (e.g. foo below). It turns out the type is a assoc array and length and keys are both giving me a headache. Without the cast I get a message like: Error: function acct.Account.__postblit () is not callable using argument types () const Below I cast away const (Dohh!). Is it safe in this case? If not is there another way? Thanks, Dan import std.stdio; import std.traits; alias Account[string] Map; struct Account { this(this){ writeln("acct copied"); } } void foo(const ref Map m) { pragma(msg, "Map is ", typeof(m)); pragma(msg, "Map is ", typeof(cast()m)); writeln("Map has ", (cast(Map)m).length); writeln("Map has ", (cast(Map)m).keys); } void main() { Map map; Account acct; foo(map); }
Nov 04 2012
prev sibling next sibling parent Tobias Pankrath <lists pankrath.net> writes:
On 11/05/2012 02:24 AM, Dan wrote:

 Thanks. The reason I'm down this path is something like below. I really
 want to keep const ref for parms on a method (e.g. foo below). It turns
 out the type is a assoc array and length and keys are both giving me a
 headache. Without the cast I get a message like:

 Error: function acct.Account.__postblit () is not callable using
 argument types () const

You probably now this, but this error message indicates that you want to call a non-const method of a const instance.
 Below I cast away const (Dohh!). Is it safe in this case?

your data.
 If not is there another way?

The code below works, too. But to be honest, I don't now why the compiler complains that the postblit is not callable in your version but does not actually get called in mine. ----- alias Account[string] Map; struct Account { this(this) const { writeln("acct copied"); } //this(this) { writeln("acct copied"); } } void foo(const ref Map m) { pragma(msg, "Map is ", typeof(m)); pragma(msg, "Map is ", typeof(cast()m)); writeln("Map has ", m.length); writeln("Map has ", m.keys); } void main() { Map map; Account acct; foo(map); Account acct2 = acct; } ----- I have added the copy construction of acct2 to verify that the const-qualified this(this) is actually used for postblit.
Nov 05 2012
prev sibling next sibling parent "Dan" <dbdavidson yahoo.com> writes:
On Monday, 5 November 2012 at 09:10:41 UTC, Tobias Pankrath wrote:

 Casting away const is okay as long as you don't change a single 
 bit of your data.

Yes, thanks. Makes sense. I need to know not only what I might be mutating, but also code I call. In this specific case, though I hope all of these things on assoc arrays are safe, const or not, so my cast is harmless. - length - keys - returning dynamic array I won't change - values - returning dynamic array I won't change - foreach I have not figured what to look at for associative arrays. For example, in .../druntime/import/object.di there is a struct called AssociativeArray which I imagine the [ "foo" : "goo" ] syntactic sugar gets me to. It has the keys property as non-const and internally it does a cast: Key[] keys() property { auto a = _aaKeys(p, Key.sizeof); return *cast(Key[]*) &a; } I'm not even sure if this is the right code to look at when dealing with associative arrays described in TDPL?
 If not is there another way?

The code below works, too. But to be honest, I don't now why the compiler complains that the postblit is not callable in your version but does not actually get called in mine.

My theory: you must use this(this) without variation for postblit to even get called until they hammer out the issues on the thread you referred to. While the signature below gets called, it is futile since incipient instance can not be patched which is the purpose. I think <this(this) const> should not be allowed but this(const ref this) or this(const this) should be allowed and preferred. I don't understand how the compiler ends up with the postblit message without the cast. I hope the lack of const on assoc array properties and foreach are oversights rendering the casts safe and on a future release of D rendering them unnecessary. Thanks Dan
 -----
 alias Account[string] Map;

 struct Account {
       this(this) const { writeln("acct copied"); }
       //this(this) { writeln("acct copied"); }
 }

Nov 05 2012
prev sibling parent Tobias Pankrath <lists pankrath.net> writes:
On 11/05/2012 12:43 PM, Dan wrote:
 On Monday, 5 November 2012 at 09:10:41 UTC, Tobias Pankrath wrote:

 I have not figured what to look at for associative arrays. For example,
 in .../druntime/import/object.di there is a struct called
 AssociativeArray which I imagine the [ "foo" : "goo" ] syntactic sugar
 gets me to. It has the keys property as non-const and internally it does
 a cast:
      Key[] keys()  property
      {
          auto a = _aaKeys(p, Key.sizeof);
          return *cast(Key[]*) &a;
      }
 I'm not even sure if this is the right code to look at when dealing with
 associative arrays described in TDPL?

AA are implemented in object_.d, rt/aaA.d and in the compiler. For example the _aaKeys function is defined in rt/aaA.d. I don't now any details though. You may take a look here: http://www.digitalmars.com/d/archives/digitalmars/D/Replacing_AA_s_in_druntime_161307.html
 The code below works, too. But to be honest, I don't now why the
 compiler complains that the postblit is not callable in your version
 but does not actually get called in mine.

While the signature below gets called, it is futile since incipient instance can not be patched which is the purpose.

 I think
 <this(this) const> should not be allowed but this(const ref this) or
 this(const this) should be allowed and preferred.

struct. So it's not clear how to define the semantics of this(const this).
 I don't understand how
 the compiler ends up with the postblit message without the cast. I hope
 the lack of const on assoc array properties and foreach are oversights
 rendering the casts safe and on a future release of D rendering them
 unnecessary.

I guess so.
Nov 05 2012