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digitalmars.D - opAssign and template class

reply "dbjdbj" <dbj dbj.org> writes:
Please consider this: http://dpaste.dzfl.pl/dc4a3c29e57f

What is my mistake ?

Thanks ...
Feb 05 2014
parent reply "Stanislav Blinov" <stanislav.blinov gmail.com> writes:
On Wednesday, 5 February 2014 at 10:33:08 UTC, dbjdbj wrote:

 Please consider this: http://dpaste.dzfl.pl/dc4a3c29e57f

 What is my mistake ?

 Thanks ...


You cannot overload identity assignment for classes. It's a 
language construct that rebinds a reference and cannot be 
overridden. Such opAssign overloading is only possible for 
structs.

See http://dlang.org/operatoroverloading.html#Assignment.
Feb 05 2014
parent reply "dbjdbj" <dbj dbj.org> writes:
 You cannot overload identity assignment for classes. It's a 
 language construct that rebinds a reference and cannot be 
 overridden. Such opAssign overloading is only possible for 
 structs.

 See http://dlang.org/operatoroverloading.html#Assignment.


I need to be able to implement "=" operator so that this works:

auto x_ = new X(), x2 = x_ ;

objects)alive needs to be 2 after.

Thanks ...
Feb 05 2014
parent reply "Stanislav Blinov" <stanislav.blinov gmail.com> writes:
On Wednesday, 5 February 2014 at 12:29:11 UTC, dbjdbj wrote:


 I need to be able to implement "=" operator so that this works:

 auto x_ = new X(), x2 = x_ ;

 objects)alive needs to be 2 after.

 Thanks ...


You can't. Classes are reference types. This just doesn't make 
sense: x2 = x_; does not create any new objects.
Feb 05 2014
parent reply "dbjdbj" <dbj dbj.org> writes:
 You can't. Classes are reference types. This just doesn't make 
 sense: x2 = x_; does not create any new objects.


*object_alive* is what I mentioned, not object_create

therefore it does "make sense" ...  so  here it is again:

auto x_ = new X() , // created:1 , alive: 1
      x2 = x_      ; // created:1 , alive: 2

I tried (in all of my D innocence) to implement simple but 
effective counter from C++ side of the "wall" ... using the CRTP 
idiom ...
Feb 05 2014
parent reply "Stanislav Blinov" <stanislav.blinov gmail.com> writes:
On Wednesday, 5 February 2014 at 13:38:08 UTC, dbjdbj wrote:

 You can't. Classes are reference types. This just doesn't make 
 sense: x2 = x_; does not create any new objects.


 *object_alive* is what I mentioned, not object_create


Yes, but in your implementation you intend to increment both 
counters in opAssign.


 therefore it does "make sense" ...  so  here it is again:

 auto x_ = new X() , // created:1 , alive: 1
      x2 = x_      ; // created:1 , alive: 2

 I tried (in all of my D innocence) to implement simple but 
 effective counter from C++ side of the "wall" ... using the 
 CRTP idiom ...


You just cannot reliably count references to class objects in D 
(at least, yet, see the ongoing discussions: 
http://forum.dlang.org/thread/lcrue7$1ho3$1 digitalmars.com, 
http://forum.dlang.org/thread/grngmshdtwqfaftefhky forum.dlang.org).

For non-class objects, there is a library implementation, 
std.typecons.RefCounted.

With it, you can roll something similar:

import std.stdio;
import std.typecons;

struct Counter(T) if (!is(T == class)) {
      disable this(this);
      disable void opAssign(ref Counter);

     private static int objectsCreated_ = 0;
     private T x_;

     this(T x) { x_ = x; ++objectsCreated_; }

     static int getObjectsCreated() { return objectsCreated_; }

      property ref auto get() inout { return x_; }

     alias get this;
}

auto counter(T,Args...)(ref auto Args args) if (!is(T == class)) {
     return RefCounted!(Counter!T)(T(args));
}

 property auto objectsCreated(O)(ref O o) if (is(O == 
RefCounted!(Counter!T), T)) {
     return o.getObjectsCreated();
}

 property auto numReferences(O)(ref O o) if (is(O == 
RefCounted!(Counter!T), T)) {
     return o.refCountedStore.refCount;
}

struct X { int v; }
struct Y { string s; }

void foo(ref const X x) {
     writefln("%s", x.v);
}

void bar(ref const Y y) {
     writefln("%s", y.s);
}

void main()
{
     auto x_ = counter!X, x2 = x_;
     writefln("objects created: %s, num references: %s", 
x_.objectsCreated, x_.numReferences);
     foo(x_);

     auto y_ = counter!Y("hello"), y2 = y_;
     y2.s = "world"; // Modifies y_.s too
     writefln("objects created: %s, num references: %s", 
y_.objectsCreated, y_.numReferences);
     bar(y_);
}


---

Note that e.g. typeof(x_) is actually RefCounted!(Counter!X).

But you cannot do similar for classes, because you cannot guard 
against escaping references.
Feb 05 2014
parent reply "dbjdbj" <dbj dbj.org> writes:
Impressive but please next time please use: 
http://dpaste.dzfl.pl/ ...

Depressive is this issue (and some more) since I was sincerely 
hoping I could live C++ "behind" ...

Eh ...
Feb 05 2014
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 02/05/2014 09:37 AM, dbjdbj wrote:>

 Impressive but please next time please use: http://dpaste.dzfl.pl/ ...


I disagree. dpaste has no association with this newsgroup and can 
disappear at any time in the future.


 Depressive is this issue (and some more) since I was sincerely hoping I
 could live C++ "behind" ...

 Eh ...


There is a significant difference between D and C++. In D, classes are 
reference types (not value types).

The equivalent in C++ of what you are trying to do would be overriding 
the assignment operator for class pointers, which is not possible in C++:

// C++ code:
X * x_ = new X() , // created:1 , alive: 1
   * x2 = x_      ; // created:1 , alive: 2 (not possible!)

Same limitation in D...

Ali
Feb 05 2014