digitalmars.D.learn - InstanceOf

Lemonfiend (72/72) Jun 23 2013 I'm trying to create a fairly generic component system, where an

Artur Skawina (8/19) Jun 23 2013 void writeB()
Chris Cain (13/24) Jun 23 2013 http://dlang.org/expression.html#CastExpression
Namespace (4/76) Jun 23 2013 foreach (I i; array) {

Lemonfiend (7/10) Jun 23 2013 Thanks all 3 of you for the quick and identical answers. :)

Timon Gehr (5/14) Jun 23 2013 Casts are as cheap as testing whether an object is an instance of a
Jacob Carlborg (14/23) Jun 23 2013 You can do something like this as well:

Lemonfiend (7/34) Jun 23 2013 Using the .classinfo is what I looked at before asking here.

Jacob Carlborg (5/10) Jun 23 2013 That might be the case, didn't think of that. In that case you need
Steven Schveighoffer (11/37) Jun 24 2013 1. Use typeid(i), not i.classinfo, classinfo is old-style.

Lemonfiend (7/49) Jun 24 2013 This method would not work for my example, since it would get the

"Lemonfiend" <lemon fie.nd> writes:

I'm trying to create a fairly generic component system, where an 
object iterates over a bunch of other objects that all implement 
a certain interface.
And this all works fine, however, I would also like to be able to 
get objects of a specific type (a la instanceOf), and I can't 
figure out how to do it.

Could anyone help me out?

[code]
interface I
{
	void update();
	void write();
}

class A : I
{
	int n;
	
	void update()
	{
		n++;
	}

	void write()
	{
		writeln(n);
	}
}

class B : I
{
	int m;
	
	void update()
	{
		m--;
	}

	void write()
	{
		writeln(m);
	}
}

class C
{
	I[] array;

	void addElem(I elem)
	{
		array ~= elem;
	}

	void loop()
	{
		foreach(elem; array)
			elem.update();
	}

	void writeAll()
	{
		foreach(elem; array)
			elem.write();
	}

	void writeB()
	{
		// Only call .write on B's
		// How do I get the B's from the array of I's?
	}
}

void main()
{
	C c = new C();

	c.addElem(new A());
	c.addElem(new B());

	c.loop();

	c.writeAll();

	c.writeB(); // This is the problem
}
[/code]

Jun 23 2013

Artur Skawina <art.08.09 gmail.com> writes:

On 06/23/13 13:04, Lemonfiend wrote:
     void writeAll()
     {
         foreach(elem; array)
             elem.write();
     }
 
     void writeB()
     {
         // Only call .write on B's
         // How do I get the B's from the array of I's?
     }

    void writeB()
    {
        foreach(elem; array)
           if (auto b = cast(B)elem)
               b.write();
    }

artur

Jun 23 2013

"Chris Cain" <clcain uncg.edu> writes:

On Sunday, 23 June 2013 at 11:04:59 UTC, Lemonfiend wrote:
 ...
 	void writeAll()
 	{
 		foreach(elem; array)
 			elem.write();
 	}

 	void writeB()
 	{
 		// Only call .write on B's
 		// How do I get the B's from the array of I's?
 	}

http://dlang.org/expression.html#CastExpression

So, this should do it:

void writeB()
{
  	foreach(elem; array)
		if(cast(B) elem)
  			elem.write();
}

You can also do some things like this, if you want to use 
something specific to Bs:

if(auto b = cast(B) elem)
	b.bSpecificMethod();

Jun 23 2013

"Namespace" <rswhite4 googlemail.com> writes:

On Sunday, 23 June 2013 at 11:04:59 UTC, Lemonfiend wrote:
 I'm trying to create a fairly generic component system, where 
 an object iterates over a bunch of other objects that all 
 implement a certain interface.
 And this all works fine, however, I would also like to be able 
 to get objects of a specific type (a la instanceOf), and I 
 can't figure out how to do it.

 Could anyone help me out?

 [code]
 interface I
 {
 	void update();
 	void write();
 }

 class A : I
 {
 	int n;
 	
 	void update()
 	{
 		n++;
 	}

 	void write()
 	{
 		writeln(n);
 	}
 }

 class B : I
 {
 	int m;
 	
 	void update()
 	{
 		m--;
 	}

 	void write()
 	{
 		writeln(m);
 	}
 }

 class C
 {
 	I[] array;

 	void addElem(I elem)
 	{
 		array ~= elem;
 	}

 	void loop()
 	{
 		foreach(elem; array)
 			elem.update();
 	}

 	void writeAll()
 	{
 		foreach(elem; array)
 			elem.write();
 	}

 	void writeB()
 	{
 		// Only call .write on B's
 		// How do I get the B's from the array of I's?
 	}
 }

 void main()
 {
 	C c = new C();

 	c.addElem(new A());
 	c.addElem(new B());

 	c.loop();

 	c.writeAll();

 	c.writeB(); // This is the problem
 }
 [/code]

foreach (I i; array) {
     if (B b = cast(B) i) { ... }
}

Jun 23 2013

"Lemonfiend" <lemon fie.nd> writes:

 foreach (I i; array) {
     if (B b = cast(B) i) { ... }
 }

Thanks all 3 of you for the quick and identical answers. :)

It had not occurred to me to use a cast for this, but indeed the 
language ref says the same:
"In order to determine if an object o is an instance of a class B 
use a cast"

It does a bit inelegant to me.. Or are casts simply extremely 
cheap?

Jun 23 2013

Timon Gehr <timon.gehr gmx.ch> writes:

On 06/23/2013 01:26 PM, Lemonfiend wrote:
 foreach (I i; array) {
     if (B b = cast(B) i) { ... }
 }

 Thanks all 3 of you for the quick and identical answers. :)

 It had not occurred to me to use a cast for this, but indeed the
 language ref says the same:
 "In order to determine if an object o is an instance of a class B use a
 cast"

 It does a bit inelegant to me.. Or are casts simply extremely cheap?

Casts are as cheap as testing whether an object is an instance of a 
certain class. Having the cast evaluate to a nullable reference is 
certainly more elegant than having instanceof and cast as separate 
constructs, where the cast may throw.

Jun 23 2013

Jacob Carlborg <doob me.com> writes:

On 2013-06-23 13:26, Lemonfiend wrote:
 foreach (I i; array) {
     if (B b = cast(B) i) { ... }
 }

 Thanks all 3 of you for the quick and identical answers. :)

 It had not occurred to me to use a cast for this, but indeed the
 language ref says the same:
 "In order to determine if an object o is an instance of a class B use a
 cast"

 It does a bit inelegant to me.. Or are casts simply extremely cheap?

You can do something like this as well:

if (i.classinfo is B.classinfo) { }

But doing the cast is more efficient if you want to use the object of as 
the type you're checking for.

You can also hide the cast in a function if you want to be a bit more 
clear of the intent:

T instanceOf (T) (Object value)
{
     return cast(T) value);
}

if (i.instanceOf!(B)) { }

-- 
/Jacob Carlborg

Jun 23 2013

"Lemonfiend" <lemon fie.nd> writes:

On Sunday, 23 June 2013 at 15:15:16 UTC, Jacob Carlborg wrote:
 On 2013-06-23 13:26, Lemonfiend wrote:
 foreach (I i; array) {
    if (B b = cast(B) i) { ... }
 }

 Thanks all 3 of you for the quick and identical answers. :)

 It had not occurred to me to use a cast for this, but indeed 
 the
 language ref says the same:
 "In order to determine if an object o is an instance of a 
 class B use a
 cast"

 It does a bit inelegant to me.. Or are casts simply extremely 
 cheap?

 You can do something like this as well:

 if (i.classinfo is B.classinfo) { }

 But doing the cast is more efficient if you want to use the 
 object of as the type you're checking for.

Using the .classinfo is what I looked at before asking here.
However, according to the specs:
".classinfo applied to an interface gives the information for the 
interface, not the class it might be an instance of."
So the i.classinfo and B.classinfo would be different?

 You can also hide the cast in a function if you want to be a 
 bit more clear of the intent:

 T instanceOf (T) (Object value)
 {
     return cast(T) value);
 }

 if (i.instanceOf!(B)) { }

This is indeed what I did :)

Jun 23 2013

Jacob Carlborg <doob me.com> writes:

On 2013-06-23 17:29, Lemonfiend wrote:

 Using the .classinfo is what I looked at before asking here.
 However, according to the specs:
 ".classinfo applied to an interface gives the information for the
 interface, not the class it might be an instance of."
 So the i.classinfo and B.classinfo would be different?

That might be the case, didn't think of that. In that case you need 
casts anyway.

-- 
/Jacob Carlborg

Jun 23 2013

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Sun, 23 Jun 2013 11:29:10 -0400, Lemonfiend <lemon fie.nd> wrote:

 On Sunday, 23 June 2013 at 15:15:16 UTC, Jacob Carlborg wrote:
 On 2013-06-23 13:26, Lemonfiend wrote:
 foreach (I i; array) {
    if (B b = cast(B) i) { ... }
 }

 Thanks all 3 of you for the quick and identical answers. :)

 It had not occurred to me to use a cast for this, but indeed the
 language ref says the same:
 "In order to determine if an object o is an instance of a class B use a
 cast"

 It does a bit inelegant to me.. Or are casts simply extremely cheap?

 You can do something like this as well:

 if (i.classinfo is B.classinfo) { }

 But doing the cast is more efficient if you want to use the object of  
 as the type you're checking for.

 Using the .classinfo is what I looked at before asking here.
 However, according to the specs:
 ".classinfo applied to an interface gives the information for the  
 interface, not the class it might be an instance of."
 So the i.classinfo and B.classinfo would be different?

1. Use typeid(i), not i.classinfo, classinfo is old-style.
2. Yes, typeid(i) will give you interface class info, or maybe even  
derived interface class info.  It's a simple indirection, whereas a cast  
must go through a pointer offset stored in the interface typeinfo in order  
to get true class info.
3. typeid(i) on one DLL may be different than typeid(i) on another.  It is  
not valid to compare the references directly

On 2, see test code:

http://dpaste.dzfl.pl/97f5866d

-Steve

Jun 24 2013

"Lemonfiend" <lemon fie.nd> writes:

On Monday, 24 June 2013 at 15:46:05 UTC, Steven Schveighoffer 
wrote:
 On Sun, 23 Jun 2013 11:29:10 -0400, Lemonfiend <lemon fie.nd> 
 wrote:

 On Sunday, 23 June 2013 at 15:15:16 UTC, Jacob Carlborg wrote:
 On 2013-06-23 13:26, Lemonfiend wrote:
 foreach (I i; array) {
   if (B b = cast(B) i) { ... }
 }

 Thanks all 3 of you for the quick and identical answers. :)

 It had not occurred to me to use a cast for this, but indeed 
 the
 language ref says the same:
 "In order to determine if an object o is an instance of a 
 class B use a
 cast"

 It does a bit inelegant to me.. Or are casts simply 
 extremely cheap?

 You can do something like this as well:

 if (i.classinfo is B.classinfo) { }

 But doing the cast is more efficient if you want to use the 
 object of as the type you're checking for.

 Using the .classinfo is what I looked at before asking here.
 However, according to the specs:
 ".classinfo applied to an interface gives the information for 
 the interface, not the class it might be an instance of."
 So the i.classinfo and B.classinfo would be different?

 1. Use typeid(i), not i.classinfo, classinfo is old-style.
 2. Yes, typeid(i) will give you interface class info, or maybe 
 even derived interface class info.  It's a simple indirection, 
 whereas a cast must go through a pointer offset stored in the 
 interface typeinfo in order to get true class info.
 3. typeid(i) on one DLL may be different than typeid(i) on 
 another.  It is not valid to compare the references directly

 On 2, see test code:

 http://dpaste.dzfl.pl/97f5866d

 -Steve

This method would not work for my example, since it would get the 
interface, not a class.
But if I were to maintain an array of some base class instead, it 
would.

Very interesting! Thanks.

Jun 24 2013

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digitalmars.D.learn - InstanceOf