• "H. S. Teoh" <hsteoh quickfur.ath.cx> Feb 10 2012
• David Nadlinger <see klickverbot.at> Feb 10 2012
• David Nadlinger <see klickverbot.at> Feb 10 2012
• Timon Gehr <timon.gehr gmx.ch> Feb 10 2012
• "H. S. Teoh" <hsteoh quickfur.ath.cx> Feb 10 2012
• "H. S. Teoh" <hsteoh quickfur.ath.cx> Feb 10 2012

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

I'm trying to write a parametrized class with a type parameter whose
existence depends on whether std.utf.decode() exists for that type.
What's the syntax to do this?

	class C(T)
		if ( /* ??? exists(decode(T)...?) */ )
	{ ... }

Thanks!


T

-- 
It is not the employer who pays the wages. Employers only handle the
money. It is the customer who pays the wages. -- Henry Ford

David Nadlinger <see klickverbot.at> writes:

On 2/10/12 7:28 PM, H. S. Teoh wrote:
 I'm trying to write a parametrized class with a type parameter whose
 existence depends on whether std.utf.decode() exists for that type.
 What's the syntax to do this?


class(T) if (__traits(compiles, { std.utf.decode(T.init); } )) { … }

David

David Nadlinger <see klickverbot.at> writes:

On 2/10/12 7:35 PM, David Nadlinger wrote:
 class(T) if (__traits(compiles, { std.utf.decode(T.init); } )) { … }


(untested, you might have to return the value from the delegate to avoid 
an expression-without-effect error)

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

On 02/10/2012 07:50 PM, H. S. Teoh wrote:
 On Fri, Feb 10, 2012 at 07:36:38PM +0100, David Nadlinger wrote:
 On 2/10/12 7:35 PM, David Nadlinger wrote:
 class(T) if (__traits(compiles, { std.utf.decode(T.init); } )) { … }


 (untested, you might have to return the value from the delegate to
 avoid an expression-without-effect error)


 Works for me. Thanks!!

 On that note, I discovered that if you want signature constraints on a
 derived class, the syntax is unbearably ugly:

 	class Derived(T)
 		if (/* conditions */)
 		: Base
 	{
 		...
 	}

 I would have expected the syntax to be like this instead:

 	class Derived(T) : Base
 		if (/* conditions */)
 	{
 		...
 	}

 But this doesn't compile.


 T


It makes more sense as it is. Consider the instantiation 'Derived!int' 
and the condition 'is(typeof(T)==class)'.

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

On Fri, Feb 10, 2012 at 08:53:58PM +0100, Timon Gehr wrote:
 On 02/10/2012 07:50 PM, H. S. Teoh wrote:


On that note, I discovered that if you want signature constraints on
a derived class, the syntax is unbearably ugly:

	class Derived(T)
		if (/* conditions */)
		: Base
	{
		...
	}

I would have expected the syntax to be like this instead:

	class Derived(T) : Base
		if (/* conditions */)
	{
		...
	}




 It makes more sense as it is. Consider the instantiation
 'Derived!int' and the condition 'is(typeof(T)==class)'.


Well, to me, the "if" condition should apply to the entire class
specification "class Derived(T) : Base", and not just to Derived(T). For
example, something like this wouldn't be in a logical sequence:

	class Derived(T)
		if (is(Base!T ...))
		: Base!T
	{
		...
	}

since you'll want the condition to restrict what values Base!T can take
on. This would be more readable:

	class Derived(T) : Base!T
		if (is(Base!T ...))
	{
		...
	}


T

-- 
"I'm running Windows '98."
"Yes."
"My computer isn't working now."
"Yes, you already said that."
-- User-Friendly

"H. S. Teoh" <hsteoh quickfur.ath.cx> writes:

On Fri, Feb 10, 2012 at 07:36:38PM +0100, David Nadlinger wrote:
 On 2/10/12 7:35 PM, David Nadlinger wrote:
class(T) if (__traits(compiles, { std.utf.decode(T.init); } )) { … }


 (untested, you might have to return the value from the delegate to
 avoid an expression-without-effect error)


Works for me. Thanks!!

On that note, I discovered that if you want signature constraints on a
derived class, the syntax is unbearably ugly:

	class Derived(T)
		if (/* conditions */)
		: Base
	{
		...
	}

I would have expected the syntax to be like this instead:

	class Derived(T) : Base
		if (/* conditions */)
	{
		...
	}

But this doesn't compile.


T

-- 
If Java had true garbage collection, most programs would delete themselves upon
execution. -- Robert Sewell