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digitalmars.D - Why Templates?

reply "Derek Parnell" <derek psych.ward> writes:
I'm sure its because I'm not from a C++ background, but I just don't get  
templates. What is the problem that Templates solve?

 From my limited experience they just seem to be a shorthand for a way of  
avoiding coding identical algorithms that only vary in the datatypes that  
are passed to them. Is that it or am I missing some fundamental elements?

And is that why people are clamoring for implicit template instantation -  
to further reduce the amount of typing to do (and remembering to  
instantiate the only the required renditions of the algorithm).

-- 
Derek Parnell
Melbourne, Australia
Mar 04 2006
next sibling parent reply Sean Kelly <sean f4.ca> writes:
Derek Parnell wrote:

 I'm sure its because I'm not from a C++ background, but I just don't get 
 templates. What is the problem that Templates solve?
 
  From my limited experience they just seem to be a shorthand for a way 
 of avoiding coding identical algorithms that only vary in the datatypes 
 that are passed to them. Is that it or am I missing some fundamental 
 elements?


That was the original intent.  As far as I know, the original idea came 
from Alexander Stepanov (http://www.stepanovpapers.com/), who is also 
responsible for generics in Ada.  But since their introduction, 
templates have also shown to be a useful high-performance alternative to 
run-time polymorphism, not to mention the more advanced code-generation 
trickery that has become so popular in the C++ world.


 And is that why people are clamoring for implicit template instantation 
 - to further reduce the amount of typing to do (and remembering to 
 instantiate the only the required renditions of the algorithm).


For the most part, yes, as template names can become quite long, and 
calling functions as:

     fn!(typeof(a),typeof(b))(a,b);

is a bit annoying and reduces readability.  ITI is also quite useful in 
C++ with respect to how overload resolution is handled, but I have no 
idea how this will translate to the far simpler overloading rules in D. 
  I think we really may have to simply want and see what Walter comes up 
with for ITI in D, as simply eliminating the need to explicitly specify 
template parameters doesn't encompass the full utility of ITI in C++. 
Will we be able to overload template functions with each other?  Will 
they also overload with non-template functions?  What happens if there 
are multiple matches?

     class C(T) {}

     template func(T) { void func( T val ) {} }
     template func(T) { void func( C!(T) val ) {} }
     void func( C!(int) val ) {}

     func( new C!(int) );

In C++, this is legal and the result is clearly defined (the "most 
specialized" overload is called), but how will this translate to D? 
Will it even be legal?


Sean
Mar 04 2006
parent reply "Walter Bright" <newshound digitalmars.com> writes:
"Sean Kelly" <sean f4.ca> wrote in message 
news:duda9o$137k$1 digitaldaemon.com...

 is a bit annoying and reduces readability.  ITI is also quite useful in 
 C++ with respect to how overload resolution is handled, but I have no idea 
 how this will translate to the far simpler overloading rules in D. I think 
 we really may have to simply want and see what Walter comes up with for 
 ITI in D, as simply eliminating the need to explicitly specify template 
 parameters doesn't encompass the full utility of ITI in C++. Will we be 
 able to overload template functions with each other?


Yes.


 Will they also overload with non-template functions?


No. This is a misfeature in C++, and the equivalent can be done with 
explicitly specialized template functions.


 What happens if there are multiple matches?

     class C(T) {}

     template func(T) { void func( T val ) {} }
     template func(T) { void func( C!(T) val ) {} }
     void func( C!(int) val ) {}

     func( new C!(int) );

 In C++, this is legal and the result is clearly defined (the "most 
 specialized" overload is called), but how will this translate to D?


It'll be the most specialized.


 Will it even be legal?


 Sean
Mar 04 2006
parent Sean Kelly <sean f4.ca> writes:
Walter Bright wrote:

 "Sean Kelly" <sean f4.ca> wrote in message 
 news:duda9o$137k$1 digitaldaemon.com...

 is a bit annoying and reduces readability.  ITI is also quite useful in 
 C++ with respect to how overload resolution is handled, but I have no idea 
 how this will translate to the far simpler overloading rules in D. I think 
 we really may have to simply want and see what Walter comes up with for 
 ITI in D, as simply eliminating the need to explicitly specify template 
 parameters doesn't encompass the full utility of ITI in C++. Will we be 
 able to overload template functions with each other?


 
 Yes.
 

 Will they also overload with non-template functions?


 
 No. This is a misfeature in C++, and the equivalent can be done with 
 explicitly specialized template functions.


Cool.  I feel pretty much the same.


 What happens if there are multiple matches?

     class C(T) {}

     template func(T) { void func( T val ) {} }
     template func(T) { void func( C!(T) val ) {} }
     void func( C!(int) val ) {}

     func( new C!(int) );

 In C++, this is legal and the result is clearly defined (the "most 
 specialized" overload is called), but how will this translate to D?


 
 It'll be the most specialized.


Very nice :-)  Yet more to look forward to!


Sean
Mar 04 2006
prev sibling next sibling parent reply Hasan Aljudy <hasan.aljudy gmail.com> writes:
Derek Parnell wrote:

 I'm sure its because I'm not from a C++ background, but I just don't 
 get  templates. What is the problem that Templates solve?
 
  From my limited experience they just seem to be a shorthand for a way 
 of  avoiding coding identical algorithms that only vary in the datatypes 
 that  are passed to them. Is that it or am I missing some fundamental 
 elements?
 
 And is that why people are clamoring for implicit template instantation 
 -  to further reduce the amount of typing to do (and remembering to  
 instantiate the only the required renditions of the algorithm).
 


I feel the same.
I do come from C++ background, but I just hate templates! (Now I hate 
C++ itself :)
I think templates were introduced in C++ to solve the "no-base-class" 
problem. In other words, it's a hack that solves a misfeature in C++.

D's templates are not really templates any more, they're more like a 
meta-programming facility. Or so I think, anyway. I don't really 
understand templates anyway, not to mention meta programming
Mar 04 2006
parent reply Ivan Senji <ivan.senji_REMOVE_ _THIS__gmail.com> writes:
Hasan Aljudy wrote:

 I feel the same.
 I do come from C++ background, but I just hate templates! (Now I hate 
 C++ itself :)
 I think templates were introduced in C++ to solve the "no-base-class" 
 problem.


It is the other way around, languages introduce common base class to 
solve the no-templates-problem. (Not a very good solution IMO).


 In other words, it's a hack that solves a misfeature in C++.
 
 D's templates are not really templates any more, they're more like a 
 meta-programming facility. Or so I think, anyway. I don't really 
 understand templates anyway, not to mention meta programming
Mar 05 2006
parent reply Hasan Aljudy <hasan.aljudy gmail.com> writes:
Ivan Senji wrote:

 Hasan Aljudy wrote:
 

 I feel the same.
 I do come from C++ background, but I just hate templates! (Now I hate 
 C++ itself :)
 I think templates were introduced in C++ to solve the "no-base-class" 
 problem.


 
 
 It is the other way around, languages introduce common base class to 
 solve the no-templates-problem. (Not a very good solution IMO).
 


No. The Object base class fits perfectly into the object oriented 
paradigm. Templates do not, hence a hack.


 In other words, it's a hack that solves a misfeature in C++.

 D's templates are not really templates any more, they're more like a 
 meta-programming facility. Or so I think, anyway. I don't really 
 understand templates anyway, not to mention meta programming
Mar 05 2006
next sibling parent Lars Ivar Igesund <larsivar igesund.net> writes:
Hasan Aljudy wrote:


 Ivan Senji wrote:

 Hasan Aljudy wrote:
 

 I feel the same.
 I do come from C++ background, but I just hate templates! (Now I hate
 C++ itself :)
 I think templates were introduced in C++ to solve the "no-base-class"
 problem.


 
 
 It is the other way around, languages introduce common base class to
 solve the no-templates-problem. (Not a very good solution IMO).
 


 
 No. The Object base class fits perfectly into the object oriented
 paradigm. Templates do not, hence a hack.
 

 In other words, it's a hack that solves a misfeature in C++.

 D's templates are not really templates any more, they're more like a
 meta-programming facility. Or so I think, anyway. I don't really
 understand templates anyway, not to mention meta programming






Luckily there is much more to programming than just OOP. :)
Mar 05 2006
prev sibling parent reply Ivan Senji <ivan.senji_REMOVE_ _THIS__gmail.com> writes:
Hasan Aljudy wrote:

 Ivan Senji wrote:
 

 Hasan Aljudy wrote:


 I feel the same.
 I do come from C++ background, but I just hate templates! (Now I hate 
 C++ itself :)
 I think templates were introduced in C++ to solve the "no-base-class" 
 problem.




 It is the other way around, languages introduce common base class to 
 solve the no-templates-problem. (Not a very good solution IMO).


 
 No. The Object base class fits perfectly into the object oriented 
 paradigm. Templates do not, hence a hack.


Maybe so, but I see a common Object base class a hack to solve many 
problems that are simple and efficient to solve with templates. For 
example containers. Containers in Java(before templates)-style and 


And one more thing <insert what Lars replyed> :)
Mar 05 2006
parent reply "Walter Bright" <newshound digitalmars.com> writes:
"Ivan Senji" <ivan.senji_REMOVE_ _THIS__gmail.com> wrote in message 
news:duf72h$f40$1 digitaldaemon.com...

 Maybe so, but I see a common Object base class a hack to solve many 
 problems that are simple and efficient to solve with templates. For 
 example containers. Containers in Java(before templates)-style and 




implement templates, not generics.

What's the difference? One way to think about it is generics are handled at 
run time (by casting), templates at compile time.
Mar 05 2006
next sibling parent reply Hasan Aljudy <hasan.aljudy gmail.com> writes:
Walter Bright wrote:

 "Ivan Senji" <ivan.senji_REMOVE_ _THIS__gmail.com> wrote in message 
 news:duf72h$f40$1 digitaldaemon.com...
 

Maybe so, but I see a common Object base class a hack to solve many 
problems that are simple and efficient to solve with templates. For 
example containers. Containers in Java(before templates)-style and 



 
 

 implement templates, not generics.
 
 What's the difference? One way to think about it is generics are handled at 
 run time (by casting), templates at compile time. 
 
 


Are you saying that all generics do is silently insert casts?
Mar 05 2006
next sibling parent James Dunne <james.jdunne gmail.com> writes:
Hasan Aljudy wrote:

 Walter Bright wrote:
 

 "Ivan Senji" <ivan.senji_REMOVE_ _THIS__gmail.com> wrote in message 
 news:duf72h$f40$1 digitaldaemon.com...


 Maybe so, but I see a common Object base class a hack to solve many 
 problems that are simple and efficient to solve with templates. For 
 example containers. Containers in Java(before templates)-style and 






 implement templates, not generics.

 What's the difference? One way to think about it is generics are 
 handled at run time (by casting), templates at compile time.


 
 Are you saying that all generics do is silently insert casts?



serialization of code into JARs and assemblies.  How would you serialize 
a template?

-- 
Regards,
James Dunne
Mar 05 2006
prev sibling parent Bruno Medeiros <daiphoenixNO SPAMlycos.com> writes:
Hasan Aljudy wrote:

 Walter Bright wrote:

 "Ivan Senji" <ivan.senji_REMOVE_ _THIS__gmail.com> wrote in message 
 news:duf72h$f40$1 digitaldaemon.com...


 Maybe so, but I see a common Object base class a hack to solve many 
 problems that are simple and efficient to solve with templates. For 
 example containers. Containers in Java(before templates)-style and 





 implement templates, not generics.

 What's the difference? One way to think about it is generics are 
 handled at run time (by casting), templates at compile time.


 
 Are you saying that all generics do is silently insert casts?


For all interested, here's an article that explains the differences in 
more detail:
http://www-128.ibm.com/developerworks/library/j-jtp01255.html
This was posted in the NG by someone else that I don't recall, some time 
ago.


-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D
Mar 05 2006
prev sibling next sibling parent Ivan Senji <ivan.senji_REMOVE_ _THIS__gmail.com> writes:
Walter Bright wrote:

 "Ivan Senji" <ivan.senji_REMOVE_ _THIS__gmail.com> wrote in message 
 news:duf72h$f40$1 digitaldaemon.com...
 

Maybe so, but I see a common Object base class a hack to solve many 
problems that are simple and efficient to solve with templates. For 
example containers. Containers in Java(before templates)-style and 



 
 

 implement templates, not generics.
 
 What's the difference? One way to think about it is generics are handled at 
 run time (by casting), templates at compile time. 
 




as templates? That sucks.
Mar 05 2006
prev sibling next sibling parent reply "John C" <johnch_atms hotmail.com> writes:
"Walter Bright" <newshound digitalmars.com> wrote in message 
news:dufcpn$p1n$2 digitaldaemon.com...

 "Ivan Senji" <ivan.senji_REMOVE_ _THIS__gmail.com> wrote in message 
 news:duf72h$f40$1 digitaldaemon.com...

 Maybe so, but I see a common Object base class a hack to solve many 
 problems that are simple and efficient to solve with templates. For 
 example containers. Containers in Java(before templates)-style and 




 implement templates, not generics.

 What's the difference? One way to think about it is generics are handled 
 at run time (by casting), templates at compile time.



do erase the type information and cast everything to an object. But the CLR 
was reworked to support generics so List<int> remains a list of ints until 
it's instantiated at runtime.
Mar 05 2006
parent reply "Walter Bright" <newshound digitalmars.com> writes:
"John C" <johnch_atms hotmail.com> wrote in message 
news:dufl8l$14au$1 digitaldaemon.com...

 "Walter Bright" <newshound digitalmars.com> wrote in message 
 news:dufcpn$p1n$2 digitaldaemon.com...


 implement templates, not generics.

 What's the difference? One way to think about it is generics are handled 
 at run time (by casting), templates at compile time.



 do erase the type information and cast everything to an object. But the 
 CLR was reworked to support generics so List<int> remains a list of ints 
 until it's instantiated at runtime.



of argument types, it implements *one* routine which handles them all via 
casting.
Mar 05 2006
next sibling parent reply Don Clugston <dac nospam.com.au> writes:
Walter Bright wrote:

 "John C" <johnch_atms hotmail.com> wrote in message 
 news:dufl8l$14au$1 digitaldaemon.com...

 "Walter Bright" <newshound digitalmars.com> wrote in message 
 news:dufcpn$p1n$2 digitaldaemon.com...


 implement templates, not generics.

 What's the difference? One way to think about it is generics are handled 
 at run time (by casting), templates at compile time.



 do erase the type information and cast everything to an object. But the 
 CLR was reworked to support generics so List<int> remains a list of ints 
 until it's instantiated at runtime.


 

 of argument types, it implements *one* routine which handles them all via 
 casting. 


It looks as though it has an optimisation for built-in types.
http://www.artima.com/intv/generics2.html


100% syntactic sugar.

There's a valid point raised there about the cryptic error messages 
generated in C++ templates. Currently D error messages for templates are 
even worse, but this is mainly because it keeps tying to instantiate 
templates after one has failed (and you can't even press ctrl-c to stop 
the torrent, which can last for over a minute). This is another area 
where D could give a great improvement over C++.
Mar 06 2006
next sibling parent "John C" <johnch_atms hotmail.com> writes:
"Don Clugston" <dac nospam.com.au> wrote in message 
news:dugufa$2udq$1 digitaldaemon.com...

 Walter Bright wrote:

 "John C" <johnch_atms hotmail.com> wrote in message 
 news:dufl8l$14au$1 digitaldaemon.com...

 "Walter Bright" <newshound digitalmars.com> wrote in message 
 news:dufcpn$p1n$2 digitaldaemon.com...


 implement templates, not generics.

 What's the difference? One way to think about it is generics are 
 handled at run time (by casting), templates at compile time.



 generics do erase the type information and cast everything to an object. 
 But the CLR was reworked to support generics so List<int> remains a list 
 of ints until it's instantiated at runtime.



 set of argument types, it implements *one* routine which handles them all 
 via casting.




I just wanted to put the record straight. For reference types, the same code 
is shared, but full type information is maintained, and there's no 
Object-to-type cast. With value types and structs, separate sections of code 
are created to explicitly avoid boxing/unboxing.

 It looks as though it has an optimisation for built-in types.
 http://www.artima.com/intv/generics2.html


There's a more technical overview here:
http://research.microsoft.com/projects/clrgen/generics.pdf



 100% syntactic sugar.

 There's a valid point raised there about the cryptic error messages 
 generated in C++ templates. Currently D error messages for templates are 
 even worse, but this is mainly because it keeps tying to instantiate 
 templates after one has failed (and you can't even press ctrl-c to stop 
 the torrent, which can last for over a minute). This is another area where 
 D could give a great improvement over C++.


Yes, template error messages are notoriously difficult to decipher. 
Compilers need to address this.
Mar 06 2006
prev sibling parent Matthias Spycher <matthias coware.com> writes:
Don Clugston wrote:



 100% syntactic sugar.


Don't forget that a dynamic compiler can do anything at runtime if the 
byte-code carries forward enough info about the original source -- class 
files are extensible. I have seen anonymous local classes reduced to 
simple functions. With decent escape analysis, allocation on the stack 
is possible for small value classes. Research on this topic, IMHO, has 
only scratched the surface.
Mar 06 2006
prev sibling parent "Andrew Fedoniouk" <news terrainformatica.com> writes:

 set of argument types, it implements *one* routine which handles them all 
 via casting.


Yep.

And more here:
Templates and Generics 
http://blogs.msdn.com/branbray/archive/2003/11/19/51023.aspx
Mar 06 2006
prev sibling parent reply Mark T <Mark_member pathlink.com> writes:
In article <dufcpn$p1n$2 digitaldaemon.com>, Walter Bright says...


implement templates, not generics.

What's the difference? One way to think about it is generics are handled at 
run time (by casting), templates at compile time. 


sorry "generics" is a generic term, let's let not pigeon-hole it, Ada uses the
term and it is compile time  

the term "template" was originally associated with patterns, 

"In computer science, generics is a technique that allows one value to take
different datatypes (so-called polymorphism) as long as certain contracts such
as subtypes and signature are kept. The programming style emphasizing use of
this technique is called generic programming."
en.wikipedia.org/wiki/Template_(programming)

"A generic term is one which picks out a class of individuals, or the prototype
of the individual, rather than the individual itself. For example, in the
sentence `The wolf has disappeared from northern Europe,' we are referring to
the genus rather than to a particular wolf."
www.informatics.susx.ac.uk/books/computers-and-thought/gloss/node1.html
Mar 08 2006
parent Georg Wrede <georg.wrede nospam.org> writes:
Mark T wrote:

 
 [T]he term "template" was originally associated with patterns,
 
 "In computer science, generics is a technique that allows one value
 to take different datatypes (so-called polymorphism) as long as
 certain contracts such as subtypes and signature are kept. The
 programming style emphasizing use of this technique is called generic
 programming." en.wikipedia.org/wiki/Template_(programming) [...]"


...

 www.informatics.susx.ac.uk/books/computers-and-thought/gloss/node1.html


Put like that, it becomes obvious how much of the generic stuff can be 
done with Interfaces.

Even with the STL, all the stuff that trickles through "my custom made 
procedures and functions" needs certain properties. C++ did a good job 
of integrating the fundamental data types and structures to this.

With anything that's objects, D can already be very powerful when one 
combines well factored interfaces with a solid set of orthogonal (not 
necessarily member-) functions.

I love D!
Mar 09 2006
prev sibling parent reply Tom <Tom_member pathlink.com> writes:
In article <op.s5wxef016b8z09 ginger.vic.bigpond.net.au>, Derek Parnell says...

I'm sure its because I'm not from a C++ background, but I just don't get  
templates. What is the problem that Templates solve?


I'm passing away... my pressure!!! :P


 From my limited experience they just seem to be a shorthand for a way of  
avoiding coding identical algorithms that only vary in the datatypes that  
are passed to them. Is that it or am I missing some fundamental elements?


Isn't this issue enough to justify templates?

I have some interesting experience with C++, I'm not an expert even though I've
coded a lot in it and I've used "nearly all" (that means excluding the infamous
ones such as those modern cast operators :), etc.). I have no doubts, I can say
that one of the best features I've ever seen working in a language is the C++
templates.

I've coded really generic functions/containers with a great success in C++ and
used them with a variety of stuff. Always worked nice!


And is that why people are clamoring for implicit template instantation -  
to further reduce the amount of typing to do (and remembering to  
instantiate the only the required renditions of the algorithm).


Tom;
Mar 04 2006
parent Sebastián E. Peyrott <as7cf yahoo.com> writes:
Well, I have no real experience with templates, either, but I do think there are
many interesting (and useful) coding techniques that require them. And even
though, I, for the most part, and as Derek, see them as a way of coding similar
algorithms based on different data-types, IMO the most important characteristic
is compile-time code generation. When to use it, and how to take advantage of
that feature, be it to create abstractions of similar algorithms based on
differing data-types, or not, is a whole other issue.
I'm not entirely sure whether there are other ways of achieving the same thing,
so   templates, for the most part, seem like an important feature for a language
such as D.

--
Sebastián.
Mar 04 2006