Why no constraint templates ?

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2 total messages Started by Valery Croizier Thu, 05 Oct 1995 18:05

#2123

Author: Valery Croizier
Date: Thu, 05 Oct 1995 18:05

30 lines
1497 bytes

Sometimes, I wish I could oblige the user of a template class to
inherit from a given class:

class Shape {
public:
  void draw() = 0;
};

template <class T inherits from Shape>
class ListOfShape {
public:
// ...
  void drawAll();
};

Of course, the above is just an example, and we can find many other
use of such a kind of template.  The main advantage is compile-time
type-checking: when you write the implementation of the template class,
you cannot call a bad method.

  o Is there a way I have not noticed to do this ?
  o Does anybody know the reason why the team working on the
standard rejected this feature ?

--
Valery

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Re: Why no constraint templates ?

#2129

Author: Steve Clamage
Date: Thu, 05 Oct 1995 18:43

33 lines
1830 bytes

In article 11m3@melimelo.enst-bretagne.fr, croizier@rennes.enst-bretagne.fr (Valery Croizier) writes:
>Sometimes, I wish I could oblige the user of a template class to
>inherit from a given class: ...
>
> ...
>  o Is there a way I have not noticed to do this ?
>  o Does anybody know the reason why the team working on the
>standard rejected this feature ?

This subject is discussed in D&E, including why C++ templates do not
have an explicit constraint mechanism.

Your particular case can be handled like this:

template< class T > // T must be derived from Shape
class C {
	C() { static_cast<Shape*>(static_cast<T*>(0)); ... }
};

The constructor won't compile unless there is an implicit conversion from
T* to Shape*, meaning that T* must be or be derived from Shape. The
result of the casts isn't used, so no code should be generated. 

If your compiler doesn't support static_cast, you can write
	Shape* p = (T*)0; // requires implicit conversion from T* to Shape*
---
Steve Clamage, stephen.clamage@eng.sun.com

[ comp.std.c++ is moderated.  Submission address: std-c++@ncar.ucar.edu.
  Contact address: std-c++-request@ncar.ucar.edu.  The moderation policy
  is summarized in http://dogbert.lbl.gov/~matt/std-c++/policy.html. ]

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