Re: C++0x static_assert(0, "") within a non-instantiated template okay?

From:
AlbertoBarbati@libero.it (Alberto Ganesh Barbati)
Newsgroups:
comp.std.c++
Date:
Sun, 1 Jul 2007 14:22:21 GMT
Message-ID:
<QfLhi.52896$U01.530279@twister1.libero.it>
Niels Dekker - no return address ha scritto:

  [ Thanks Greg, for revitalizing the discussion ]

Alberto Ganesh Barbati wrote:

So it's not true that sizeof(T) > 0 for *every* type. It's true only
for *complete* types (those that can be instantiated to object).


I hope you still agree that there is *no* type T, for which
(sizeof(T)==0) evaluates to true...


Correct.

The compiler cannot "optimize" the expression sizeof(T) == 0 assuming that
it always evaluates to false, because it might be ill-formed for some T
(for example: void). This is enough to force the compiler to treat the
expression as type-dependent.


Interesting. Do you mean to say that if a program is ill-formed because
of a static_assert(sizeof(T)==0, "") within an uninstantiated template,
the compiler would not be allowed to say so? If so, the program would
be "ill-formed, no diagnostic /allowed/" !!!


No. The diagnostic is still required: the compiler should complain that
the sizeof operator is applied to an invalid type ([expr.sizeof] para
1). So the program is ill-formed in both cases, but reason of
ill-formedness is different: in order to provide the correct diagnostic
the compiler must treat the expression as type-dependent.

BTW, I guess that by definition sizeof(T) > 0 is true for every
*DefaultConstructible* type T. So a C++0x compiler would be *allowed*
to diagnose the following template definition, even without being
instantiated:

  #include <concepts> // For the std::DefaultConstructible concept.
  
  template<std::DefaultConstructible T>
  void TemplateFunc(void) {
    static_assert(sizeof(T)==0, "Ill-formed!");
  }

I would rather have the C++0x Standard stating explicitly that a static
assert declaration has no effect within an uninstantiated template.


Hmm... I haven't fully grasped all the details of concepts, so I may be
wrong, but can't you declare a type to be DefaultConstructible before
providing a definition for such type? In that case you would have a
DefaultConstructible type which is still incomplete and therefore you
cannot apply sizeof() on it and we are still in the same situation as
above. I agree that's a corner case, however...

  template<int> struct int_dependent_false {
    enum { value = false }; };

  template<const void*> struct ptr_dependent_false {
    enum { value = false }; };

  template<template<class> class> struct template_dependent_false {
    enum { value = false }; };

Unfortunately the list could go on and on, for each possible kind of
template template argument:

  template<template<class, class> class>
  struct template_dependent_false2 {
    enum { value = false }; };

  template<template<class, class, class> class>
  struct template_dependent_false3 {
    enum { value = false }; };

Is there a more "generic" technique to implement an "always false
struct" for each kind of template argument? Gennaro Prota and I
recently had an e-mail discussion on this approach as well, but we
couldn't think of an alternative...


These looks very much like tr1 type_traits to me... Those, together with
MPL techniques seem a very good alternative to me.

Ganesh

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