Re: Function returning a literal string, always the same pointer?

Thanks to all of you for your replies. I'm concluding the following:

Suppose std::exception::what() is implemented by just returning a string
literal:

  inline const char* std::exception::what() const throw()
  {
    return "std::exception";
  }

Such a what() implementation might not always return a pointer to one and
the same object, when it is called multiple times during the execution of a
program. And that is because those what() calls might take place in
different "modules" that are linked at runtime (e.g., dynamically linked
libraries), that might have different instances of the literal string. This
/might/ look like a violation of [dcl.fct.spec]/4 (7.1.2/4 in N2914), but it
is not, because the C++ Standard simply does not deal with such modules.

The Standard C++ Library Working Group has discussed whether to specify that
a copy of an std::exception would return the same pointer as the original,
in Bellevue: www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#471 It
was considered "probably an overspecification", and I probably agree. :-)
Still for an implementation that just returns a literal, it's safe to say
that the following condition holds, for any two objects lhs and rhs of type
std::exception (or a class derived from std::exception):

   lhs.std::exception::what() == rhs.std::exception::what()

Just because both what() calls would be in the very same translation unit,
so [dcl.fct.spec]/4 would certainly apply. :-)

Kind regards, Niels
--
Niels Dekker
http://www.xs4all.nl/~nd/dekkerware
Scientific programmer at LKEB, Leiden University Medical Center

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