Re: libstdc++/9679: Strange behaviour of valarray::apply method

[Gabriel Dos Reis <gdr@integrable-solutions.net>]

The following reply was made to PR libstdc++/9679; it has been noted by GNATS.

From: Gabriel Dos Reis <gdr@integrable-solutions.net>
To: dens@stl.sarov.ru
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: libstdc++/9679: Strange behaviour of valarray::apply method
Date: 12 Feb 2003 23:33:04 +0100

 dens@stl.sarov.ru writes:
 
 | >Number:         9679
 | >Category:       libstdc++
 | >Synopsis:       Strange behaviour of valarray::apply method
 
 strange but standard conforming with C++98, not C++0x...
 
 [...]
 
 | #include <valarray>
 | #include <string.h>
 | char bar(char arg)
 | {
 |     return tolower(arg);
 | }
 | 
 | char foo(char * x)
 | {
 |     return toupper(*x);
 | }
 | 
 | int main(void)
 | {
 |         char * str = new char[7];
 |         strcpy(str,"STRING");
 | 
 |         std::valarray<char> y(str, 6);
 | ///   Strange behaviour    
 |         char some = foo(&(y.apply(bar)[0]));
 |         return 0;
 | }
 | 
 | If I understood correctly then apply method should return valarray<char> object which operator [] should return reference to char (char &). (C++ standard 26.3.2.3 and 26.3.2.7 8.)
 | 
 | But in our case this operator returns non-lvalue and I can't get it address. It is very strange for me.
 
 Here is what is happening.  Firstly, let's recall
 
 26.3.1/3:
   Any function returning a valarray<T> is permitted to return an
   object of another type, provided all the const member functions of
   valarray<T> are also applicable to this type. This return type shall
   not add more than two levels of template nesting over the most
   deeply nested argument type.
 
 apply() being a const member function actually returns something that
 is -not- a valarray.  Subsequent application of operator[] to that
 object is considered to be application of "operator[] const" which is
 defined by C++08 to return an rvalue.  Therefore, pedantically
 speaking your program is not required to compile.
 
 However, I happen to be the other guy that champions 
 "valarray<T>::operator[] const" returning a const T&; that, in effect,
 conflicts with loosened return type.  Too bad :-(
 
 -- Gaby