🚀 go-pugleaf

Greg McGary (gkm@magilla.cichlid.com) wrote:

|> According to sec. 4.4: Qualification conversions, paragraph 3, the
|> following is NOT a const-safe conversion:

|>  char * *  --> char const * *

|> whereas this is safe:

|>  char * *  --> char const *const *

|> Why?  I don't see how *adding* `const' an any level of a multi-level
|> pointer could lessen safety.

Consider the following:

    char const      c = 'a' ;
    char*           p ;
    char**          pp = &p ;
    char const**    ppc = pp ;      //  Supposing that this were not
                                    //      illegal
    *pp = &c ;						//	Oops: where does p point
    *p = 'b' ;                      //  And what is wrong here?

This was first pointed out during the development of the C standard, and
the standard was changed to eliminate the implicit casts for all but the
first level below the top.  During the C++ standardization, it was
pointed out that this went too far, and so the slightly looser rules of
C++ were adapted.  One of the members of the standards committee
actually proved that the current rules are optimal; everything safe is
allowed, and everything dangerous (like the above) is forbidden.
-- 
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
Conseils, études et réalisations en logiciel orienté objet --
              -- A la recherche d'une activité dans une region francophone

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