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Message-ID: <20150417193320.GA11349@wilbur.25thandClement.com> Date: Fri, 17 Apr 2015 12:33:20 -0700 From: William Ahern <william@...handClement.com> To: musl@...ts.openwall.com Subject: Re: Explicit casts in ctype.h suppress compiler warnings On Fri, Apr 17, 2015 at 12:52:38PM -0400, Rich Felker wrote: > On Fri, Apr 17, 2015 at 06:49:54PM +0200, Jens Gustedt wrote: <snip> > > I generally think that casts are a bad idea, anyhow, and should only > > be used where it must be done, that is basically for pointer to > > integer conversion (and back). Code like this > > > > #define isdigit(a) (((unsigned)(a)-'0') < 10) > > > > can easily be replaced by > > > > #define isdigit(a) (((unsigned const){a}-'0') < 10) > > > > to change the explicit conversion to an implicit one in the > > initializer of the compound literal. Then, any compiler would have to > > diagnose if "a" would be a pointer. > > In another place (math.h) I removed this type of compound literal > usage because it was incompatible with C++, but the macros are > suppressed in C++ anyway. Still they might break -pedantic with > -std=c89. I do like this approach best in principle if it works > though, because the rules for when an error occurs are basically the > same as the rules for a real function. I think C++11 brace initialization int { 42 } works nearly identically to C99 compound literal definition (int){ 42 } The big difference is that the lifetime of C++ temporaries have expression scope, whereas compound literals have block scope. But that's irrelevant where the values will be copied and no pointer is derived.* A simple macro could be used to select the syntax. However, I don't think something like `unsigned char { 42 }' will work. The type name needs to be a single identifier, so a typedef would be needed: u_char { 42 }. * Unfortunately, by default g++ accepts C99-style compound literals, but for whatever reason gives them expression-scoped lifetimes as-if they were C++ temporaries. This gave me and some other people grief when when using pointers to compound literals, either explicitly or implicitly through array-to-pointer decay). See Bug #53220. Everything seemed to work when compiled as C++, unless you were fortunate enough to get a crash near the offending code. More recent versions of g++ now at least warn when a pointer is derived from a compound literal. I think clang++ has the same behavior.
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