Re: ({

Am 20.03.2025 um 19:27 schrieb Alan Mackenzie:

Hello, C++.
 I'm having some difficulty (amending Emacs's C++ Mode) reconciling the
two conflicting uses in C++ of ({.
 Firstly, it is used as a "statement expression", a GCC enhancement also
found in C, allowing a more relaxed and natural way to write an
expression as the end result of a sequence of statements:
       ({ int y = foo (); int z;
         if (y > 0) z = y;
else z = - y;
z; })

If the sign of y is badly predictible you may chose:
z = (y >> sizeof(y) * CHAR_BIT - 1) * y;

.  I think this usage is very old.
 Secondly, there's initialisation expressions like:
    void f4 (int a, int b, int c)
   {
     std::vector<ABC> abcList2(
       {{a+6,
         b+6,
         c+6}
       }
       );
   ....
   }
 .  Here the ( on the std::vector line, together with the next {, can be
confused as a statement expression, though it's clearly not meant that
way.  I think this syntax is much newer than the other one, though I may
be wrong here.
 In calculating the indentation for source lines in these constructs, the
ambiguity causes mis-indentation for one or the other of them.
 Now to my question: how common is GCC's statement expression in the wide
world of C++ source code?  How much would be lost if I simply removed the
statement expression from C++ Mode's parsing functions?