Re: ({

On 2025-03-21 11:54, Paavo Helde wrote:

On 20.03.2025 20:27, Alan Mackenzie wrote:

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; })
>
.  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.

 This braced initialization (called "List-initialization") was introduced in C++11, it is standardized and in wide and growing use.
 

Actually, it is from K&R C, except (perhaps) the option to put the list expression in a pair of regular parenthesis with the usual identical meaning.  The basic K&R form is:
int smallprimes[4] = { 2, 3, 5, 7 };
int identitymatrix[2][2] = { { 1, 0 }, { 0, 1 } };
In ANSI C, these can be declared const and placed in a read-only part of the resulting program file.  This was broken by the desire to allocate all data on the heap via heap-centric classes like vector<int> .
   Enjoy
   Jakob
--
Jakob Bohm, MSc.Eng., I speak only for myself, not my company
This public discussion message is non-binding and may contain errors
All trademarks and other things belong to their owners, if any.