• Jason House <jason.james.house gmail.com> Sep 23 2008

Jason House <jason.james.house gmail.com> writes:

Sergey Gromov Wrote:

 Sean Kelly <sean invisibleduck.org> wrote:
 Sergey Gromov wrote:
 Steven Schveighoffer <schveiguy yahoo.com> wrote:
 oops, D1 doesn't support that type of enum ;)


 I think you've got the idea. The code
 
 version (D_Version2)
 {
   mixin("some very specific code");
 }
 
 works in any version of D, current or future.


 String mixins don't work directly for everything though.  If you want to 
 change a return value from non-const to const, for example, you need to 
 make an alias for it using a versioned string mixin and then use the 
 alias in the function declaration.  And then there are things like this:


 template Const(T) {
   version (D_Version2) {
     alias const(T) Const;
   } else {
     alias T Const;
   }
 }
 
 then
 
 Const!(int) bar;
 
 is just one char more than
 
 const(int) bar;


This style of solution won't work for function declarations... There's no way
to mixin const/invariant/pure function declarations without taking the function
body along for the ride. If only we could find a way to make the D1 compiler
ignore those extra function attributes.




 


 
 // D1
 const x = "hello";
 
 // D2
 enum x = "hello";
 
 With string mixins you end up having to duplicate the entire line, and 
 this can be a disaster if you're trying to maintain large header files 
 with tons of declarations.


 const keyword can be used to declare manifest constants in D2:
 
 const x = "foo";
 pragma(msg, typeof(x).stringof);	// invariant(char[3u])
 
 Finally, I think the version(D_Version2) idea is backwards.  It should 
 be version(D_Version1).  The current method isn't forwards-compatible, 
 so all the code with these version statements in it will break when we 
 get D version 3.


 I agree here, there's not enough flexibility.  The version identifiers 
 should also have a numeric value, so that you can write:
 
 version (D_Version, 2)
 {
   ...
 }
 
 and the versioned code compiles only if D_Version has value of 2 or 
 greater.