Optional extra return value? Multiple return values with auto?

[Ali Çehreli]

On 08/11/2012 03:48 PM, ReneSac wrote:
 > On Tuesday, 24 July 2012 at 05:30:49 UTC, Ali Çehreli wrote:

 >> - Use an out parameter, which can have a default lvalue:
 >>
 >> int g_default_param;
 >>
 >> void foo(ref int i = g_default_param)
 >> {
 >> if (&i == &g_param) {
 >> // The caller is not interested in 'i'
 >>
 >> } else {
 >> // The caller wants 'i'
 >> i = 42;
 >> }
 >> }
 >>
 >> void main()
 >> {
 >> foo();
 >>
 >> int i;
 >> foo(i);
 >> assert(i == 42);
 >> }
 >
 > This is not working inside a class. I'm not sure what default value I
 > should put when I don't know the type entered:
 >
 > class a (T) {
 > T dummy = T.init;
 > bool foo(int a, out T optional = dummy)
 > {
 > return true;
 > }
 > }
 >
 > void main () {
 > auto c = new a!uint();
 > c.foo(5);
 > }
 >
 > I get the following error:
 >
 > Error: need 'this' to access member dummy
 >

I am not a fan of this solution either. To make the code to compile, 
define dummy as static:

     static T dummy = T.init;  // <-- this works

That way there will be just one copy for the entire type, instead of one 
copy per object.

I also tried to define it as immutable but the following line fails to 
compile:

     static immutable T dummy = T.init;  // <-- compilation error

I thought that a solution would be to define 'static this()':

class a (T){
     static immutable T dummy;

     static this()
     {
         dummy = T.init;
     }

     bool foo(int a, out T optional = dummy)
     {             // <-- compilation error on this line
         return true;
     }
}

Still fails to compile:

   Error: cast(uint)dummy is not an lvalue

The error is on the line that I have pointed in the code. I think this 
is a compiler bug. T is not a reference type and 'cast(uint)dummy' not 
being an lvalue should not matter.

I tried an enum too but a different error on the same line:

     static enum T dummy = T.init;

   Error: constant 0u is not an lvalue

Ali