const - How can I return imutable reference types without break constness? -

i have following code in d:

import std.stdio; import std.container.array;  class reftype { }  class mycontainer {     private array!reftype test;     reftype result() const {  // want const on this, not on return type         return test[0];       // use opindex() array(t)         // error: cannot implicitly convert expression (this.test.opindex(0u))          // of type const(reftype) main.reftype     } }  int main(string[] argv) {     auto c = new mycontainer; auto r = c.result(); return 0; } 

as can see want return reference type custom container class. opindex() of array not giving permission that, why?

i think opindex() should return reftype instead of const(reftype) value because array array!reftype.

is bug? or design intention? if intented design, how can want?

i think opindex() should return reftype instead of const(reftype) value because array array!reftype.

no, wrong. when define memthod const, have specified members of class const. means that, no matter type member has in definition, inside result method (at least) const. such, compiler justified in returning error returning.

the usual (e.g. c++) way resolve define 2 (or, in d, three) overloads:

reftype result() {     return test[0]; } reftype result() const {     return test[0]; } reftype result() immutable {     return test[0]; } 

so, 1 case class mutable, returns mutable reference. 1 case class const, returns const reference, , 1 case class immutable, returns immutable reference.

you notice, however, aside method decoration, 3 implementations precisely same. prevent defining 3 identical implementations, d has inout modifier:

reftype result() inout {     return test[0]; } 

this single definition replaces 3 of definitions above. rule of thumb, write definition usual, place inout place you'd otherwise place const.