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digitalmars.D - const member function synatx?

reply im <no body.com> writes:
I read the page: http://www.digitalmars.com/d/2.0/const3.html
under section "Const Member Functions"

It didn't give any example, but following the example of 'invariant', it
should be:

const ReturnType memberFunc(param) {}

I think this is really confusing: is 'const' trying to specify the
'ReturnType' or the memberFunc?

And if I want to specify both, then I have to write:

const const ReturnType  memberFunc(param) {}, or
const const(ReturnType) memberFunc(param) {}

The 2 leading const in a row looks ugly and confusing.

Why not just use the C++'s postfix syntax for const member function? to put
'const' after the param, and before the body {}

ReturnType memberFunc(param) const {}

So I tried it (although undocumented); and to my glad, it worked (see code
below).

But there seems to have a semantic bug: I'm modifying class field in void f(A
o) const {...}, but the compiler didn't complain about it.

I think we should fix the confusing syntax, and use C++'s postfix syntax for
const member function; and also fix this bug.


$ cat const.d
===============================
class A{
public:
  A m_a;
}

class B{
public:
  A m_a;
  A a() const {
    return m_a;
  }
  void f(A o) const {  // without 'const' won't compile
    m_a = o;           // BUG, I'm modifying m_a
  }
/*
  const A v() {
    m_a = null;
    return m_a;
  }
*/
}

int main() {
  const B b = new B();

  A a = b.a();
  a.m_a = null;
  b.f(a);
//b.v();

  return 0;
}
===============================
$ dmd.exe const.d
d:\project\dmd\bin\..\..\dm\bin\link.exe const,,,user32+kernel32/noi;
Feb 15 2008
next sibling parent reply "Janice Caron" <caron800 googlemail.com> writes:
On 15/02/2008, im <no body.com> wrote:
  I think this is really confusing: is 'const' trying to specify the
  'ReturnType' or the memberFunc?

  And if I want to specify both, then I have to write:

  const const ReturnType  memberFunc(param) {}, or
  const const(ReturnType) memberFunc(param) {}

Actually, only const const(ReturnType) memberFunc(param) {} would work. This has been mentioned before, and for what it's worth, I agree with you. Just some comments though. Walter likes the existing syntax, because it means you can do: const { /* several functions */ } It is unusual ever to need to return a const(ReturnType), since why would you want to prevent the callee from modifying their copy of the return value? That said, it is very common to want to return a const(T)[] for some T, so the problem doesn't go away. const-at-the-end syntax does work. I don't know if this is a permanent feature or not, but currently you can say: const(ReturnType) memberFunc(param) const {} And finally, I have argued in the past (and still believe) that the D syntax should be const(this) const(ReturnType) memberFunc(param) {} For an extra six characters, you get complete clarity. const(this) would mean that the symbol "this" will (transitively) not be modified within its scope. Walter's desire to encompass multiple functions would thus be preserved. i.e. const(this) { /* several functions */ } and in addition, the new syntax lends itself to future expansion (such as const(outer), etc.). Unfortunately, last I heard, Walter wasn't too keen on this suggestion, possibly because it means typing six more characters. But either way, I definitely believe that const ReturnType memberFunc(param) {} /must/ have exactly one interpretation, and that interpretation should be const(ReturnType) memberFunc(param) {} and not, as is currently the case ReturnType memberFunc(param) const {} I agree that that /is/ confusing, and is one of those little creases in the new const system that we still need to iron out.
Feb 15 2008
parent Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Janice Caron wrote:
 It is unusual ever to need to return a const(ReturnType), since why
 would you want to prevent the callee from modifying their copy of the
 return value?

How about returning a class reference that you still have your own copy to? One common example of this might be getters for properties of class type, especially if the class instance is part of the internal state of the owning class and shouldn't be modified without going through the methods of the owner.
Feb 16 2008
prev sibling parent "Janice Caron" <caron800 googlemail.com> writes:
On 16/02/2008, Frits van Bommel <fvbommel remwovexcapss.nl> wrote:
 Janice Caron wrote:
  > It is unusual ever to need to return a const(ReturnType), since why
  > would you want to prevent the callee from modifying their copy of the
  > return value?


 How about returning a class reference that you still have your own copy to?

Good point. I'd forgotten about that one. Not that that makes any difference to im's original point, or to my reply, of course. The unintuitive meaning of const ReturnType f(params...) still remains.
Feb 16 2008