Immutable member functions and private members

[Jonathan M Davis]

On Wednesday 03 August 2011 11:44:27 simendsjo wrote:
> On 03.08.2011 10:52, Jonathan M Davis wrote:
> > On Wednesday 03 August 2011 10:37:58 simendsjo wrote:
> >> I have a struct with a private member that is only ever accessed
> >> through
> >> a single property method - even from within the struct.
> >> As this property fills the value on the first access, it cannot be
> >> immutable, and as such, none of the many methods accessing this
> >> property
> >> can be immutable methods.
> >> 
> >> This is according to specification, but I thought that since the
> >> single
> >> write to the property is done at one, and only one, access point, that
> >> it would be safe?
> >> 
> >> I could fill this value in the constructor, but it's a bit slow, so
> >> I'd
> >> rather do it only if needed.
> >> 
> >> And is there any potential performance optimizations done by the
> >> compiler, or is it "only" for safety?
> >> Is there a way to hack around this, and more importantly, is it safe
> >> to
> >> do so, or will I open Pandora's box?
> >> 
> >> 
> >> Small example:
> >> 
> >> int len(const char[] c) {
> >> 
> >>       return c.length;
> >> 
> >> }
> >> 
> >> struct S {
> >> 
> >>       private immutable(char)[] _v;
> >>       @property immutable(char[]) v() { // Cannot be immutable
> >>       method
> >>       
> >>           if(!_v)
> >>           
> >>               _v = "init"; /* or from external function
> >>               */
> >>           
> >>           return _v;
> >>       
> >>       }
> >>       
> >>       @property int a() { // and so this cannot be immutable
> >>       method
> >>       
> >>           return len(v); /* notice the property function v
> >>           that might
> >> 
> >> modify _v */
> >> 
> >>       }
> >> 
> >> }
> >> 
> >> void main() {
> >> 
> >>       S s;
> >>       s.a;
> >> 
> >> }
> > 
> > You're basically looking for logical const - albeit a subset which would
> > be much easier to implement were we to implement it (that is, a lazy
> > initialized const or immutable member variable). D has no support for
> > logical const. Even worse, you're looking for logical immutable (which
> > makes no sense at all beyond perhaps lazy initialization and probably
> > doesn't even make sense there).
> > 
> > The thing is that immutable methods are pointless unless you make the
> > struct immutable (if you want to be able to call them with both a
> > mutable and immutable instance of the struct, then you need the
> > functions to be const, not immutable). And if you make the struct
> > immutable, the compiler is free to put it in read-only memory if it so
> > chooses, at which point setting _anything_ in the struct after the
> > constructor has run is likely to blow up. So, even if you can get
> > around the issue via casts and get both lazy initialization and
> > immutable methods, there's a good chance that it'll blow up at some
> > point (as in segfault or worse).
> > 
> > If you were trying to do this with const, you might get away with it
> > (though you'd be stepping outside of the type lsystem by casting away
> > const and then altering anything - it's undefined behavior). But with
> > immutable, there's no way that this is a good idea.
> > 
> > Lazy initialization with const or immutable member variables just is
> > _not_ a good idea in D. D provides no type-safe way to do this. You
> > must break the type system by casting away const or immutable to even
> > attempt it. Convievably, in the case of const, the language could be
> > extended to allow for lazy initialization of member variables, but
> > there's no way that it could do that with immutable (because the
> > variable could conceivably be put in read- only memory), and even if it
> > were done, it would likely have to be a D3 feature. Syntactically, it
> > would probably be something like this:
> > 
> > lazy int v = initFunc();
> > 
> > and then when v was first accessed, initFunc would be called and v set
> > to that value. But that could be ugly and inefficient to implement even
> > if it's theoretically possible, so I wouldn't bet on anything like that
> > making it into the language. Regardless, it wouldn't be until D3. For
> > now, D doesn't support any kind of logical const.
> > 
> > http://stackoverflow.com/questions/4219600/logical-const-in-d
> > 
> > - Jonathan M Davis
> 
> Thanks!
> 
> I'm not really sure the compiler could put my struct in ROM.
> My lazy parameter is immutable(char)[], so the compiler should see that
> I have a non-immutable reference.
> 
> The entire struct is immutable without this lazy variable though.
> It only has two handles for passing to external functions.
> I really would like to always use it only as immutable s = S(123). It
> makes no sense for it to be mutable at all.
> 
> Below is an exact example of what I want to do.
> If I move the handle2 calculation to the ctor and use const methods, I
> still cannot call the methods using a const variable though.. Bug?
>    const s = S(100);
>    s.a; // function t.S.a () immutable is not callable using argument type
> It says immutable when it should say const..?
> 
>    immutable s = S(100);
>    s.a; // works on both const and immutable

If a variable is const, you should only be able to call const functions on it. 
If it's immutable, you can call either const or immutable functions. If it's 
mutable, then you can call either const or non-const, non-immutable functions. 
If it's complaining about being unable to call a function on an immutable 
variable when the variable is const, then it's a bug.

> ----
> 
> import std.conv, std.exception;
> 
> // external expensive function
> extern(System) char[] getHandle2(const int handle) {
>      return to!(char[])(handle);
> }
> 
> // other external functions taking string handle instead of int
> extern(System) int extFunc1(string handle2) {
>      return to!int(handle2);
> }
> 
> struct S {
>      immutable int handle;
>      private immutable(char)[] _handle2;
> 
>      this(int handle) {
>          this.handle = handle;
>      }
> 
>      @property immutable(char[]) handle2() {
>          if(!_handle2) {
>              auto buf = getHandle2(handle);
>              _handle2 = assumeUnique(buf);
>          }
>          return _handle2;
>      }
> 
>      @property int a() {
>          return extFunc1(handle2);
>      }
> 
>      // many more properties like this
> }
> 
> void main() {
>      auto s = S(100);
>      assert(s.a == 100);
> 
>      immutable s2 = S(100);
>      //assert(s2.a == 100); // oops, a not immutable
> }

You can't call a non-const, non-immutable function on an immutable variable. a 
must either be const or immutable to be callable here.

In the general case, I'd advise against trying to have member variables of 
structs which are either const or immutable, because then you can't assign to 
them, which means that you can't use them in arrays and the like - only as 
local variables which are directly initialized. As long as the struct is 
capable of being immutable, you can then have immutable variables of that type 
if you want to, but you can then also stick it in arrays and the like if need 
be. But until http://d.puremagic.com/issues/show_bug.cgi?id=4867 is fixed, 
dealing with const structs which need a postblit doesn't work, so depending on 
what you're doing using const or immutable with structs won't necessarily 
work. It'll usually work, but you could run into trouble if a struct has any 
kind of indirection in it.

In any case, you pretty much either have to completely initialize a struct in 
its constructor or you can't have its member functions be const or immutable, 
and there are definite issues with having const or immutable structs (both in 
terms of bugs in the current implementation and because anything using init is 
stuck with the init value). So, I'd be careful about trying to force a struct 
to always be const or immutable. It's generally doable, but there can be 
negative consequences to it.

- Jonathan M Davis