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digitalmars.D.learn - Overzealous immutable and classes

reply Gareth Charnock <gareth.charnock gmail.com> writes:
So having got a collectors' edition TDPL, I though I'd have a try at 
writing some concurrent code. The idea was a worker thread(s) would do 
some work and write the results to some immutable objects. These would 
get passed to an indexer thread that would do neat stuff like indexing 
the references to the objects in things like hash tables and such, 
performing any reduce type operations along the way. These views would 
then be passed back to workers in due course. Unfortunately I hit a 
snag. It appears you can't do this:

class A {
    ...
}
immutable A a1 = new immutable(A);  //pointer to a real, immutable object
immutable A a2;  //pointer to null
a2 = a1;   //error, guess I doomed a2 to a life of being useless

I thought that as such assignments don't change the underlying objects 
in memory, they would be fine. Compare to:

immutable(char)[] s1 = "hello world"; //fat pointer to a section of 
immutable memory
immutable(char)[] s2;  //fat pointer to null
s2 = s1; //fine

The only solutions I can think of:
1) Use a pointer to a class. Works, but that just seems very unsafe and 
just plain un-D-ish. We also have to dereference two pointers.
2) Work entirely with immutable(A)[], which is not quite as crazy as it 
seems as we are copying about arrays of pointers rather than arrays of 
A. It's still quite crazy.
3) Write some sort of wrapper struct to hide the pointer.
4) Perhaps ref works like C++ int&?
ref immutable(A) ref_a //error
Nope.

So is this intended behavior? Am I missing something obvious?
Jul 15 2010
parent reply Jonathan M Davis <jmdavisprog gmail.com> writes:
On Thursday, July 15, 2010 17:40:26 Gareth Charnock wrote:
 So having got a collectors' edition TDPL, I though I'd have a try at
 writing some concurrent code. The idea was a worker thread(s) would do
 some work and write the results to some immutable objects. These would
 get passed to an indexer thread that would do neat stuff like indexing
 the references to the objects in things like hash tables and such,
 performing any reduce type operations along the way. These views would
 then be passed back to workers in due course. Unfortunately I hit a
 snag. It appears you can't do this:
 
 class A {
     ...
 }
 immutable A a1 = new immutable(A);  //pointer to a real, immutable object
 immutable A a2;  //pointer to null
 a2 = a1;   //error, guess I doomed a2 to a life of being useless
 
 I thought that as such assignments don't change the underlying objects
 in memory, they would be fine. Compare to:
 
 immutable(char)[] s1 = "hello world"; //fat pointer to a section of
 immutable memory
 immutable(char)[] s2;  //fat pointer to null
 s2 = s1; //fine
 
 The only solutions I can think of:
 1) Use a pointer to a class. Works, but that just seems very unsafe and
 just plain un-D-ish. We also have to dereference two pointers.
 2) Work entirely with immutable(A)[], which is not quite as crazy as it
 seems as we are copying about arrays of pointers rather than arrays of
 A. It's still quite crazy.
 3) Write some sort of wrapper struct to hide the pointer.
 4) Perhaps ref works like C++ int&?
 ref immutable(A) ref_a //error
 Nope.
 
 So is this intended behavior? Am I missing something obvious?

First off, you're using using a reference, not a pointer. They're similar but quite different. If you were using a pointer, you could do immutable (A)* a1; and the object would be immutable while the pointer would be mutable. It's a mutable pointer to an immutable object of type A. The problem with a reference is that it has no syntactic way to differentiate between what's doing the refering and what's being referred. Because immutable is transitive, immutable A a1; is an immutable reference to an immutable object of type A. As soon as you try and make a reference immutable, what it refers to is immutable as well. There is no syntactic way to fix the problem. The solution is Rebindable!(T) in std.typecons. Rebindable!(T) is a wrapper struct. It allows you to have const and immutable references and still change what they refer to. Take this example from the docs: // Rebindable references to const and immutable objects void bar() { const w1 = new Widget, w2 = new Widget; w1.foo(); // w1 = w2 would not work; can't rebind const object auto r = Rebindable!(const Widget)(w1); // invoke method as if r were a Widget object r.foo(); // rebind r to refer to another object r = w2; } By wrapping the const (or immutable) Widget, you can "rebind" what is "bound" by Rebindable!(T) and effectively get a mutable reference to a const or immutable object. It's not terribly pretty, but apparently no one could come up with a satistfactory way of doing it in the language itself given the syntax for references. So, Rebindable!(T) is the solution. - Jonathan M Davis
Jul 15 2010
next sibling parent Gareth Charnock <gareth.charnock gmail.com> writes:
On 16/07/10 02:08, Jonathan M Davis wrote:
 On Thursday, July 15, 2010 17:40:26 Gareth Charnock wrote:
 So having got a collectors' edition TDPL, I though I'd have a try at
 writing some concurrent code. The idea was a worker thread(s) would do
 some work and write the results to some immutable objects. These would
 get passed to an indexer thread that would do neat stuff like indexing
 the references to the objects in things like hash tables and such,
 performing any reduce type operations along the way. These views would
 then be passed back to workers in due course. Unfortunately I hit a
 snag. It appears you can't do this:

 class A {
      ...
 }
 immutable A a1 = new immutable(A);  //pointer to a real, immutable object
 immutable A a2;  //pointer to null
 a2 = a1;   //error, guess I doomed a2 to a life of being useless

 I thought that as such assignments don't change the underlying objects
 in memory, they would be fine. Compare to:

 immutable(char)[] s1 = "hello world"; //fat pointer to a section of
 immutable memory
 immutable(char)[] s2;  //fat pointer to null
 s2 = s1; //fine

 The only solutions I can think of:
 1) Use a pointer to a class. Works, but that just seems very unsafe and
 just plain un-D-ish. We also have to dereference two pointers.
 2) Work entirely with immutable(A)[], which is not quite as crazy as it
 seems as we are copying about arrays of pointers rather than arrays of
 A. It's still quite crazy.
 3) Write some sort of wrapper struct to hide the pointer.
 4) Perhaps ref works like C++ int&?
 ref immutable(A) ref_a //error
 Nope.

 So is this intended behavior? Am I missing something obvious?

First off, you're using using a reference, not a pointer. They're similar but quite different. If you were using a pointer, you could do immutable (A)* a1; and the object would be immutable while the pointer would be mutable. It's a mutable pointer to an immutable object of type A. The problem with a reference is that it has no syntactic way to differentiate between what's doing the refering and what's being referred. Because immutable is transitive, immutable A a1; is an immutable reference to an immutable object of type A. As soon as you try and make a reference immutable, what it refers to is immutable as well. There is no syntactic way to fix the problem. The solution is Rebindable!(T) in std.typecons. Rebindable!(T) is a wrapper struct. It allows you to have const and immutable references and still change what they refer to. Take this example from the docs: // Rebindable references to const and immutable objects void bar() { const w1 = new Widget, w2 = new Widget; w1.foo(); // w1 = w2 would not work; can't rebind const object auto r = Rebindable!(const Widget)(w1); // invoke method as if r were a Widget object r.foo(); // rebind r to refer to another object r = w2; } By wrapping the const (or immutable) Widget, you can "rebind" what is "bound" by Rebindable!(T) and effectively get a mutable reference to a const or immutable object. It's not terribly pretty, but apparently no one could come up with a satistfactory way of doing it in the language itself given the syntax for references. So, Rebindable!(T) is the solution. - Jonathan M Davis

Thanks, that should work just fine. Looking at the phobos source for Rebindable, I was nowhere near close to reinventing that wheel.
Jul 15 2010
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
Jonathan M Davis:
 It's not terribly pretty, but apparently no one could come up with a 
 satistfactory way of doing it in the language itself given the syntax for 
 references. So, Rebindable!(T) is the solution.

A helper function can help: import std.stdio, std.typecons, std.traits; template isImmutable(T) { // I don't know if this works well in all cases const bool isImmutable = is(const(T) == T) || is(immutable(T) == T); } Rebindable!T rebindable(T)(T obj) if ((is(T == class) || is(T == interface) || isArray!T) && isImmutable!T) { return Rebindable!T(obj); } class Foo { int x, y; } void main() { auto a = rebindable(new immutable(Foo(1, 2))); assert(a.sizeof == 4); a = new immutable(Foo(3, 4)); } A template like isImmutable can be useful in Phobos... Bye, bearophile
Jul 15 2010