Kagamin wrote: > Simen kjaeraas Wrote: > >> Worth adding: >> Even if non-null references/pointers cannot be perfectly implemented in a >> library, they may still be a worthwhile addition to Phobos, to mark >> function arguments and the like. > > Hmm... this doesn't work: > > struct NonNull(T) > { > T value; > this(T v) > { > assert(value); > value=v; > } > } > > class A {} > > void foo(NonNull!A a){} > > void goo() > { > A a=new A(); > foo(a); > } > > test.d(23): Error: function test.foo (NonNull!(A) a) is not callable using > argument types (A) test.d(23): Error: cannot implicitly convert expression > (a) of type test.A to NonNull!(A) The above seems correct to me. You are assigning a nullable to a non- nullable so you force the user to assess that is correct and provide an override. Based on that I've had a crack at this myself.
http://myweb.tiscali.co.uk/adz21c/nonnull.d http://myweb.tiscali.co.uk/adz21c/test.d Upto now I've only concentrated on mutable class references, but I figured I would post what I have up to now and see what people think. I've pulled NonNull (NN to save my fingers) construction out into a separate function. toNN performs the runtime null check rather than the struct constructor. This allows us to use opAssign to convert NN!Derived to NN!Super without a null check, you only check for nulls when crossing from the nullable world to non-nullable using toNN. It also means code like below is not possible without explicitly saying I want to cross from nullable to non-nullable (using toNN) which offers some sort of compile time check (not as good as a compiler checking for "if is null" but it is something). A a = null; NN!A b = a; // will not compile NN!A b = toNN(a); // inserts the null runtime check NN!A c = b; // No null check as there is no need. I've attempted where possible to try and keep it looking like a reference (as you can see above). So far I have come across a few snags in my plan. 1. Default struct constructor. This means the NN can be created without assigning a value. I have tried to get around this issue somewhat by adding a null check in the invariant but it seems the invariant is not called when using the default constructor. Should it be or should it not? If it is then atleast while contracts are enabled we get to know about uninitialised NNs. 2. opAssign and alias this on function parameters a. For assignment I used opAssign to perform a conversion from NN!B to NN!A. This doesn't work for function parameters of NN!A when passing NN!B so I am forced to use toNN adding unnecessary runtime check and nasty looking code. b. When passing NN!A to A I assume the alias this kicks in and passes NN!A.value to A. However on function parameters this is not happening so I am forced to manually call NN!A.value. 3. Assigning new gets checked Below is annoying, clearly it does not need a null check. NN!A a = toNN(new A()); I figure a function that does not do a null check *could* be offered for this situation, but unfortunately that also destroys whatever guarantuees you get with NN. The other idea I have had is somehow passing in the "new A()" as an expression and evaluating if the expressions purpose is to create an object then remove the null check, but I don't know how to do that. 4. Double checking nulls foo(A a) { if (a is null) // check one doSomething(); else bar(toNN(a)); // check two } bar(NN!A a) {} If the user checks for null before assigning nullables to NN then 2 null checks are performed, one by the user and another by toNN. Would the compiler be smart enough to remove the redundancy? If not then the only way round it I can see is allow a delegate to be passed to toNN which holds the isnull code path. Problem with this is I think it would make code quite difficult to read. Otherwise we suck it up. 5. Consider if someone is altering code with toNN in it, the toNN will hide a bug (if they removed a required check) till runtime that could be identified at compile time. But I guess thats the diff between a library version and compiler version. At least the code will die earlier at runtime though. Thoughts up to now? Am I barking up completely the wrong tree?