Dynamically tagged expression not allowed. Why?
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From: Dmitry A. Kazakov on 3 Jun 2010 03:48 On Thu, 03 Jun 2010 04:37:42 +0200, Yannick Duch�ne (Hibou57) wrote: > Le Sat, 29 May 2010 20:23:36 +0200, Dmitry A. Kazakov > <mailbox(a)dmitry-kazakov.de> a �crit: >>> Conn : Class_Type := New_Class_Instance; >> >> Should Conn be mutable? Otherwise you could use a function instead. >> > Good note (formally talking). > > May be the reason was a matter of efficiency ? A renaming could also go, but it would not because of the "unseen body" problem. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de
From: Dmitry A. Kazakov on 3 Jun 2010 03:57 On Thu, 03 Jun 2010 04:36:12 +0200, Yannick Duch�ne (Hibou57) wrote: > Not necessarily, as he can move the body of the function returning > Class_Type before the declaration of the variable initialized from this > function. You cannot have bodies in the package specifications. This is a difficult problem, since present construction model does not support deferred initialization. In short, you cannot have this in a single package. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de
From: Yannick Duchêne (Hibou57) on 3 Jun 2010 04:21 Le Thu, 03 Jun 2010 09:57:33 +0200, Dmitry A. Kazakov <mailbox(a)dmitry-kazakov.de> a écrit: > You cannot have bodies in the package specifications. This is a difficult > problem, since present construction model does not support deferred > initialization. In short, you cannot have this in a single package. OK. I had though the example was an exact extract from the real thing. So indeed, if the variable is to be in a spec, it needs two packages, one containing the function, and one containing the variable, so that one package -- the one containing the function -- can be elaborated before the other -- containing the variable. A child package would be a good choice to hold that variable. -- There is even better than a pragma Assert: a SPARK --# check. --# check C and WhoKnowWhat and YouKnowWho; --# assert Ada; -- i.e. forget about previous premises which leads to conclusion -- and start with new conclusion as premise.
From: Adam Beneschan on 3 Jun 2010 12:03 On Jun 2, 7:36 pm, Yannick Duchêne (Hibou57) <yannick_duch...(a)yahoo.fr> wrote: > Le Wed, 02 Jun 2010 01:36:17 +0200, Randy Brukardt <ra...(a)rrsoftware.com> > a écrit:> You have to explicitly use a > > type conversion here: > > > Conn : Class_Type := Class_Type (New_Class_Instance); > > This one is not legal, as Class_Type is limited and conversion are not > allowed on limited type due to their copy based semantic. > [ARM 2005 4.6] says about type conversions > Neither the target type nor the operand type shall be limited Umm, no, that sentence (4.6(24.7)) applies only to array types. (And, I think, only in cases where there is no common ancestor of the operand and target type.) However, 7.5(2.1-2.2) makes it illegal. Type conversions on limited types are allowed in some cases; e.g. if T1 is a limited tagged type, and T2 is a type extension, and X2 is an object of type T2, you can use T1(X2) as an actual parameter to a procedure that takes an IN OUT parameter of type T1. This is a view conversion and doesn't involve a copy. -- Adam
From: Marc A. Criley on 5 Jun 2010 14:47
On 06/01/2010 06:36 PM, Randy Brukardt wrote: > "Marc A. Criley"<mcNOSPAM(a)mckae.com> wrote in message > news:c2ead$4c0150a0$4ca4a823$11809(a)API-DIGITAL.COM... > ... >> Stripping out a test case, I see that it has nothing to with the C++ >> binding per se, but it's an Ada issue that's flummoxing me. Here's the >> test code: >> >> procedure Dyty_Test is >> >> type Class_Type is tagged limited record >> null; >> end record; >> >> function New_Class_Instance return Class_Type'Class; >> >> Conn : Class_Type := New_Class_Instance; > > This is illegal because the function returns a class-wide object and you are > assigning it into an object of a specific type. You have to explicitly use a > type conversion here: > > Conn : Class_Type := Class_Type (New_Class_Instance); > > to specify that you want to truncate the returned object, or make the object > classwide: > > Conn : Class_Type'Class := New_Class_Instance; > > But notice that while both of these are legal, they'll both raise > Program_Error because an access-before-elaboration error (the body of > New_Class_Instance hasn't been elaborated at the point of this call). Actually, stripping out the C++ interface pragmas was probably a bad idea on my part. (Though the thread *was* educational :-) I thought that to create an instance of the C++ class I had to invoke the New_Class_Instance function myself, from which arose this issue. Turns out I don't. GNAT's handling of the C++ pragmas does it all for me automagically. The documentation on creation an Ada binding to C++ is somewhat...terse when it comes to describing the mechanics of what is going on "under the hood", from whence came my misunderstanding. So this part is all better now, I can step in and verify that my constructors are being executed. I'm having other problems, but that's just the usual banging the head against the wall stuff. Marc A. Criley www.mckae.com |