make-instance without initialize-instance
in [Lisp]
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From: Tamas K Papp on 13 May 2010 13:23 Hi, I want to allocate an instance of a class without running initialize-instance (which would perform some checks on initargs, those I want to bypass because I am initializing the slots manually). I think that I have to call allocate-instance, but when I try (allocate-instance (find-class 'my-class)) I get an error condition (which SBCL is "unable to display", so I must have screwed up heavily). What's the proper way to do what I want? Thanks, Tamas
From: Zach Beane on 13 May 2010 13:34 Tamas K Papp <tkpapp(a)gmail.com> writes: > Hi, > > I want to allocate an instance of a class without running > initialize-instance (which would perform some checks on initargs, > those I want to bypass because I am initializing the slots manually). This question seems upside-down to me. What checks cause problems when you want to initialize the slots manually? Zach
From: Captain Obvious on 13 May 2010 14:15 TKP> I think that I have to call allocate-instance, but when I try TKP> (allocate-instance (find-class 'my-class)) TKP> I get an error condition (which SBCL is "unable to display", so I must TKP> have screwed up heavily). What's the proper way to do what I want? Well, allocate-instance itself works fine: CL-USER> (defclass test1 () (a)) #<STANDARD-CLASS TEST1> CL-USER> (allocate-instance (find-class 'test1)) #<TEST1 {C5C9AA1}> If it doesn't work with your class, it is some problem you have in your code. E.g. one way to implement this error: CL-USER> (defmethod print-object ((o test1) stream) (format stream "#<TEST1 a=~a>" (slot-value o 'a))) CL-USER> (allocate-instance (find-class 'test1)) Unable to display error condition [Condition of type UNBOUND-SLOT] 0: ((SB-PCL::FAST-METHOD SLOT-UNBOUND (T T T)) #<error printing object>) 1: (SB-PCL::SLOT-UNBOUND-INTERNAL #<error printing object>) 2: ((SB-PCL::FAST-METHOD PRINT-OBJECT (TEST1 T)) #<error printing object>) 3: (SB-PRETTY:OUTPUT-PRETTY-OBJECT #<error printing object>) 4: (SB-IMPL::STRINGIFY-OBJECT #<error printing object>) 5: (PRIN1-TO-STRING #<error printing object>) That is, forget to initialize a slot and write a print-object which uses that slot, and you'll get "Unable to display" thing. By the way, you might want to use shared-initialize (it is more flexible than initialize-instance) after allocate-instance, but it is not strictly necessary, depends on your goal.
From: Tamas K Papp on 14 May 2010 14:08 On Thu, 13 May 2010 13:34:42 -0400, Zach Beane wrote: > Tamas K Papp <tkpapp(a)gmail.com> writes: > >> Hi, >> >> I want to allocate an instance of a class without running >> initialize-instance (which would perform some checks on initargs, those >> I want to bypass because I am initializing the slots manually). > > This question seems upside-down to me. What checks cause problems when > you want to initialize the slots manually? A particular object represents a multivariate t distribution, and is initialized with the mean, sigma (which is proportional to the variance), and the degrees of freedom. The INITIALIZE-INSTANCE :AFTER method checks that the arguments make sense. But internally, the mean and sigma are used to create a multivariate normal distribution (in the INITIALIZE-INSTANCE :AFTER method), and t variates are drawn using the standard algorithm. In some cases, I have this object directly from intermediate calculations, so instead of (make-instance 'mv-t :mean (mean mv-normal) :sigma (variance mv- normal) :nu nu) I can just use (let ((rv (allocate-instance (find-class 'mv-t)))) (setf (slot-value rv 'mv-normal) mv-normal) ...) Tamas
From: Tamas K Papp on 14 May 2010 14:11
On Thu, 13 May 2010 21:15:33 +0300, Captain Obvious wrote: > TKP> I think that I have to call allocate-instance, but when I try > > TKP> (allocate-instance (find-class 'my-class)) > > TKP> I get an error condition (which SBCL is "unable to display", so I > must TKP> have screwed up heavily). What's the proper way to do what I > want? > > Well, allocate-instance itself works fine: > > CL-USER> (defclass test1 () (a)) > #<STANDARD-CLASS TEST1> > CL-USER> (allocate-instance (find-class 'test1)) #<TEST1 {C5C9AA1}> > > > If it doesn't work with your class, it is some problem you have in your > code. > > E.g. one way to implement this error: > > CL-USER> (defmethod print-object ((o test1) stream) > (format stream "#<TEST1 a=~a>" (slot-value o 'a))) > > CL-USER> (allocate-instance (find-class 'test1)) > > Unable to display error condition > [Condition of type UNBOUND-SLOT] > 0: ((SB-PCL::FAST-METHOD SLOT-UNBOUND (T T T)) #<error printing > object>) 1: (SB-PCL::SLOT-UNBOUND-INTERNAL #<error printing object>) > 2: ((SB-PCL::FAST-METHOD PRINT-OBJECT > (TEST1 T)) #<error printing object>) > 3: (SB-PRETTY:OUTPUT-PRETTY-OBJECT #<error printing object>) 4: > (SB-IMPL::STRINGIFY-OBJECT #<error printing object>) 5: > (PRIN1-TO-STRING #<error printing object>) > > > That is, forget to initialize a slot and write a print-object which uses > that slot, and you'll get "Unable to display" thing. Thanks, my problem was something similar. > By the way, you might want to use shared-initialize (it is more flexible > than initialize-instance) after allocate-instance, but it is not > strictly necessary, depends on your goal. I think it makes my code more standard and compact, so it was useful. Tamas |