Xref: utzoo comp.lang.c++:13594 comp.std.c++:926
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From: dsouza@gwen.cad.mcc.com (Desmond Dsouza)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: Constructor Limitation: [Was: Re: Constructor question]
Message-ID: <DSOUZA.91May22131958@gwen.cad.mcc.com>
Date: 22 May 91 18:19:58 GMT
References: <1991Apr2.110623.22219@and.cs.liv.ac.uk> <20164@alice.att.com>
	<HAYDENS.91Apr17113619@natasha.juliet.ll.mit.edu>
	<1991Apr18.005426.21863@dragon.wpd.sgi.com> <71952@microsoft.UUCP>
	<DSOUZA.91May3150307@gwen.cad.mcc.com> <72461@microsoft.UUCP>
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In-reply-to: jimad@microsoft.UUCP's message of 20 May 91 20:59:57 GMT

In article <72461@microsoft.UUCP> jimad@microsoft.UUCP (Jim ADCOCK) writes:

   >  You cannot compute any temporaries (e.g. common subexpressions) 
   >  which are needed for initialization of such data members.

   Consider:

   ----

   int F1(int j) { /* something really really big and horrible here */ return j;}

   class A {
     public: A(int m, int n);
   };

   class B {
   public:
     B (int j);
   private:
     int f1;
     A a1;	// initialized with F1(j), F1(j)+1 for some horribly complex F1
   };

   B::B(int j)
   // how can I share the computation of F1(j) here ??? !!! Try the following:
   : f1(F1(j)), a1(f1, f1+1)
   {
     // this part does not help: I cannot modify a1 at this point.
   }


No good. You cannot afford to add a data member per object instance to 
simply store an intermediate constructor variable.

(There is also the subtle point about order of initialization of data 
 members -- it follows the order in the class definition, so you had it right).

-- Desmond.
--

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 Desmond D'Souza, MCC CAD Program | ARPA: dsouza@mcc.com | Phone: [512] 338-3324
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