Newsgroups: comp.lang.c++
Path: utzoo!utgpu!cunews!csi.uottawa.ca!news
From: hitz@sim5.csi.uottawa.ca (Martin Hitz)
Subject: Re: Semantics of default assignment operator
Message-ID: <1991Apr2.201510.311@csi.uottawa.ca>
Summary: It is defined, but ain't clear.
Keywords: assignment operator
Sender: news@csi.uottawa.ca
Nntp-Posting-Host: sim5
Organization: University of Ottawa
References: <1991Mar22.172508.1881@csi.uottawa.ca> <71616@microsoft.UUCP>
Distribution: na
Date: Tue, 2 Apr 91 20:15:10 GMT

In article <71616@microsoft.UUCP> jimad@microsoft.UUCP (Jim ADCOCK) writes:
>In article <1991Mar22.172508.1881@csi.uottawa.ca> hitz@sim5.csi.uottawa.ca (Martin Hitz) writes:
>|I recently came across the following incompatibility between
>|C and C++:
>|If a structure X contains an array, then the assignment
>|	
>|	struct X a, b;
>|	/* ..... */
>|	b = a;
>|
>|is well defined in C and older C++ versions (bitwise copy).
>| 
>|Now, as C++ adopted the memberwise assignment semantics, it is not clear
>|to me what should happen, as there is no such thing as assignment to
>|arrays.
>
>I agree that ARM is not very clear on this issue.  However, see section
>12.8  
>
>Generated copy constructors and op= now use memberwise assignment.  So
>the generated op= should come down to a part that does a:
>
>b.array = a.array;
>
>Which is undefined.  Therefore, I claim that at compile-time the compiler
>should squawk that the array assignment is undefined in the generated
>copy constructor.
>
>One way around this dilemma would be for C++ to support the pure extension over
>C to allow array assignment.  Such could be enabled in C++ since in C++
>arrays and references to arrays include a concept of the size of the 
>array.
>	 [paragraph on benefits deleted]


I asked Bjarne Stroustroup for his opinion on the assignment of arrays
nested within structures:

MH>What does "memberwise assignment" (in the absence of an explicit 
MH>assignment operator) mean for class members that are (ordinary) arrays?

BS> that every element of the member array is copied.

So, I believe it *is* defined (BTW, g++ did it correctly, Zortech did it wrong).
However, to my knowledge the ARM lacks an explicit mention of the fact that 
memberwise assignment is *recursively* applied (which is implicitely clear 
for structures, but not for arrays).

Martin (hitz@csi.UOttawa.CA)