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From: drk@athena.mit.edu (David R Kohr)
Newsgroups: comp.lang.c++
Subject: Re: C++ coding standards (Comment needed)
Summary: You can't jump over an "embedded" declaration
Keywords: standard,variables
Message-ID: <1990Aug19.160307.20972@athena.mit.edu>
Date: 19 Aug 90 16:03:07 GMT
References: <2161@runxtsa.runx.oz.au> <425@mole-end.UUCP>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: Massachusetts Institute of Technology
Lines: 30


In this discussion of the benefits and drawbacks of declaring variables
at their point of use in a block in C++, rather than at the beginning 
of a block as is required in C, I don't recall anyone making the
following point: you can't jump (using "goto") past a declaration with an 
initializer unless that declaration is within a block nested within the
current block.  (This is according to Stroustrup, _The_C++_Programming_
Language_, p. 296, section 9.11 of the C++ reference manual chapter;
I assume it's still true in AT&T C++ 2.0, but I don't have any
reference for that version on hand.)

There are those of us who occassionally use "goto" for efficiency 
reasons (or even to make the code clearer).  It seems awkward and
error-prone to me to have to consciously avoid jumping over declarations
with initializers each time one uses a "goto", and to have to check
that each declaration with an initializer is not in a code segment which 
can possibly be skipped over by a "goto".  Such awkwardness does not
lend itself to making code more maintainable.

This subtle mistake probably does not appear very often in practice, since the
"goto" is not used with great frequency in C++, but when it does appear
it is probably fairly difficult for the programmer to detect, much like use 
of an uninitialized variable.  However, compilers can easily detect 
instances of the use of uninitialized variables, while detecting the "goto" 
error requires a more extensive analysis of the code's flow-of-control.

--
David R. Kohr     M.I.T. Lincoln Laboratory      Group 45 ("Radars 'R' Us")
    email:    DRK@ATHENA.MIT.EDU (preferred)  or  KOHR@LL.LL.MIT.EDU
    phone:    (617)981-0775 (work)	      or  (617)527-3908 (home)