Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hplsla!jima
From: jima@hplsla.HP.COM (Jim Adcock)
Newsgroups: comp.lang.c++
Subject: Re: variable declaration
Message-ID: <6590251@hplsla.HP.COM>
Date: 13 Sep 89 17:46:16 GMT
References: <953@ncratl.Atlanta.NCR.COM>
Organization: HP Lake Stevens, WA
Lines: 70

> Has anybody developed any style rules for this?  I'm especially worried 
> about the tradeoff of readability for optimization. (This becomes a real
> issue in loop indices)  

There shouldn't be any issue of optimization no matter where one declares
one's variables using any modern optimizing compiler.  The optimizing compiler
will analyze what scope of the code your variable is actually used in,
and will free that variable's register for other usage elsewhere.
So feel free to do whatever style of declaration makes your code most readable.

---

If a variable is widely used throughout a procedure, declare and initialize
it at the start of a procedure.

If a variable is only used locally in a procedure, perhaps only in a couple
lines of code, then declare and initialize it where first used.

But *don't* make the mistake of introducing a variable in the middle of 
your code, then using it again later.  Of course, one isn't tempted to do
this is one keeps one's routines small.

Other no-nos: don't use the same variable name more than once in a routine
with differing meanings, or with differing types.

As the following example demonstrates, for-loop scope [for historical reasons]
is not what one would wish when declaring dummy loop variables.  You could
handle this by adding curly braces to make a local scope, but this is not
a common style.  Maybe one should continue in the C style of declaring
standard dummy loop variables at the start of a routine?

#include <stdio.h>

void doSmethingWith(int i) {printf("%d\n",i);}

void aLoopyRoutine()
{
  for (int i=0; i<10; ++i) doSomethingWith(i);

// more code

  for (int i=0; i<10; ++i) doSomethingWith(i*i); //error: two declarations of i
}

void aLoopyRoutine2()
{
  {for (int i=0; i<10; ++i) doSomethingWith(i);}   // curly braces make i local

// more code

  {for (int i=0; i<10; ++i) doSomethingWith(i*i);} //okay; but not common style
}

void aLoopyRoutine3()
{
  int i;	//maybe just declare common dummy loop variables at start?

  for (i=0; i<10; ++i) doSomethingWith(i);

// more code

  for (i=0; i<10; ++i) doSomethingWith(i*i);
}

void main()
{
  aLoopyRoutine();
  aLoopyRoutine2();
  aLoopyRoutine3();
}