James Newton, Host wrote:

>>>     for (expression1; expression2; expression3)
>>>         <statement>
>>>
>>> is handled by the compiler as
>>>
>>>     expression1;
>>>     while (expression2)
>>>     {
>>>         <statement>;
>>>         expression3;
>>>     }
>>>
>>> and <statement> can be either a simple or a compound statement.
>> 
>> Microsoft's latest C++ is interpreting it as:
>> 
>>   {
>>     expression1;
>>     while (expression2)
>>     {
>>        <statement>;
>>        expression3;
>>     }
>>   }
> 
> Huh? I don't see any difference. Does the extra set of braces matter?

Yes. You can declare variables in expression1, and the presence or not of
the (invisible) extra set of braces determines the scope of the variables
that are defined in expression1. Without the extra set, they are valid
until the end of the enclosing block (whatever that is); with it, they are
valid only within the for loop.


The problem is that the definition with the extra set of braces has been
standard for ten years or more, but some compilers didn't handle it this
way and may now be moving towards the standard. See for example
http://www.csci.csusb.edu/dick/c++std/cd2/stmt.html (dated 11/1996) and
browse to section 6.5.3:

----- quote -----

  6.5.3  The for statement                                    [stmt.for]

1 The for statement
          for ( for-init-statement conditionopt ; expressionopt ) statement
  is equivalent to

          {
                  for-init-statement
                  while ( condition ) {
                          statement
                          expression ;
                  }
          }
  except  that  names declared in the for-init-statement are in the same
  declarative-region as those declared in the condition, and except that
  a  continue in statement (not enclosed in another iteration statement)
  will execute expression before re-evaluating condition.   [Note:  Thus
  the  first statement specifies initialization for the loop; the condi-
  tion (_stmt.select_) specifies a test,  made  before  each  iteration,
  such  that  the  loop  is exited when the condition becomes false; the
  expression often specifies incrementing that is done after each itera-
  tion.  ]

2 Either  or both of the condition and the expression can be omitted.  A
  missing  condition  makes  the  implied  while  clause  equivalent  to
  while(true).

3 If  the  for-init-statement is a declaration, the scope of the name(s)
  declared extends to the end of the for-statement.  [Example:
          int i = 42;
          int a[10];

          for (int i = 0; i < 10; i++)
                  a[i] = i;

          int j = i;        // j = 42
   --end example]

----- end quote -----


Some VC++ background on this issue: http://vcfaq.mvps.org/lang/1.htm

There is probably a compiler switch that allows selecting one or the other
interpretation.

Gerhard

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist