Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #67632

Re: Parentheses as operators

Show all headers | View raw

ram@zedat.fu-berlin.de (Stefan Ram) writes:
> Richard Heathfield <rjh@cpax.org.uk> writes:
>>On 17/08/2015 21:17, Keith Thompson wrote:
>><snip>
>>> N1570 6.5p1 says:
>>>      An *expression* is a sequence of operators and operands that
>>>      specifies computation of a value, or that designates an object or a
>>>      function, or that generates side effects, or that performs a
>>>      combination thereof.
>>> which is not accurate, since `42` is clearly an expression, but it has
>>> no operators and therefore no operands.
>>It could be argued that it is a sequence of 0 operators and 1 operand 
>>that specifies the computation of the value 42.
>
>   In
>
> sizeof( double )
>
>   , by the above, »double« seems to be an operand, but it is
>   not an expression itself. So then, operands are not always
>   expressions.

The operand is not `double`, but `(double)`, and the operator is
`sizeof`.

>   I'd only call the »sizeof« as in
>
> sizeof 19
>
>   an »operator«, while I'd call expressions such as:
>
> ( double )2
>
>   and
>
> sizeof( double )
>
>   to be »special forms«. But »special form« is no C term IIRC,
>   it might stem from LISP.

I agree that that would be a more consistent description, and one that
I'd prefer, but it's not what the standard says.

>   In the case of a cast ("cast operator"), I always wondered
>   whether
>
> ( double )2
>
>   is a binary operator with »double« and »2« as operands,
>   or whether it is a unary operator with »( double )« being
>   the operator and »2« being its sole operand.

Neither.  `(double)` is a cast operator, and `(double)2` is a
cast-expression.

The phrase "unary operator" doesn't just mean an operator with one
operand (postfix ++ takes one operand but is not a unary operator), nor
does "binary operator" just mean an operator with two operands ([] takes
two operands but is not a binary operator).  Both are specific forms
defined by the grammar.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Back to comp.lang.c | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

Re: Parentheses as operators Keith Thompson <kst-u@mib.org> - 2015-08-17 13:17 -0700
  Re: Parentheses as operators Richard Heathfield <rjh@cpax.org.uk> - 2015-08-17 20:57 +0100
    Re: Parentheses as operators Keith Thompson <kst-u@mib.org> - 2015-08-17 14:36 -0700
    Re: Parentheses as operators Keith Thompson <kst-u@mib.org> - 2015-08-17 14:40 -0700

csiph-web