Re: "gcc" complains about a constant being non constant...

[Andrew Haley <aph at redhat dot com>]

Linda A. Walsh wrote:
> 
> Bill McEnaney wrote:
>> Maybe we need to distinguish between a const variable and a literal.

That would IMO be a mistake.  We should use the ISO terminology, which
is "constant expression".

>>>> if, in line 8, sizeof(days) is a constant,  AND in line 9,
>>>> sizeof(String) is
>>>> a constant, then how can their division (in line 10) NOT be a constant?
>>> Simple: sizeof(days) is a constant, but sizeof_days is not.  Reason:
>> because
>>> the C language standard says so.  A const variable is not a constant.
>>  It'd be
> ---
>    Sorry to say, but whoever the 'rocket scientist' that thought up
> this nonsense was, should be shot.

Gosh, I hate sentences that begin "Sorry, but ..."   :-)

> I can use sizeof(days)/sizeof(String)-1 as a direct initializer into
> a 'const' variable -- and THAT works:
> typedef const char * String;
> 
> static const String suffixes [] = {"B", "KB", "MB", "GB", "TB"};
> 
> static const int index_of_last_suffix = sizeof(suffixes)/(sizeof
> (String)) - 1;
> 
> As long as I don't try to assign "sizeof(suffixes)/sizeof(String) to
> a 'const' declared integer, first, and then use that as an intermediate
> step, I'm fine.  But If I want to first assign the quotient to a const int
> for 'num_items', and then use that value with "-1" to init a 2nd
> constant, it fails.
> 
> Sure seems like a C-design bug -- i.e. may be designed that way, but
> it's certainly non-intuitive to see

> const int a=b/c;
> const int d=a-1; FAIL, while
> const int d=b/c-1; works
> 
> Certainly breaks or voids the concept of equivalent expressions being
> equal.

But they're not equivalent.

The key thing to remember here:

    A const variable is not a constant.

const in C is just a type qualifier.

If there's one thing wrong here, it's that const is badly named: it really
means "readonly".

> Do I remember incorrectly, or is this fixed in C++?

Yes, it is different in C++.

Here's the C definition:

"An integer constant expression shall have integer type and shall
only have operands that are integer constants, enumeration constants,
character constants, sizeof expressions whose results are integer
constants, and ïoating constants that are the immediate operands of
casts. Cast operators in an integer constant expression shall only
convert arithmetic types to integer types, except as part of an
operand to the sizeof operator."

and the C++ definition:

"An integral constant-expression can involve only literals (2.13),
enumerators, const variables or static data members of integral or
enumeration types initialized with constant expressions (8.5),
non-type template parameters of integral or enumeration types, and
sizeof expressions. Floating literals (2.13.3) can appear only if they
are cast to integral or enumeration types. Only type conversions to
integral or enumeration types can be used. In particular, except in
sizeof expressions, functions, class objects, pointers, or references
shall not be used, and assignment, increment, decrement,
function-call, or comma operators shall not be used."

Andrew.