Re: Why use C++?

[Ian Collins <ian-news@hotmail.com>]

On 08/13/11 09:52 PM, Dmitry A. Kazakov wrote:
> On Sat, 13 Aug 2011 21:29:05 +1200, Ian Collins wrote:
>
>> It's pretty rare for the application domain to be concerned about
>> the semantics of an integer beyond those specified by the language.
>
> If buffer overflows and code quality is not a concern, then yes.

Most variables have limited scope and a well defined range.  So if I use 
an int in a for loop over a small set of data, I'm not writing poor 
quality code.

>>>>> In each such case I will have to use a type different from the machine
>>>>> type. This is just another argument why types need to be precisely
>>>>> specified. I addressed this issue above.
>>>>
>>>> But they are precisely specified on any given platform.
>>>
>>> 1. They are not. In order to see the specification of int, you have to read
>>> the compiler documentation. My point was that the program code does not
>>> specify the properties of int. The answer that it is defined somewhere
>>> somehow is not an argument at all, anything works in this or that way.
>>
>> If you want to get the best performance out of a given platform (which
>> many C and C++ programs do), you have to program to the characteristics
>> of the platform.
>
> First of all, performance is not a functional requirement.

It very often is the first one for most of the embedded systems I write!

> If functional
> requirements are not met, non-functional ones are irrelevant.

The functional requirements are irrelevant if they can not be met in the 
required time.

> Thus the
> question stands: what does int implement? Where in the program code is any
> specification of the *functional* requirements which the implementation
> chosen to be int is supposed to fulfill [in the best etc possible way,
> which in its turn requires validation]? How do I know if int is OK, not
> unsigned, not long, not short and so on?

In general, you can't.  Where it matters, you have to measure.

> Secondly, how do you know that int gives you the best performance across
> all possible implementations fulfilling the functional requirements? By
> choosing int you prohibit the compiler to select any other machine type,
> which also does the things you want. The point is, specify what you want
> and let the compiler to implement it in a best possible way. Choosing an
> implementation is an optimization issue, you can aim at memory use, at
> time, at best lock-free sharing and so on. Fixing a machine type makes your
> program potentially less efficient. More precisely you specify the
> semantics of required type, more knowledge the compiler has to generate
> efficient code.

I do see your point and for C or C++ the best we can do knowing a 
variable is signed or unsigned of a given range is to use one of the C 
fast_xx types.  Far from ideal I know.

Yes I agree being able to specify a type without reference to an 
underlying fundamental type is a powerful feature C and C++ lack.

-- 
Ian Collins