std.algorithm.remove and principle of least astonishment

[Rainer Deyke]

On 11/21/2010 11:23, Andrei Alexandrescu wrote:
> On 11/20/10 9:42 PM, Rainer Deyke wrote:
>> On 11/20/2010 16:58, Andrei Alexandrescu wrote:
>>> The parallel does not stand scrutiny. The problem with vector<bool>  in
>>> C++ is that it implements no formal abstraction, although it is a
>>> specialization of one.
>>
>> The problem with std::vector<bool>  is that it pretends to be a
>> std::vector, but isn't.  If it was called dynamic_bitset instead, nobody
>> would have complained.  char[] has exactly the same problem.
> 
> char[] does not exhibit the same issues that vector<bool> has. The
> situation is very different, and again, trying to reduce one to another
> misses a lot of the picture.

I agree that there are differences.  For one thing, if you iterate over
a std::vector<bool> you get actual booleans, albeit through an extra
layer of indirection.  If you iterate over char[] you might get chars or
you might get dchars depending on the method you use for iterating.

char[] isn't the equivalent of std::vector<bool>.  It's worse.  char[]
is the equivalent of a vector<bool> that keeps the current behavior of
std::vector<bool> when iterating through iterators, but gives access to
bytes of packed booleans when using operator[].

> vector<bool> hides representation and in doing so becomes non-compliant
> with vector<T> which does expose representation. Worse, vector<bool> is
> not compliant with any concept, express or implied, which makes
> vector<bool> virtually unusable with generic code.

The ways in which std::vector<bool> differs from any other vector are
well understood.  It uses proxies instead of true references.  Its
iterators meet the requirements of input/output iterators (or in boost
terms, readable, writable iterators with random access traversal).  Any
generic code written with these limitations in mind can use
std::vector<T> freely.  (The C++ standard library doesn't play nicely
with std::vector<bool>, but that's another issue entirely.)

std::vector<bool> is a useful type, it just isn't a std::vector.  In
that respect, its situation is analogous to that of char[].

>>> It may be wise in fact to start using D2 and make criticism grounded in
>>> reality that could help us improve the state of affairs.
>>
>> Sorry, but no.  It would take a huge investment of time and effort on my
>> part to switch from C++ to D.  I'm not going to make that leap without
>> looking first, and I'm not going to make it when I can see that I'm
>> about to jump into a spike pit.
> 
> You may rest assured that if anything, strings are not a problem.

I'm not concerned about strings, I'm concerned about *arrays*.  Arrays
of T, where T may or not be a character type.  I see that you ignored my
Vector!char example yet again.

Your assurances aren't increasing my confidence in D, they're decreasing
my confidence in your judgment (and by extension my confidence in D).


-- 
Rainer Deyke - rainerd at eldwood.com