std.algorithm.remove and principle of least astonishment

[Rainer Deyke]

On 11/20/2010 05:12, spir wrote:
> On Fri, 19 Nov 2010 22:04:51 -0700 Rainer Deyke <rainerd at eldwood.com>
> wrote:
>> You don't see the advantage of generic types behaving in a generic 
>> manner?  Do you know how much pain std::vector<bool> caused in
>> C++?
>> 
>> I asked this before, but I received no answer.  Let me ask it
>> again. Imagine a container Vector!T that uses T[] internally.  Then
>> consider Vector!char.  What would be its correct element type?
>> What would be its correct behavior during iteration?  What would be
>> its correct response when asked to return its length?  Assuming you
>> come up with a coherent set of semantics for Vector!char, how would
>> you implement it?  Do you see how easy it would be to implement it
>> incorrectly?
> 
> Hello Rainer,
> 
> The original proposal by Bruno would simplify some project I have in
> mind (namely, a higher-level universal text type already evoked). The
> issues you point to intuitively seem relevant to me, but I cannot
> really understand any. Would be kind enough and expand a bit on each
> question? (Thinking at people who about nothing of C++ -- yes, they
> exist ;-)

std::vector<bool> in C++ is a specialization of std::vector that packs
eight booleans into a byte instead of storing each element separately.
It doesn't behave exactly like other std::vectors and technically
doesn't meet the C++ requirements of a container, although it tries to
come as close as possible.  This means that any code that uses
std::vector<bool> needs to be extra careful to take those differences in
account.  This is especially an issue when dealing with generic code
that uses std::vector<T>, where T may or may not be bool.

The issue with Vector!char is similar.  Because char[] is not a true
array, generic code that uses T[] can unexpectedly fail when T is char.
 Other containers of char behave like normal containers, iterating over
individual chars.  char[] iterates over dchars.  Vector!char can,
depending on its implementation, iterate over chars, iterate over
dchars, or fail to compile at all when instantiated with T=char.  It's
not even clear which of these is the correct behavior.

Vector!char is just an example.  Any generic code that uses T[] can
unexpectedly fail to compile or behave incorrectly used when T=char.  If
I were to use D2 in its present state, I would try to avoid both
char/wchar and arrays as much as possible in order to avoid this trap.
This would mean avoiding large parts of Phobos, and providing safe
wrappers around the rest.


-- 
Rainer Deyke - rainerd at eldwood.com