Re: [Boost-users] Library Interface Design

From: Matthias Hofmann (hofmann_at_[hidden])
Date: 2006-09-18 08:11:55

"Peter Dimov" <pdimov_at_[hidden]> schrieb im Newsbeitrag 
news:00b401c6d7f2$7e28fa30$6507a8c0_at_pdimov2...

> No, a singular iterator is not a valid object and it fulfills no 
> invariants.
> You can't even copy it without UB. The ofstream can be made to fulfill its
> invariant if we define its invariant as "true", I guess, or something with 
> a
> similar utility.

A singular iterator is a valid object. It's use is limited, however: You can 
only assign a valid value to it. It may be useful to distinguish between an 
invalid and an uninitialized object.

> A default-constructed std::string is fine. You are not breaking the
> invariant with invoking op[], you are supplying an invalid index to it. 
> BTW,
> did you know that the const version of op[] is required to return 0 for 
> s[0]
> :-) How exactly is it supposed to return 0 when the return type is a char
> const& is left as an exercise for the implementer.

That's not true, see Section 21.3.4 of the 2003 C++ Standard for the 
definition of operator[] for std::basic_string<> (note that std::string is a 
typedef for std::basic_string<char>):

[quote]
const_reference operator[](size_type pos) const;
reference operator[](size_type pos);

Returns: If pos < size(), returns data()[pos]. Otherwise, if pos == size(), 
the const
version returns charT(). Otherwise, the behavior is undefined.
[/quote]

This behaviour does not seem to be difficult to implement. The default 
constructor only needs to allocate an array of size 1 and initialize its 
single element with charT(). The constant version of operator[] can then 
return a constant reference to that element.

-- 
Matthias Hofmann
Anvil-Soft, CEO
http://www.anvil-soft.com - The Creators of Toilet Tycoon
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