Re: [Boost-users] Library Interface Design

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-09-14 07:38:12

Scott Meyers wrote:
> Marshall Clow wrote:
>> At 9:06 PM -0700 9/13/06, Scott Meyers wrote:
>> [ snip ]
>>> I'll note that C++ itself allows "uninitialized" objects with
>>> constructors to be created all the time:
>>>
>>>    std::ofstream ofs;
>>>    std::vector<int>::iterator i;
>>>    std::string s;
>>
>> Just a nit - I think that your third example is not like the others.
>> A std::string, AFAIK, constructed with the default constructor, is
>> perfectly valid - just empty.
>
> In each case, a default constructor is invoked.  They're all valid
> objects that presumably fulfill their invariants, it's just that you
> can't safely invoke very many operations on them.

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 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.