[boost] Re: Formal Review: Circular Buffer

From: Alberto Barbati (abarbati_at_[hidden])
Date: 2004-03-07 06:44:52

Jan Gaspar wrote:
>>>4. Type Requirements: don't T need to be
>>>Assignabletoo?
>>Just no!
>>---------
>>Then how is an element overwritten?
> Look at the replace() method in the source code.

This strikes me. All standard containers require T to be Assignable, so 
I don't understand the rationale for dropping such requirement. Does it 
give more freedom? Perhaps yes, but not so much IMHO. Users of 
containers are probably going to provide assignment anyway.

Suppose that T *is* assignable, is destroy+copy construction better than 
assignment? Probably no, for several reasons, including:

1) assigment is probably more efficient. In particular it might avoid 
destruction/reconstruction of sub-objects non related with the class 
invariant (for example: buffers, mutexes, etc.).

2) with the dtor/ctor idiom, if the ctor throws an exception, the old 
element is lost and you can't do anything about it, so the container 
cannot provide more than the basic guarantee for any method that might 
overwrite elements. However, the user might implement assigment with the 
strong guarantee, the container might leverage on that and provide the 
strong guarantee too at least for methods that might overwrite a single 
element such as push_back/push_front. (Of course, the user might 
implement assignment so badly that it doesn't provide even the basic 
guarantee and the container could not recover from that, but a library 
such not try to outsmart the user... too much ;).

Just my opinion,

Alberto