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