Message du 06/05/11 01:19 De : "Vladimir Batov" A : boost@lists.boost.org Copie à : Objet : Re: [boost] [review] string convert
Vicente BOTET wanadoo.fr> writes: ...
template <> default_value { static example apply() { return example(3); } };
quite bothersome as I know I really do not have to. In the original wacky design the fallback and the default were synonyms. Now you are introducing an additional piece of machinery/code that is needed to deploy a class with conversion and I have to provide the default and the fallback. With that in mind I am not sure I am personally warming up for default_value concept yet.
I was not talking here of the fallback.
Note that you don't need to do that for the classes having a default constructor, which are most of them.
Vicente,
If I were proposing something for a potentially wide user-base, then I'd personally be probably cautious with "most of them" statements. The classes you happen to write/come across might indeed have default constructors. My projects do not. In fact, only a small fraction of (relatively low-level) classes will have default constructors. Our classes happen to be quite complex and require parameters to be created. Those classes cannot be instantiated with default constructors. One could argue that the default constructor *could* be still provided for those classes by creating invalid instances. I argue that that's a bad idea as from the design perspectives that code pollution to work around limitations of some other library.
I suspect that these complex classes you are talking off don't define the input/ouput stream operator to work with your conversion framework. Or did them? The classes I expect to be used are regular classes with value semantics. In any case, you have two options: the default value is created via a default_value function implicitly or explicitly via a constructor with parameters . As a user of a class I will prefer to don't have to create it explicitly. Of course this force the designer of the class to define a default value. Could you give some more examples of regular classes that don't have a default constructor, neither a valid instance that can be constructed by a default value function?
Even simple classes do not often have default constructors. Take the example from my 'convert' documentation -- the direction class with up and dn values. What do you consider the default constructor would do?
I could try to follow the builtin behavior. I will define one of them as 0 and init the default constructor with it.
Or you'd suggest introducing another 'not-defined' state just to be user in the default constructor? Then, it'll have far-reaching ripples through all the code as now I never know if my direction object is valid or not and, therefore, have to always write error-prone "if (dir != undefined) proceed".
No please. Do not do that. Best, Vicente