[C++-sig] Re: FW: Injected constructors

[Nicodemus]

Roman Yakovenko wrote:

>class MyClass
>{
>...
>	MyClass(); //after constructing object stays in state 1
>	MyClass( int i ); //after constructing object stays in state 2
>	MyClass( const YourClass &x); //after constructing object stays in state 3
>...
>};
>
>Now if I understand right there is a need in other state of the MyClass object.
>
>MyClass make_myclass_from_other( const SomeOther &y );
>
>BOOST_PYTHON_MODULE(staticmethod_ext)
>{
>    class_<MyClass>
>		...
>        .staticmethod(make_myclass_from_other)
>...
>
>While in python I'll write
>obj = MyClass.make_myclass_from_other( third_party_class )
>
>As for me it is much clear then adding an other constructor for the class.
>At the moment I wrote I thought about class that already has a few constructors. 
>Now I understand that it is not the only case. May be you have nice small class with 
>one constructor and you want to add an other one.
>  
>

On a side note, this is a good reason to use classmethods in Python:

class ConfigurationLoader(object):
    '''loads configuration from a filename or a file-like object.
    '''

    def __init__(self, file_object):
         self.file = file_object

    def FromFileName(cls, filename):
         self = cls(file(filename))
         return self

    FromFileName = classmethod(FromFileName)


# usage
config = ConfigurationLoader.FromFileName('test')


This avoids having to do type checking in the __init__ method, plus it 
makes the code a little more readable. 8)