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Re: Virtual functions in constructors. Is it possible?

From Anonymous <anonymous@no.net>
Newsgroups comp.lang.c++
Subject Re: Virtual functions in constructors. Is it possible?
Date 2011-07-02 11:25 +0200
Organization Aioe.org NNTP Server
Message-ID <iumo6l$hs6$1@speranza.aioe.org> (permalink)
References (1 earlier) <iulhlm$tji$2@dont-email.me> <iulkmt$fvq$1@speranza.aioe.org> <iumejo$kv6$1@dont-email.me> <iumm5c$di7$1@speranza.aioe.org> <iumn7a$veh$1@dont-email.me>

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Alf P. Steinbach /Usenet ha scritto:
> * Anonymous, on 02.07.2011 10:50:
>> Alf P. Steinbach /Usenet ha scritto:
>>> * Anonymous, on 02.07.2011 01:19:
>>>>> See the FAQ
>>>>>
>>>>> <url: 
>>>>> http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.6> 
>>>>>
>>>>
>>>> That is an ugly solution in my opinion.
>>>
>>> It's not "a" solution, it's a discussion of the possible ways in 
>>> current C++.
>>
>> Ok, to be precise, the ways mentioned in the FAQ have the following 
>> limitations:
>>
>> "In this variation, the code that is creating the object *explicitly 
>> executes
>> both phases*. When executing Phase I, the code creating the object 
>> either knows
>> the object's exact class (e.g., new Derived() or perhaps a local Derived
>> object), or doesn't know the object's exact class (e.g., the virtual 
>> constructor
>> idiom or some other factory). The "doesn't know" case is strongly 
>> preferred when
>> you want to make it easy to plug-in new derived classes."
>>
>> and
>>
>>
>> "The next approach does not rely on a two-phase initialization, 
>> instead using a
>> second hierarchy whose only job is to house methods foo() and bar(). 
>> *This
>> approach doesn't always work, and in particular it doesn't work in 
>> cases when
>> foo() and bar() need to access the instance data declared in Derived*, 
>> but it is
>> conceptually quite simple and clean and is commonly used."
>>
>> In a way or the other static member functions are used there, which 
>> make the
>> approaches just "ugly".
>>
>> They key to the *perfect solution* is to profit by the lifetime of 
>> temporary
>> objects passed to constructors: you can invoke a virtual function 
>> init() after
>> an object has been constructed by calling it from a destructor of a 
>> temporary
>> object passed to the constructor of the class that has to be initialized.
> 
> So, if I do
> 
>   p = new YourClass()
> 
> and init() fails, do I have a memory leak or not?

  no, because init() is called from a destructor of an object passed as 
argument to the constructor. From top down, if init() throws an 
exception, then the destruction fails, then the construction fails..

> 
> But I think the scheme that you're suggesting should be in that FAQ item.

where exactly?

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Thread

Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-02 00:19 +0200
  Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-02 00:37 +0200
    Re: Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-02 01:19 +0200
      Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-02 08:50 +0200
        Re: Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-02 10:50 +0200
          Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-02 11:17 +0200
            Re: Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-02 11:25 +0200
              Re: Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-02 11:31 +0200
              Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-02 12:00 +0200
              Re: Virtual functions in constructors. Is it possible? Werner <werasm@gmail.com> - 2011-07-04 01:10 -0700
                Re: Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-04 20:41 +0200
                Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-04 21:35 +0200
                Re: Virtual functions in constructors. Is it possible? Leo Equinox Gaspard <ekinox1995@gmail.com> - 2011-07-04 23:59 +0200
                Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-05 11:09 +0200
                Re: Virtual functions in constructors. Is it possible? Leo Equinox Gaspard <ekinox1995@gmail.com> - 2011-07-05 13:15 +0200
                Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-05 14:51 +0200
                Re: Virtual functions in constructors. Is it possible? Leo Equinox Gaspard <ekinox1995@gmail.com> - 2011-07-05 19:55 +0200
                Re: Virtual functions in constructors. Is it possible? Anonymous <anonymous@no.net> - 2011-07-05 20:46 +0200
                Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-05 21:23 +0200
                Re: Virtual functions in constructors. Is it possible? Werner <werasm@gmail.com> - 2011-07-06 00:14 -0700
  Re: Virtual functions in constructors. Is it possible? Virchanza <virtual@lavabit.com> - 2011-07-05 14:58 -0700
    Re: Virtual functions in constructors. Is it possible? "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-06 01:28 +0200

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