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Re: static_cast and std::vector

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"john"  wrote in message news:jfbveg$ae0$1@speranza.aioe.org...
>
>On 1/20/2012 9:00 AM, Leigh Johnston wrote:
>> On 20/01/2012 13:56, john wrote:
>>> Hi,
>>>
>>> I am attempting to replace lots of C-style casts with static_cast's in
>>> our code base. Most of these are simple, but there are a few that I'm
>>> not sure what to do.
>>>
>>> Basically, the problem is shown in the code below. GetD0/GetD1 are the
>>> original C-style casts that work fine. I would like to change these to
>>> C++ style casts, but the GetDD0/GetDD1 functions do not compile.
>>>
>>> Is there a way to do this? Are there any potential problems I should be
>>> aware of?
>>>
>>> Thanks.
>>>
>>> #include <vector>
>>>
>>> class Base {};
>>> class Derived0 : public Base {};
>>> class Derived1 : public Base {};
>>>
>>> class Y
>>> {
>>> std::vector<Base*> m_vb[2];
>>>
>>> public:
>>>
>>> void Add(Derived0* d0) { m_vb[0].push_back(d0); }
>>> void Add(Derived1* d1) { m_vb[1].push_back(d1); }
>>>
>>> const std::vector<Derived0*>* GetD0()
>>> { return (const std::vector<Derived0*>*) &m_vb[0]; }
>>> const std::vector<Derived1*>* GetD1()
>>> { return (const std::vector<Derived1*>*) &m_vb[1]; }
>>>
>>> const std::vector<Derived0*>* GetDD0()
>>> { return static_cast<const std::vector<Derived0*>*>(&m_vb[0]); }
>>> const std::vector<Derived1*>* GetDD1()
>>> { return static_cast<const std::vector<Derived1*>*>(&m_vb[1]); }
>>>
>>> };
>>
>> Both the C-style cast and the static_cast are wrong; you cannot cast a
>> std::vector<base*> to a std::vector<derived*>.
>>
>> Instead have these getter functions return a std::vector<base*> and
>> downcast (using static_cast or dynamic_cast) the vector's elements
>> instead elsewhere in your code.
>>
>> /Leigh
>
>
>I do not see why the C-style cast is wrong.  It works flawlessly across a 
>wide range of compilers and systems.

It may work in the case of simple inheritance, but it usually fails when 
multiple and/or virtual inheritance is going to be used.

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static_cast and std::vector john <gh14tq5@yahoo.com> - 2012-01-20 08:56 -0500
  Re: static_cast and std::vector Leigh Johnston <leigh@i42.co.uk> - 2012-01-20 14:00 +0000
    Re: static_cast and std::vector john <gh14tq5@yahoo.com> - 2012-01-20 09:58 -0500
      Re: static_cast and std::vector Leigh Johnston <leigh@i42.co.uk> - 2012-01-20 15:02 +0000
        Re: static_cast and std::vector john <gh14tq5@yahoo.com> - 2012-01-20 10:28 -0500
      Re: static_cast and std::vector "Fred Zwarts \(KVI\)" <F.Zwarts@KVI.nl> - 2012-01-20 16:48 +0100
        Re: static_cast and std::vector Leigh Johnston <leigh@i42.co.uk> - 2012-01-20 16:02 +0000
          Re: static_cast and std::vector Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-20 14:30 -0800
      Re: static_cast and std::vector Juha Nieminen <nospam@thanks.invalid> - 2012-01-20 19:04 +0000
  Re: static_cast and std::vector Tobias Müller <troplin@bluewin.ch> - 2012-01-21 08:56 +0000
    Re: static_cast and std::vector john <gh14tq5@yahoo.com> - 2012-01-23 12:47 -0500
      Re: static_cast and std::vector Ian Collins <ian-news@hotmail.com> - 2012-01-24 09:22 +1300
  Re: static_cast and std::vector Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-01-21 23:02 -0500

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