Re: static_cast and std::vector

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From	Ian Collins <ian-news@hotmail.com>
Newsgroups	comp.lang.c++
Subject	Re: static_cast and std::vector
Date	Tue, 24 Jan 2012 09:22:54 +1300
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On 01/24/12 06:47 AM, john wrote:
>>> #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]); }
>>>
>>> };
>>
>> Why don't you just write:
>> std::vector<Derived0*>   m_vd0;
>> std::vector<Derived1*>   m_vd1;
>> and use those without any casting?
>>
>> This would be the most obvious solution and you are losing nothing.
>
> We have looked into this, but we also really need functionality like
>
>     const std::vector<Base*>* GetBase(int which)
>    	{ return&m_vb[which]; }
>
> So, changing as you suggested would preclude this.  Would it be possible
> to replace the UB casts
>
>        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]; }
>
> with some type of specialized iterator?

What do you do with the returned vectors?

You don't have a vector of Derived0* to return, which is why others have 
told you the casts are UB.  If you are iterating through them, you could 
add a (template) method to perform the iteration with the appropriate 
cast of the vector elements (not the vector) to your container class.

-- 
Ian Collins

Thread

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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