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Re: Confused about casts

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Em 21-12-2011 02:28, Alf P. Steinbach escreveu:
> On 21.12.2011 02:54, Paulo da Silva wrote:
>>
>> using namespace std;
>>
>> #include<iostream>
>> #include<fstream>
>>
>> int main(int argc,char **argv)
>> {    ifstream *in_p;
>>     int val;
>>     if (argc==2)
>>     {    in_p=new ifstream(argv[1]);
>>         // Test in_p->good()
>>     }
>>     else
>>         in_p=static_cast<ifstream*>(&cin);
>>     *in_p>>  val;
>>     cout<<  "val="<<  val<<  endl;
>>     if (in_p!=&cin)
>>         delete in_p;
>>     return 0;
>> }
>>
>> In the line "in_p=static_cast<ifstream*>(&cin);" both static and
>> reinterpret casts seem to work. Which is (more) correct?
> 
> They are both technically allowed, and both are *wrong*.
> 
> static_cast is allowed because ifstream is derived from istream so that
> the opposite direction would convert implicitly.
> 
> reinterpret_cast is allowed because just about any data pointers can be
> reinterpret_cast'ed, although for cases outside the conversions listed
> here and there in the standard the result (and whether it's allowed)
> depends on the compiler and compiler options.
> 
> You're doing a so called *downcast*, to a more derived class.
> 
> That's only proper if the referred to object is of the derived class (or
> more derived).
> 
> Here's what you should do, given the apparent goals of your code:
> 
> 
> <code>
> using namespace std;
> 
> #include <iostream>
> #include <fstream>
> 
> void do_things( istream& in )
> {
>     int val;
> 
>     in >> val;
>     cout << "val=" << val << endl;
> }
> 
> int main( int argc,char *argv[] )
> {   
>     if (argc==2)
>     {
>         ifstream f( argv[1] );
>         // Test in_p->good()
>         do_things( f );
>     }
>     else
>     {
>         do_things( cin );
>     }
> }
> </code>
Thanks!
But using the same logic, may be a little change fixes the problem.
Isn't this the same?

<code>
using namespace std;

#include <iostream>
#include <fstream>

int main(int argc,char **argv)
{	istream *in_p;
	int val;
	if (argc==2)
	{	in_p=new ifstream(argv[1]);
		// Test in_p->good()
	}
	else
		in_p=&cin;
	*in_p >> val;
	cout << "val=" << val << endl;
	if (in_p!=&cin)
		delete in_p;
	return 0;
}
</code>



> 
> 
> Disclaimer: code untouched by compiler's dirty hands.
> 
> 
> Cheers & hth.,
> 
> - Alf
> 
> PS: I liked your placing "using namespace std;" at the very top. I'll
> start doing that. It's never to late to pick up a good idea! :-)

Yes :-) For small/medium projects no need for namespaces.
Thank you

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Thread

Confused about casts Paulo da Silva <p_s_d_a_s_i_l_v_a@netcabo.pt> - 2011-12-21 01:54 +0000
  Re: Confused about casts "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2011-12-21 03:28 +0100
    Re: Confused about casts Paulo da Silva <p_s_d_a_s_i_l_v_a@netcabo.pt> - 2011-12-21 18:56 +0000
      Re: Confused about casts Leigh Johnston <leigh@i42.co.uk> - 2011-12-21 19:10 +0000
      Re: Confused about casts "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2011-12-21 20:32 +0100
    Re: Confused about casts Paulo da Silva <p_s_d_a_s_i_l_v_a@netcabo.pt> - 2011-12-21 19:32 +0000
    Re: Confused about casts Paulo da Silva <p_s_d_a_s_i_l_v_a@netcabo.pt> - 2011-12-21 19:34 +0000
  Re: Confused about casts (sorry) Paulo da Silva <p_s_d_a_s_i_l_v_a@netcabo.pt> - 2011-12-21 19:45 +0000

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