[ale] Q: directing stream I/O _the_C++_way_

[Joe Knapka]

Hi John,

Your code is failing due to the semantics of references.

(1) References must always be initialized when declared, and

(2) Assigning to a reference is exactly the same as assigning to the
referenced object; since ostreams aren't assignable (operator== is
private), assigning to a reference to an ostream won't work either.

You can use pointers, but it's ugly:

#include <iostream>
#include <fstream>
using namespace std;

int main(int argc, char* argv[]) {
	ostream* out = &cout;
	if (argc>1) {
		out = new fstream(argv[1]);
	}
	ostream& ref_out = *out;
	ref_out << "Hello, world!" << endl;
    if (out != &cout) delete out; // Ick-a-rama...
}

Cheers,

-- Joe

John Mills <johnmills at speakeasy.net> writes:

> Fletch --
> 
> 
> On Sat, 27 Nov 2004, Fletch wrote:
> 
> > I want to say that you should be able to declare an ostream and assign
> > either an ofstream or cout as desired.
> > 
> > ostream& log;  <-------[initializing to 'cout' => same whine]-----+
> >                                                                   |
> > if( logfilename ) {                                               |
> >   ofstream logfile( logfilename.c_str() );                        |
> >   log = logfile;                                                  |
>     ^^^^^^^^^^^^^---- This doesn't compile. I get:                  |
>                                                                     V
> Switchbox.cpp:38: `logging' declared as reference but not initialized
> /usr/include/c++/3.2.3/bits/ios_base.h: In member function 
>    `std::basic_ios<char, std::char_traits<char> >& std::basic_ios<char, 
>    std::char_traits<char> >::operator=(const std::basic_ios<char, 
>    std::char_traits<char> >&)':
> /usr/include/c++/3.2.3/bits/ios_base.h:427: `std::ios_base& 
>    std::ios_base::operator=(const std::ios_base&)' is private
> Switchbox.cpp:133: within this context
> make: *** [Switchbox.o] Error 1
> 
> > } else {
> >   log = cout;
> > }
> 
>  - John Mills
>    john.m.mills at alum.mit.edu
> 
> _______________________________________________
> Ale mailing list
> Ale at ale.org
> http://www.ale.org/mailman/listinfo/ale
> 
> 

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--
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