copy std::cin to a ifstream - c++

Ralf Goertz wrote:
> Hi,
>
> consider the following program
>
> #include <iostream>
> #include <fstream>
>
> using namespace std;
>
> int main(int argc, char *argv[]){
> if (argc<2) exit(1);
> ifstream infile;
> if (argv[1]!="-")
> infile.open(argv[1]);
> else
> infile=cin; //doesn't compile
> // read from infile
> return 0;
> }
>
> In c I can just assign stdin to a FILE*. What would be the c++ way of
> doing this? Do I have to use a pointer here, too? I could use
> infile.open("/dev/stdin") but that's probably not portable.
>
> Thanks,
>
> Ralf

I'd use a pointer

istream* input;
ifstream infile;
if (argv[1]!="-")
{
infile.open(argv[1]);
input = &infile;
}
else
{
input = &cin;
}
// read from *input

john

John Harrison wrote:
> Ralf Goertz wrote:
>> Hi,
>>
>> consider the following program
>>
>> #include <iostream>
>> #include <fstream>
>>
>> using namespace std;
>>
>> int main(int argc, char *argv[]){
>> if (argc<2) exit(1);
>> ifstream infile;
>> if (argv[1]!="-")
>> infile.open(argv[1]);
>> else
>> infile=cin; //doesn't compile
>> // read from infile
>> return 0;
>> }
>>
>> In c I can just assign stdin to a FILE*. What would be the c++ way of
>> doing this? Do I have to use a pointer here, too? I could use
>> infile.open("/dev/stdin") but that's probably not portable.
>>
>> Thanks,
>>
>> Ralf
>
> I'd use a pointer
>
> istream* input;
> ifstream infile;
> if (argv[1]!="-")
> {
> infile.open(argv[1]);
> input = &infile;
> }
> else
> {
> input = &cin;
> }
> // read from *input
>
> john

If you like you could use a reference (untested code)

ifstream infile;
istream& input = pick_input(infile, argv);


istream& pick_input(char** argv, ifstream& infile)
{
if (argv[1]!="-")
{
infile.open(argv[1]);
return infile;
}
else
{
return cin;
}
}

John Harrison wrote:

> John Harrison wrote:
>
> If you like you could use a reference (untested code)
>
> ifstream infile;
> istream& input = pick_input(infile, argv);
>
>
> istream& pick_input(char** argv, ifstream& infile)
> {
> if (argv[1]!="-")
> {
> infile.open(argv[1]);
> return infile;
> }
> else
> {
> return cin;
> }
> }

That would be an idea, thanks.

I just thought of doing it the other way around, that is always using
cin for reading after reopening it with the filename in argv[1] if that
is not "-". However, std::istream doesn't have either open() or close().
FAQ 15.13 mentions that it is in principle possible to close and reopen
std::cin. But probably only in a nonstandard way. Just because I'm
curious: Is there a reason why it would be undesirable to have a program
decide on it's own what it's standard input would be?

Ralf