How to print an entire istream to standard out and string

Question

How do you print an istream variable to standard out. [EDIT] I am trying to debug a scenario wherein I need to ouput an istream to a log file

Answer 1

You ouput the istream's streambuf.

For example, to output an ifstream to cout:

std::ifstream f("whatever");
std::cout << f.rdbuf();

Answer 2

Edit: I'm assuming that you want to copy the entire contents of the stream, and not just a single value. If you only want to read a single word, check 1800's answer instead.

---

The obvious solution is a while-loop copying a word at a time, but you can do it simpler, as a nice oneliner:

#include <iostream>
#include <iterator>

...

std::istream i;
std::copy(std::istream_iterator<char>(i), std::istream_iterator<char>(), std::ostream_iterator<char>(std::cout));

The stream_iterators use operator << and >> internally, meaning they'll ignore whitespace. If you want an exact copy, you can use std::istreambuf_iterator and std::ostreambuf_iterator instead. They work on the underlying (unformatted) stream buffers so they won't skip whitespace or convert newlines or anything.

You may also use:

 i >> std::noskipws;

to prevent whitespace from disappearing. Note however, that if your stream is a binary file, some other characters may be clobbered by the >> and << operators.

Answer 3

This will print the whole stream, 1 character at a time:

char c;
c = my_istream.get();
while (my_istream)
{
    std::cout << c;
    c = my_istream.get();
}

This will print the whole thing, but discard whitespace:

std::string output;
while(my_istream >> output)
    std::cout << output;

Answer 4

You need to read from it, and then output what you read:

istream stm;
string str;
stm >> str;
cout << str;