CODEWITHSUNDEEP
c++stream
kal
kal

Reputation: 29411

How to print an entire istream to standard out and string

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

Upvotes: 23

Views: 37623

Answers (5)

Stack Overflow is garbage
Stack Overflow is garbage

Reputation: 248219

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.

Upvotes: 12

Shakiba Moshiri
Shakiba Moshiri

Reputation: 23914

std::ifsream overloading and std::ostringstream

c++11 or upper

Pay attention to && in std::ifstream that allow you to direct using

#include <iostream>
#include <sstream>
#include <fstream>

std::ostream& operator<<(std::ostream& os, std::basic_ostringstream&& iss){
    return os<<iss.str();
}

std::ostream& operator<<(std::ostream& os, std::ifstream&& ifs){
    return std::cout<<ifs.rdbuf();
}


int main()
{
    std::cout<<std::ostringstream("Test ostringstream overloading")<<std::endl;
    std::ofstream("fstream.txt")<<"Test fstream overloading"<<std::endl;
    std::cout<<std::ifstream("fstream.txt")<<std::endl; // overloading okay
}

output:

Test ostringstream overloading
Test fstream overloading


Process returned 0 (0x0)   execution time : 0.012 s
Press ENTER to continue.

Upvotes: 0

&#201;ric Malenfant
&#201;ric Malenfant

Reputation: 14148

You ouput the istream's streambuf.

For example, to output an ifstream to cout:

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

Upvotes: 28

rlbond
rlbond

Reputation: 67839

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;

Upvotes: 7

1800 INFORMATION
1800 INFORMATION

Reputation: 135433

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

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

Upvotes: 1

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