I/O Redirection in C++


In C, we could use the function freopen() to redirect an existing FILE pointer to another stream. The prototype for freopen() is given as

FILE * freopen ( const char * filename, const char * mode, FILE * stream );

For Example to redirect the stdout to say a textfile, we could write

freopen ("text_file.txt", "w", stdout);

While this method is still supported in C++, this article discusses another way to redirect I/O streams.

C++ being an object-oriented programming language gives us the ability to not only define our own streams but also redirect standard streams. Thus in C++, a stream is an object whose behavior is defined by a class. Thus anything that behaves like a stream is also a stream.

Streams Objects in C++ are mainly of three types :



  • istream : Stream object of this type can only perform input operations from the stream
  • ostream : Theese objects can only be used for output operations.
  • iostream : Can be used for both input and output operations

All these classes, as well as file stream classes, derive from the classes: ios and streambuf. Thus filestream and IO stream objects behave similarly.

All stream objects also have an associated data member of class streambuf. Simply put streambuf object is the buffer for the stream. When we read data from a stream, we don’t read it directly from the source, but instead, we read it from the buffer which is linked to the source. Similarly, output operations are first performed on the buffer, and then the buffer is flushed (written to the physical device) when needed.

C++ allows us to set the stream buffer for any stream. So the task of redirecting the stream simply reduces to changing the stream buffer associated with the stream. Thus the to redirect a Stream A to Stream B we need to do

  1. Get the stream buffer of A and store it somewhere
  2. Set the stream buffer of A to the stream buffer of B
  3. If needed reset the stream buffer of A to its previous stream buffer

We can use the function ios::rdbuf() to perform two opeations.

1) stream_object.rdbuf(): Returns pointer to the stream buffer of stream_object
2) stream_object.rdbuf(streambuf * p): Sets the stream buffer to the object pointed by p

Here is an example program below to show the steps

// Cpp program to redirect cout to a file
#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main()
{
    fstream file;
    file.open("cout.txt", ios::out);
    string line;

    // Backup streambuffers of  cout
    streambuf* stream_buffer_cout = cout.rdbuf();
    streambuf* stream_buffer_cin = cin.rdbuf();

    // Get the streambuffer of the file
    streambuf* stream_buffer_file = file.rdbuf();

    // Redirect cout to file
    cout.rdbuf(stream_buffer_file);

    cout << "This line written to file" << endl;

    // Redirect cout back to screen
    cout.rdbuf(stream_buffer_cout);
    cout << "This line is written to screen" << endl;

    file.close();
    return 0;
}

Output:

This line is written to screen
Contents of file cout.txt:
This line written to file

Note:
The above steps can be condensed into a single step

auto cout_buf = cout.rdbuf(file.rdbuf())

// sets couts streambuffer and returns the old 
streambuffer back to cout_buf

References:
CPP IOS



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