std::istream_iterator and std::ostream_iterator in C++ STL

std::istream_iterator and std::ostream_iterator are stream iterators provided by the C++ Standard Library that allow streams to be treated like containers. They provide a convenient way to read data from input streams and write data to output streams using STL algorithms. 

• std::istream_iterator is used to read values from an input stream.
• std::ostream_iterator is used to write values to an output stream.

Stream Iterators

Stream iterators are special iterators that work with input and output streams. They act as a bridge between STL algorithms and stream objects, allowing operations such as copying, filtering, and transforming data using standard algorithms.

To use stream iterators, include the <iterator> header file:

#include <iterator>

There are two types of stream iterators:

std::istream_iterator

std::istream_iterator is an input iterator that reads values sequentially from an input stream.

• Supports only sequential reading.
• Behaves like an input iterator.
• Provides a special end-of-stream iterator.
• Automatically advances after reading.

Class Template

template <
class T,
class CharT = char,
class Traits = std::char_traits<CharT>
>
class istream_iterator;

Syntax

istream_iterator<T> itr(stream);

where:

• T is the type of data to read.
• stream is an input stream object.

std::ostream_iterator

std::ostream_iterator is an output iterator that writes values sequentially to an output stream.

• Supports only writing operations.
• Behaves like an output iterator.
• Can be used with STL algorithms such as copy().
• Supports optional delimiters.

Class Template

template <
class T,
class CharT = char,
class Traits = std::char_traits<CharT>
>
class ostream_iterator;

Syntax

ostream_iterator<T> itr(stream);
ostream_iterator<T> itr(stream, delimiter);

where:

• T is the type of data to write.
• stream is an output stream object.
• delimiter is an optional separator inserted after each element.

Examples of Stream Iterators

Example 1: Copy Input Stream to Output Stream

#include <algorithm>
#include <iostream>
#include <iterator>

using namespace std;
int main()
{

    // Get input stream and end of stream iterators
    istream_iterator<int> cin_it(cin);
    istream_iterator<int> eos;

    // Get output stream iterators
    ostream_iterator<int> cout_it(cout, " ");

    // We have both input and output iterators, now we can treat them
    // as containers. Using copy function we transfer data from one
    // container to another.
    // Copy elements from input to output using copy function
    copy(cin_it, eos, cout_it);

    return 0;
}

Input: 1 2 3 4 5 
Output: 1 2 3 4 5

Explanation: The copy() algorithm reads integers from cin using istream_iterator and writes them to cout using ostream_iterator.

Example 2: Read Data from File and Sort It

#include <algorithm>
#include <fstream>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

using namespace std;
int main()
{

    // Define a vector to store the strings received from input
    vector<string> strings_v;

    // Define the filestream object used to read data from file
    ifstream fin("input_file.txt");

    // Get input stream and end of stream iterators
    istream_iterator<string> fin_it(fin);
    istream_iterator<string> eos;

    // Get output stream iterators
    ostream_iterator<string> cout_it(cout, " ");

    // Copy elements from input to vector using copy function
    copy(fin_it, eos, back_inserter(strings_v));

    // Sort the vector
    sort(strings_v.begin(), strings_v.end());

    // Copy elements from vector to output
    copy(strings_v.begin(), strings_v.end(), cout_it);

    return 0;
}

Contents of File "input_file.txt": quick brown fox jumps over the lazy dog
Output: brown dog fox jumps lazy over quick the 

Example 3: Filter and Print Even Numbers

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

using namespace std;
int main()
{

    // Define a vector to store the even integers received from input
    vector<int> vi;

    // Get input stream and end of stream iterators
    istream_iterator<int> cin_it(cin);
    istream_iterator<int> eos;

    // Get output stream iterators
    ostream_iterator<int> cout_it(cout, " ");

    // Copy even integer elements from input to vector using for_each function
    for_each(cin_it, eos, [&](int a) {
        if (a % 2 == 0) {
            // if a is even push it to vector
            vi.push_back(a);
        }
    });

    // Sort the vector
    sort(vi.begin(), vi.end());

    // Copy elements from vector to output
    copy(vi.begin(), vi.end(), cout_it);

    return 0;
}

Input: 1 4 3 2 6 8 31 52 
Output: 2 4 6 8 52 

Difference Between istream_iterator and ostream_iterator

References: 

istream_iterator 
ostream_iterator