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Stream iterators

Stream iterator将stream和iterator联系起来，通过iterator，进而连接了algorithm，因此通过stream iterator，可以将algorithm应用于stream。

关于stream，参见C++\Library\Standard-library\Input-output-library。

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

NOTE: 这篇文章论述得非常好

ostream_iterator

把这两种output stream iterator放到一起来进行介绍。

cppreference std::ostream_iterator

cppreference std::ostreambuf_iterator

std::ostream_iterator VS std::ostreambuf_iterator

在std::ostream_iterator中有这样的描述：

When writing characters, std::ostreambuf_iterator is more efficient, since it avoids the overhead of constructing and destructing the sentry object once per character.

显然std::ostream_iterator需要sentry object，那sentry object是什么呢？

而std::ostreambuf_iterator是通过：

a boolean flag indicating if the the end of file condition has been reached

istream_iterator

cppreference std::istream_iterator

std::istream_iterator is a single-pass input iterator that reads successive objects of type T from the std::basic_istream object for which it was constructed, by calling the appropriate operator>>.

NOTE: std::istream_iterator的特性：

  • single pass
  • input iterator

The default-constructed std::istream_iterator is known as the end-of-stream iterator. When a valid std::istream_iterator reaches the end of the underlying stream, it becomes equal to the end-of-stream iterator. Dereferencing or incrementing it further invokes undefined behavior.

NOTE: 在下面的example中，就用到了“The default-constructed std::istream_iterator is known as the end-of-stream iterator. ”来标志end-of-stream。

Notes

When reading characters, std::istream_iterator skips whitespace by default (unless disabled with std::noskipws or equivalent), while std::istreambuf_iterator does not. In addition, std::istreambuf_iterator is more efficient, since it avoids the overhead of constructing and destructing the sentry object once per character.

Example

#include <iostream>
#include <sstream>
#include <iterator>
#include <numeric>
#include <algorithm>

int main()
{
    std::istringstream str("0.1 0.2 0.3 0.4");
    std::partial_sum(std::istream_iterator<double>(str),
                     std::istream_iterator<double>(),
                     std::ostream_iterator<double>(std::cout, " "));

    std::istringstream str2("1 3 5 7 8 9 10");
    std::cout << "\nThe first even number is " <<
        *std::find_if(std::istream_iterator<int>(str2),
                      std::istream_iterator<int>(),
                      [](int i){return i%2 == 0;})
        << ".\n";
    //" 9 10" left in the stream
}

NOTE: std::istream_iterator<double>()原来标识end-of-stream。

cppreference std::istreambuf_iterator

Example: output container

Output to std out

取自: stackoverflow How to use ostream_iterator<> to output result to a newfile?

// ostream_iterator example
#include <iostream>     // std::cout
#include <iterator>     // std::ostream_iterator
#include <vector>       // std::vector
#include <algorithm>    // std::copy

int main()
{
    std::vector<int> myvector;
    for (int i = 1; i < 10; ++i)
    {
        myvector.push_back(i * 10);
    }

    std::ostream_iterator<int> out_it(std::cout, ", ");
    std::copy(myvector.begin(), myvector.end(), out_it);
    return 0;
}
// g++ test.cpp

取自: https://www.tenouk.com/cpluscodesnippet/cplusstlostream_iteratorconstructor.html

// C++ STL ostream_iterator::ostream_iterator, constructor

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

using namespace std;

int main(void)

{

    // C++ STL ostream_iterator for stream cout
    cout << "Inserting & printing elements: " << endl;
    ostream_iterator<int> intOut(cout, "\n");
    // insert data
    *intOut = 12;
    intOut++;
    *intOut = 33;
    intOut++;
    int i;

// vector container
    vector<int> vec;
// push/insert data into vec vector
    for (i = 10; i <= 15; ++i)
    {
        vec.push_back(i);
    }
    cout << "Operation: with and without delimiter..." << endl;

// write elements to standard output stream

    cout << "Elements output without delimiter: ";
    copy(vec.begin(), vec.end(), ostream_iterator<int>(cout));
    cout << endl;

// write elements with delimiter " " to output stream
    cout << "Elements output with delimiter: ";
    copy(vec.begin(), vec.end(), ostream_iterator<int>(cout, " "));
    cout << endl;

    return 0;
}
// g++ test.cpp

Output to file

取自: https://stackoverflow.com/a/22876942

// ostream_iterator example
#include <iostream>     // std::cout
#include <iterator>     // std::ostream_iterator
#include <vector>       // std::vector
#include <algorithm>    // std::copy
#include <fstream>      // std::ofstream

int main()
{
    std::vector<int> myvector;
    for (int i = 1; i < 10; ++i)
    {
        myvector.push_back(i * 10);
    }

    std::ofstream of("myoutput.txt");

    std::ostream_iterator<int> out_it(of, ", ");
    std::copy(myvector.begin(), myvector.end(), out_it);
    return 0;
}
// g++ test.cpp

Read from std in and print

源自: https://www.geeksforgeeks.org/stdistream_iterator-stdostream_iterator-c-stl/

// Cpp program to illustrate
// Read a bunch of integers from the input stream
// and print them 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;
}
// g++ test.cpp

Read from file and sort

源自: https://www.geeksforgeeks.org/stdistream_iterator-stdostream_iterator-c-stl/

// Cpp program to illustrate
// Read a bunch of strings from a file
// sort them lexicographically and print them to output stream

#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;
}
// g++ test.cpp

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

Read from std in and sort

源自: https://www.geeksforgeeks.org/stdistream_iterator-stdostream_iterator-c-stl/

// Cpp program to illustrate

// Read a bunch of integers from the stream
// print the sorted order of even integers only

#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;
}
// g++ --std=c++11 test.cpp

使用下面的方式进行测试:

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ctr+D // 表示EOF