C++: how to get input using two kind of ways,like cin and file?

Question

I have written a c++ program which can get info from a file, but it must support two ways of reading the data.

The first way is by passing the filename as a command line parameter: ./a.out a.txt

The second one is by reading from a pipe: cat a.txt | ./a.out

The second way is more important and more difficult for me. How can it be done?

Here is detail about my question:

There is a class,which i defined like this:

class API_EXPORT SamReader{

// constructor / destructor

public: SamReader(void); ~SamReader(void);

// closes the current SAM file
bool Close(void);
// returns filename of current SAM file
const std::string GetFilename(void) const;

// return if a SAM file is open for reading
bool IsOpen(void) const;

// opens a SAM file
// *** HERE　IS MY QUESTION
bool Open(const std::string& filename);
bool Open(std::istream cin); //HERE~~~~

// retrives next available alignment
bool LoadNextAlignment(BamAlignment& alignment);

private: Internal::SamReaderPrivate * d; };

I have got the way to input a file and get the result, But I need to add a func in this class, make it can input from stdin...

Thanks for help, I am new guy here and I am really appreciate people who help me just now.

Answer 1

To read from a pipe, your program just simply need to read from stdin. That's how pipe in shell works, if you run a | b, it simply passes all output on stdout of a to stdin of b.

So basically your program needs to cover 2 cases: (1) read from a file if one is provided and (2) read from stdin otherwise.

To avoid code repetition, the easiest way is always read from stdin. That obviously will cover (2). To cover (1), when a file is provided as a param, use freopen to override stdin with that file.

#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main(int argc, char** argv) {
  if (argc > 1) {
    freopen(argv[1], "r", stdin);
  }

  string s;
  while (cin >> s) {
    cout << s << endl;
  }

  return 0;
}

Demonstrating how this works

$ g++ test.cc
$ cat content
This
is
a
test
$ ./a.out content
This
is
a
test
$ cat content | ./a.out
This
is
a
test

Answer 2

Just like @Sander De Dycker said. This is none of program's business. it depends on the shell you're using, I guess you using the classic sh.

You can write a very simple c++ program, such as:

#include <string>
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<string> v;
    string s;

    while (cin >> s)
    v.push_back(s);

    for (int i = 0; i != v.size(); ++i)
    cout << v[i] << endl;

    return 0;
}

then, you can type ./a.out a.txt for using the a.txt as output, or type cat a.txt | ./a.out for using the a.txt as input. or you also can mix that two commands.