流对象代表文件输入，然后代表标准输入？

Question

As a noob C++ project, I am building a CLI game with a game loop that relies upon user input. For the purpose of testing, I would like to be able to pass a file name as a command line argument to the program, which will be treated "like standard input" until it is read through.

Consequently, I need a way to encapsulate an object that represents a file's contents and then std::cin once the file has been read through - basically prepending a file's contents to standard input. At least, this seems like it would be very nice, so that I can avoid having something like

std::istringstream retrieve_input(std::ifstream &file) {
  std::string line;
  if (std::getline(file, line))
    return std::istringstream{line};
  std::getline(std::cin, line);
  return std::istringstream{line};
}

Is replacing this with some kind of custom class a good approach? Or can I mess with std::cin somehow to prepend my file contents to it?

---

To clarify : My game loop might have say, 20 iterations. If I pass a file with five lines, I want to use those five lines for the first five iterations of my game loop, and then drop back to standard input for the remaining fifteen. I understand how to do this with a bunch of conditions, but I think there must be a way to nicely have this sort of behavior in a class. My problem is - what should it inherit from? std::streambuf ? Is this a good idea in principle?

My (probably bad attempt)

class InputGrabber {
public:
  virtual std::istringstream retrieve_line() = 0;
  virtual ~InputGrabber() {}
};

class BaseInputGrabber : public InputGrabber {
public:
  BaseInputGrabber(std::istream &_in): in{_in} {}
  std::istringstream retrieve_line() override {
    std::string line;
    std::getline(in, line);
    return std::istringstream{line};
  }

private:
  std::istream ∈
};

class VarInputGrabber : public InputGrabber {
public:
  VarInputGrabber(std::istream &_in, const std::string &f_name) :
    in{_in}, init{std::ifstream(f_name)} {}
  std::istringstream retrieve_line() override {
    std::string line;
    if (std::getline(init, line)) {
      return std::istringstream{line};
    }
    std::getline(in, line);
    return std::istringstream{line};
  }
private:
  std::istream ∈
  std::ifstream init;
};

Answer 1

Would the below work? You open the file, redirect std::cin to read from the file. Use std::getline on std::cin so that it reads from the file stream. Some time before the program ends or if the file is finished being read, restore std::cin back to its original state.

#include <iostream>
#include <fstream>

int main(int argc, const char * argv[]) {

    std::fstream file("/users/Brandon/Desktop/testingInput.txt", std::ios::in);

    //Redirect cin to a file.
    std::streambuf* cinBuffer = std::cin.rdbuf();
    std::cin.rdbuf(file.rdbuf());

    //Read line from std::cin (which points to the file) with std::getline.
    std::string line;
    std::getline(std::cin, line);

    std::cout<<"INPUT LINE: "<<line<<std::endl;

    //Redirect cin to standard input.
    std::cin.rdbuf(cinBuffer);

    //Read some line from the user's input..
    std::getline(std::cin, line);
    std::cout<<"USER INPUT LINE: "<<line<<std::endl;
    std::cin.get();

    return 0;
}

Answer 2

You could always redirect from file via argv. Treat the whole thing as std::cin from the beginning.

Answer 3

Is the following example that what you are looking for?

std::string readNextLine(std::istream& is)
{
    std::string line;

    if(!std::getline(is, line))
        std::getline(std::cin, line);

    return line;
}

int main()
{
    std::stringstream ss ("a\nb\nc\n");

    for(int i = 0; i < 5; ++i)
    {
        auto line = readNextLine(ss);

        std::cout << "got: " << line << std::endl;
    }

    return 0;
}

The stringstream can be replaced with fstream or any other type that implements std::istream.

You can test it here http://cpp.sh/5iv65