Stream object to represent file input and then standard input? - c++

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