Transparent read from stdin or a file in C++? - c++

Question

I would like to parse either an stdin stream or a file. So I want a function/method to accept either of this.
Note that my goal is not to call read twice!
As istream is the base class for cin and ifstream` I should be able write this:
#include <iostream>
#include <fstream>
void read(std::istream &fp) {
while(!fp.eof()) {
std::string line;
std::getline(fp, line);
std::cout << line << std::endl;;
}
}
int main(int argc, char *argv[])
{
std::ifstream fp;
if (argc >= 2) {
fp.open(argv[1]);
if (!fp) abort();
}
else {
fp = std::cin;
}
read(fp);
if (fp.is_open()) {
fp.close();
}
return 0;
}
In C I can do the following with calling it with either read_content(stdin) or read_content(fp):
void read_content(FILE *file)
What is the proper way to do this in C++?

Answer 1

std::cin is an instance of std::istream and not derived from std::ifstream but the opposite is true.
Inheritance graph of Stream-based I/O:
(Taken from cppreference.com - Input/output library)
So, OPs intention can be performed using a reference or pointer to std::istream.
Demo:
#include <iostream>
#include <fstream>
void read(std::istream &fp) {
while(!fp.eof()) {
std::string line;
std::getline(fp, line);
std::cout << line << std::endl;;
}
}
int main(int argc, char *argv[])
{
std::ifstream fp;
std::istream &in = (argc >= 2)
? [&]() -> std::istream& {
fp.open(argv[1]);
if (!fp) abort();
return fp;
}()
: std::cin;
read(in);
if (fp.is_open()) {
fp.close();
}
return 0;
}
Compiled on coliru
Note:
while (!fp.eof()) {
should be replaced by
while (fp) {
The reason for this has been thoroughly discussed in
SO: Why is iostream::eof inside a loop condition (i.e. while (!stream.eof())) considered wrong?.

Answer 2

In C I can do the following with calling it with either read_content(stdin) or read_content(fp):
Yes, and in C++ you should just call either read(std::cin) or read(fp). It's exactly the same.
The line
fp = std::cin;
is the wrong thing to do, as std::cin is only declared as a std::istream. There is no std::ifstream constructor taking an istream, you don't want an independent object here anyway, and if std::cin is really an object of some type derived from std::ifstream, you'd slice it.