Capture file via stdin/cin with < and read to EOF

Question

My main.cpp

#include <iostream>
#include <fstream>

int main(int argc, char* argv[]) {
   std::string filename;
   std::cin >> filename;
   std::ifstream inf(filename);
   std::string line;
   while(std::getline(inf, line)) {
         std::cout << line << std::endl;
   }

   return 0;
}

Trying to read:

main < somefile.txt

I was old that when you write, < or > it's a directive for stdout/stdin (cin/cout). I want to read line by line, and check for EOF, in C++.

EDIT: Nothing really happens. Are there any C++ 20 recommendations for faster C++, I know C is normally faster.

file.txt:

300 55 12
56 99 -21

Nothing happens, even if the file under same directory contains content. — John Smith, Aug 29 '20 at 19:13
Currently, what your code appears to be doing is reading a line from stdin, and then opening a file with that name. Is that what you expect it to be doing? — SuperStormer, Aug 29 '20 at 19:14
Yes, indeed. I wrote it this way. But nothing comes out from the `cout` inside the `while`-loop. — John Smith, Aug 29 '20 at 19:14
@SuperStormer, `main < somefile.txt` I expect it to open that file or any file I pass to it like that. I tried, `argv[1]` but nothing. — John Smith, Aug 29 '20 at 19:15
@drescherjm, no it's under the same directory as the executable. — John Smith, Aug 29 '20 at 19:15
I tested it locally and it works. Does your file have a newline after the filename input? — SuperStormer, Aug 29 '20 at 19:16
https://pastebin.com/3KH8xBYK `somefile.txt` itself contains a filename. — SuperStormer, Aug 29 '20 at 19:18
File redirection with < sends it directly to stdin, so you don't need to create a ifstream. — SuperStormer, Aug 29 '20 at 19:19
@SuperStormer, I know. But, I have to somehow keep track of EOF of the FILE? — John Smith, Aug 29 '20 at 19:20
@SuperStormer, I know. But, I want to know how to check EOF manually in C++ and break! Thx. — John Smith, Aug 29 '20 at 19:23

SuperStormer · Accepted Answer · 2020-08-29T19:22:19.397

3

File redirection with < sends it directly to stdin, so you don't need to create a std::ifstream.

#include <iostream>

int main(int argc, char* argv[]) {
   std::string line;
   while(std::getline(std::cin, line)) {
         std::cout << line << std::endl;
   }

   return 0;
}

edited Aug 29 '20 at 19:22

answered Aug 29 '20 at 19:20

SuperStormer

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I wrote code like this, the writing of code isn't a big issue mate. But, I am trying to figure out how I can check EOF and break. – John Smith Aug 29 '20 at 19:22
2

Your code already does that right here: `while(std::getline(std::cin, line)) {` std::getline returns false when you hit the end of the file. – drescherjm Aug 29 '20 at 19:22
I know. But, `while(std::getline(std::cin, line) != std::cin::eof())` ? – John Smith Aug 29 '20 at 19:25
1

The current code is correct and it will avoid this issue: [https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons) – drescherjm Aug 29 '20 at 19:27
1

getline returns false on eof – SuperStormer Aug 29 '20 at 19:27

Capture file via stdin/cin with < and read to EOF

1 Answers1