using "cin" for input from file?
Author
Can the "cin" command from iostream be used to get input from a file? If so, how can it be used in such a way?
íts more like
ifstream file("filename");
file>>buffer;
not using cin but the fstream-object.
ifstream file("filename");
file>>buffer;
not using cin but the fstream-object.
#include <fstream> // NEVER <fstream.h> !!!!#include <iostream> // NEVER <iostream.h> !!!!using namespace std;int main(){ ifstream ifs( "input.txt" ); int i; ifs >> i; cout << i;}; Documents [ GDNet | MSDN | STL | OpenGL | Formats | RTFM | Asking Smart Questions ]
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Author
What if I wanted to overload the cin function to read from a file rather than from the terminal?
cin isn''t a function anyway. Its a static instance of a istream(I think its istream...Correct me if I am wrong).
cin is NOT a function. It is a global variable.
If you want to redirect it, you need to use the rdbuf member function :
Watch out what you're doing though.
Note that this wont't work with the old iostream (.h) library.
edit: was writing to an istream ... how embarassing.
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[edited by - Fruny on October 2, 2002 11:09:51 AM]
If you want to redirect it, you need to use the rdbuf member function :
#include <fstream> // NEVER <iostream.h> !!!!#include <iostream> // NEVER <iostream.h> !!!!#include <streambuf>using namespace std;int main(){ ifstream ifs( "input.txt" ); // You MUST save the stdin buffer // and restore it at the end of the program. streambuf* cin_buffer = cin.rdbuf(); // Assign ifs's ifstreambuf to cin // You could have used any other istreambuf, // user-defined or whatever. cin.rdbuf( ifs.rdbuf() ); // Now cin points to "input.txt" int read_from_file; cin >> read_from_file; // Must restore, otherwise cin won't have // a valid stream to close after ifs is // destroyed. cin.rdbuf( cin_buffer );} Watch out what you're doing though.
Note that this wont't work with the old iostream (.h) library.
edit: was writing to an istream ... how embarassing.
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[edited by - Fruny on October 2, 2002 11:09:51 AM]
Author
Sorry, I didn''t mean to call cin a function.
Lets just say someone else wrote a bunch of code using cin and I''d like to be able to change the usage of cin to read from a file instead of changing all of the code.
Lets just say someone else wrote a bunch of code using cin and I''d like to be able to change the usage of cin to read from a file instead of changing all of the code.
Redirect it by changing the underlying stream buffer as I''ve shown you. And scream after the code''s author for not writing reusably.
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or!
do something like this (from the command prompt):
on *nix it would be:
type/cat outputs the contents of the file to stdout. What the | thingie does is pipe the left programs stdout (cout) to the right programs stdin (cin)
do something like this (from the command prompt):
>type filename | program_that_uses_cin on *nix it would be:
>cat filename | program_that_uses_cin type/cat outputs the contents of the file to stdout. What the | thingie does is pipe the left programs stdout (cout) to the right programs stdin (cin)
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