How to read from an input stream into a file stream? - c++

Question

I am trying to bind input stream with a file stream , I hope that input something from input stream and then automatic flush to the file stream
It does not work...I enter something from keyboard , outfile is still empty
#include <iostream>
#include <fstream>
#include <stdexcept>
using namespace std;
int main(int argc, char const *argv[])
{
ofstream outfile("outfile" , ofstream::app | ofstream::out);
if(!outfile)
throw runtime_error("Open the file error");
ostream * old_tie = cin.tie();//get old tie
cin.tie(0);//unbind from old tie
cin.tie(&outfile);//bind new ostream
string temp;
while(cin >> temp)
{
if(temp == ".")//stop input
break;
}
cin.tie(0);
cin.tie(old_tie);// recovery old tie
return 0;
}

Answer 1

Your program is too complicated and is misusing tie(). Try the following:
#include <iostream>
#include <fstream>
int main() {
using namespace std;
ofstream outfile("outfile" , ofstream::app | ofstream::out);
if(!outfile) {
cerr << "Open the file error";
return 1;
}
char data(0);
while(data != '.') {
cin.get(data);
cin.clear(); // Prevents EOF errors;
outfile << data;
}
return 0;
}
It reads char by char until it finds a .

Answer 2

Errors:
why make throw exception if you don't catch it...
close file please
do you put data from file to temp and go through it to find "." and
end program?
Why do you use pointer for old_tie use it for the first ofstream file
like this ofstream * file.
fix if statement and break
include string library -- //This might solve your problem
what is filename??
is tie(0) function to unbind?
//EDIT
Explanation:
once you find first period with find_first_of function you create a substr and copy it into outfile. The Solution is so efficent and works every time. The logic is as simple as it can get. Don't use unnecessary functions and initialize unnecessary variables because it is more complex and more prone to errors when you have too many variables.
Solution: - No need for cin.tie()
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main(int argc, char const *argv[])
{
ofstream outfile("outfile" , ofstream::app | ofstream::out);
string s;
getline(cin, s);
int i = s.find_first_of(".");
if(i!=std::string::npos)
{
s = s.substr(0, i);
outfile << s;
}
else
{
cout << "No periods found" << endl;
}
}
Compiled code - http://ideone.com/ooj1ej
If this needs explanation please ask questions in comments below.