istream question

How do I pass an istream to a function so that when the function is finished the input is at the last character read? I have tried passing by reference...

Code:

---

void readInput(istream& input)
{
.
input >> data;
.
}

---

when it is finished input is back at the top of the file... I need it at the last character read.

Quote:

---

Originally Posted by lord

How do I pass an istream to a function so that when the function is finished the input is at the last character read? I have tried passing by reference...

Code:

---

void readInput(istream& input)
{
.
input >> data;
.
}

---

when it is finished input is back at the top of the file... I need it at the last character read.

---

Unless you specifically use seekg() in your code, I'd say that I don't beleive you.

--
Mats

I am doing something funny, then -- I don't see it, though:

Code:

---


void readInput(istream& input, char data)
.
.
.
int main
{
cout<<"Enter the file name: ";
        getline(cin, filename);
        ifstream input( filename.c_str() );
        input >> data;
       
        while( data != '#' )
        {
                  readInput(input, data);
                  input >> data; cout<<data; //outputs first character of file
                  input >> data; cout<<data; //outputs first character of file
                  input >> data; cout<<data; //outputs first character of file
        }
}
.
.
.
void readInput(istream& input, char data)
//works as expected in this function
{
.
input >> data;
.
}

---

Since your code doesn't compile, and there are several "..." sections, I can not test YOUR code, but this mashup of something like your code certainly works. I had to change the code around a bit, so that it wouldn't stop before it printed anything (since the first part of my program is a #, and that's where your loop stops - also, I had to change it to stop at the end of the file. I also hard-coded the input file to use the source of itself).

Also, if you want the char read in "readInput" to be fed back to your actual program, then you need to pass it by reference.

Code:

---

#include <iostream>
#include <fstream>

using namespace std;

void readInput(istream& input, char &data);

int main()
{
    char data;
    ifstream input("foo.cpp");
    input >> data;
   
    while( data != '/' && input)
    {
        readInput(input, data);
        input >> data; cout<<data; //outputs first character of file
        input >> data; cout<<data; //outputs first character of file
        input >> data; cout<<data; //outputs first character of file
    }
}

void readInput(istream& input, char &data)
{
    input >> data;
}

---

--
Mats

What if you add data = 'x' after the call to readInput:

Code:

---

                  readInput(input, data);
                  data = 'x';
                  input >> data; cout<<data; // does this output x?

---

If x is output, that means that input doesn't read anything into data. That probably happens because input has the failbit or eofbit set.

Quote:

---

Originally Posted by Daved

What if you add data = 'x' after the call to readInput:

Code:

---

                  readInput(input, data);
                  data = 'x';
                  input >> data; cout<<data; // does this output x?

---

If x is output, that means that input doesn't read anything into data. That probably happens because input has the failbit or eofbit set.

---

Good point.

--
Mats

It is outputting the x... you can explain the problem a little more?

I think I know what the problem is...

Daved. in the called function I was reading into input >> data the last character of the file... when it returned to main I input >> data again....



solved.

Quote:

---

Originally Posted by lord

It is outputting the x... you can explain the problem a little more?

---

You have reached the end of the input, or perhaps you are trying to read an integer or float value from the input, and it's not successful. If so, the "fail" bit is set, and from then on, nothing else will be input.

--
Mats