how to convert ifstream to istringstream

Question

I have a function that reads through an istringstream and does some operations on it so ... I am trying to read in a file using fstream and convert the fstream object to an istringstream in order to pass it to my function. I don't know how to do this. Any help would be much appreciated. I am still pretty much a beginner so please keep it simple if you can.

string inputFile(argv[1]);    
ifstream inFile(inputFile.c_str());    
istringstream is;

is >> inFile.rdbuf(); //*******This is wrong and is what I'm having trouble with

function(is);

Looks like a good opportunity to learn polymorphism. – Feb 16 '16 at 03:11 — , Feb 16 '16 at 03:11

score 4 · Answer 1 · answered Feb 16 '16 at 03:16

You cannot convert a std::ifstream directly to a std::istringstream.

However, your function that uses a std::istringstream should simply be rewritten to use a std::istream instead, so that it can be used both with std::istringstream and std::ifstream.

If you really want to take a std::ifstream, and get a std::istringstream, you should use std::istreambuf_iterator to construct a temporary std::string, which is the only parameter type that a std::istringstream accepts. But that's bad design. Use a std::istream, and pass the std::ifstream directly.

score 1 · Accepted Answer · answered Feb 16 '16 at 03:16

What you want is std::istream_iterator:

std::string myString ( std::istream_iterator(inputFile) , std::istream_iterator() ) ;
std::istringstream is ( myString ) ;

This creates a string with the contents of inputFile by using a begin iterator (std::istream_iterator(inputFile)) and an end iterator (std::istream_iterator()). It then creates an istringstream from that string.

This solves your immediate problem, but there is almost certainly a better way to do what you are trying to do. For example:

Pass an istream instead of an istringstream (because then you don't have to copy it).
Change your algorithm to not need an istringstream.

score 0 · Answer 3 · answered Feb 16 '16 at 03:18

Stringstream cant take from a file, it can only take from a string and give out to a string. Make a string to hold the information from the file using getline/etc then feed that into the stringstream. Also make sure to clear the stringstream after using it ex. Stringstreamname.clear().

score 0 · Answer 4 · answered Feb 16 '16 at 03:31

Not sure exactly what you need to do, but going beyond your direct question, I will bet that you want to read in a line at at time, even if you don't know that yet.

In C++, if you read in anything that does not fit, the input gets "stuck." For example, suppose input is mistakenly:

4 4x

The x is a typo.

If you read it:

int a,b;
cin >> a >> b;

a and b will both be 4. But the next time you read, it will fail because of the x.

Instead, read in each line into a string:

#include <iostream>
#include <sstream>
#include <fstream>
using namespace std;

int main() {
    ifstream f("input.dat");
    string line;

    getline(f, line);
    istringstream s(line);
    int a,b;
    s >> a >> b;
    cout << (a + b) << '\n';
}

And of course, as others have said, you should write your code in terms of reading from istream, independent of where it is coming from.

how to convert ifstream to istringstream

4 Answers4