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);

Answer 1

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 .

Answer 2

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.

Answer 3

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().

Answer 4

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.