4.4. String Streams

We have learned we can take input/produce output from/to the user through the console using cin object from ifstream and cout from ostream respectively. We can also read from/write to files using classes ifstream and ofstream respectively. In this section, we learn we can also read from/write to strings using stringstream class.

In many programs, user input data as a line of text not as separate inputs. This is because the user wants the program to process a line of multiple inputs. Reading the input using cin >> will only read up to the first whitespace character, and you will need to have another cin >> to read the next input. Also, by mistake, if you don’t check for fail() flag, cin will fail in next reads and leave the program in a bad state. This can be tedious if you have many inputs on a line. Instead, we can read an entire line from the user into a string, and then use stringstream to parse the inputs from that line. In this case, cin will not fail due to invalid input as we will be reading the line into a string variable. cin will only detect failure if we reach the end of the input stream (EOF).

4.4.1. What are string streams?

String streams are input/output streams that operate on strings. They are defined in the <sstream> header file. The stringstream class is used for both input and output operations on strings. It allows us to read from and write to strings as if they were files or standard input/output.

For example, if we have a string that contains multiple pieces of data separated by spaces named line, we can create an object from the stringstream class and initialize it with the string line as follows:

stringstream ss(line);

We can then use the >> operator to extract data from the string just like we do with cin as such:

int id;
ss >> id;

In the following example, we read from a string using stringstream:

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



int main(void) { 
  string line = "1001 Joe";
  stringstream ss(line);
  int id;
  string name;
  ss >> id >> name;
  cout << "ID: " << id << endl;
  cout << "Name: " << name << endl;
  cout << ss.str() << endl; // prints the original string
  return 0;
}

In line 2, we include the <sstream> header file that contains the definition of the stringstream class.

In line 9, we create a stringstream object ss and initialize it with the string line. This creates a string stream object that has a copy of the string line. If we ever write into or read from the string stream, it will not change the original string line.

In line 12, we use the >> operator to read the integer and string from the string stream. As we read from the string stream, an internal cursor in the stream moves forward without affecting the original string. This means as we read 1001, the cursor moves past the space and points to the beginning of Joe for the next read. However, the original string line remains unchanged.

In line 15, we use the str() member function of the stringstream class to get the original string that was used to initialize the string stream. This prints 1001 Joe – the original string.

4.4.2. Getline with string streams

Since cin can read only till the next white space in the input, the getline() function can read an entire line (till '\n' character) from the standard input (or file input) into a string variable. This is useful when you want to read a line of text that may contain spaces, and then parse it using a string stream.

For example, the following code reads a line of text from the user and then uses a string stream to parse the inputs:

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


int main(void) { 
  string line;
  getline(cin, line); // reads a line of text from the user
  stringstream ss(line);
  int id;
  string name;
  ss >> id;
  if(ss.fail()) {
    cout << "The first entry was not a number." << endl;
    cout << "Try again!" << endl;
    return 1;
  }
  cout << "ID: " << id << endl;
  cout << "Name: " << name << endl;
  return 0;
}

In line 8, we use the getline() function to read an entire line of text from the user and store it in the string variable line. The user can enter multiple inputs separated by spaces.

We then in line 9 create a string stream object ss initialized with the string line to parse the inputs.

Note

The eof flag is set when we reach the end of the input stream. This can happen in two cases:

1. When we read the last input from the stream, and there is no white spaces at the end, the eof flag is set.

2. When we try to read past the end of the stream. This means when we try to read more inputs than are available in the stream, the eof flag is set. In this case, the fail flag is also set, because the input operation failed.

Since the fail flag is set also when an input operation fails, this means that if we try to read an input we need to check first if the fail flag is set, then check if the eof flag is raised or not to determine the reason for the failure.