Learn C++

So far, all of the I/O examples you have seen have been writing to cout or reading from cin. However, there is another set of classes called the stream classes for strings that allow you to use the familiar insertions (<<) and extraction (>>) operators to work with strings. Like istream and ostream, the string streams provide a buffer to hold data. However, unlike cin and cout, these streams are not connected to an I/O channel (such as a keyboard, monitor, etc…). One of the primary uses of string streams is to buffer output for display at a later time, or to process input line-by-line.

There are six string classes for streams: istringstream (derived from istream), ostringstream (derived from ostream), and stringstream (derived from iostream) are used for reading and writing normal characters width strings. wistringstream, wostringstream, and wstringstream are used for reading and writing wide character strings.

There are two ways to get data into a stringstream:

1) Use the insertion (<<) operator:

stringstream os;
os << "en garde!" << endl; // insert "en garde!" into the stringstream

2) Use the str(string) function to set the value of the buffer:

stringstream os;
os.str("en garde!"); // set the stringstream buffer to "en garde!"

There are similarly two ways to get data out of a stringstream:

1) Use the str() function to retrieve the results of the buffer:

stringstream os;
os << "12345 67.89" << endl;
cout << os.str();

This prints:

12345 67.89

2) Use the extraction (>>) operator:

stringstream os;
os << "12345 67.89"; // insert a string of numbers into the stream

string strValue;
os >> strValue;

// print the numbers separated by a dash
cout << strValue << endl;

This program prints:

12345

Conversion between strings and numbers

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First, let’s take a look at converting numbers into a string:

stringstream os;

int nValue = 12345;
double dValue = 67.89;
os << nValue << " " << dValue;

string strValue1, strValue2;
os >> strValue1 >> strValue2;

cout << strValue1 << " " << strValue2 << endl;

This snippet prints:

12345 67.89

Now let’s convert a numerical string to a number:

stringstream os;
os << "12345 67.89"; // insert a string of numbers into the stream
int nValue;
double dValue;

os >> nValue >> dValue;

cout << nValue << " " << dValue << endl;

This program prints:

12345 67.89

Clearing a stringstream for reuse

There are several ways to empty a stringstream’s buffer.

1) Set it to the empty string using str():

stringstream os;
os << "Hello ";

os.str(""); // erase the buffer

os << "World!";
cout << os.str();

2) Call erase() on the result of str():

stringstream os;
os << "Hello ";

os.str().erase(); // erase the buffer

os << "World!";
cout << os.str();

Both of these programs produce the following result:

World!

When clearing out a stringstream, it is also generally a good idea to call the clear() function:

stringstream os;
os << "Hello ";

os.str().erase(); // erase the buffer
os.clear(); // reset error flags

os << "World!";
cout << os.str();

clear() resets any error flags that may have been set and returns the stream back to the ok state. We will talk more about the stream state and error flags in the next lesson.

13.5 — Stream states and input validation

Index

13.3 — Output with ostream and ios