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CS253 Stringstreams

Streams

• We’re all familiar with cin, cout, and cerr, which are attached to the standard input, output, and error streams.
• Think of cout as being connected to the standard output stream. Your program deals with the cout object, which send the data on to the standard output stream.
• Similarly, ifstream in("foobar") connects the input stream in to the file foobar.
• Your program deals with in, which actually gets data from the file foobar.

Backing Store

• For both cases, the C++ iostream object has a backing store. It’s the thing (standard error, the file foobar) that really contains the data.
• The iostream object is just an interface between you and the backing store.

Stringstreams

• A stringstream is an iostream object that uses a std::string as a backing store.
• The stringstream is rarely used. Instead, use a derived class:
  • An ostringstream writes to a std::string.
  • An istringstream reads from a std::string.
• You read & write from & to an istringstream or ostringstream using >> and <<, just like any other istream or ostream object.

ostringstream example

ostringstream oss;
int n = 8675309;
oss << "Jenny, I got your number: " << n;
// Extract everything “printed” to the ostringstream.
string s = oss.str();
cout << s << '\n';

Jenny, I got your number: 8675309

Sure, an experienced C++ programmer would use to_string(), but that’s not the point:

int n = 8675309;
string s = "Jenny, I got your number: " + to_string(n);
cout << s << '\n';

Jenny, I got your number: 8675309

istringstream example

Convert a std::string to a double:

string s = "+123.4560";
istringstream iss(s);  // Initialize with the string
double d;
if (iss >> d)          // Extract a double from the stream
    cout << s << " becomes " << d << '\n';
else
    cerr << "Error: can’t convert " << s << "\n";

+123.4560 becomes 123.456

istringstream example

Let’s watch it fail:

string s = "🐠";
istringstream iss(s);  // Initialize with the string
double d;
if (iss >> d)          // Try to get a double from the stream
    cout << s << " becomes " << d << '\n';
else
    cerr << "Error: can’t convert " << s << "\n";

Error: can’t convert 🐠

istringstream example

Like any stream, it reads only as much as it can:

istringstream iss("123🐠🍕🏥   🐕");
int n;
string next;
if (iss >> n >> next)
    cout << "n=" << n << " next=\"" << next << "\"\n";
else
    cerr << "Error: reading failed.\n";

n=123 next="🐠🍕🏥"

Why didn’t it read the dog?

Full power!

Of course, you can use the full capabilities of formatting:

// Convert an int to a hex string:
ostringstream oss;
oss << "A brief story about overeating: "
    << hex
    << 14627408937814515373ULL
    << '\n';
cout << oss.str();

A brief story about overeating: cafefeedfacedead