Lesson 38 min read

Input & Output with Streams

Streams are like conveyor belts — data flows in one direction, and you just place items on or pick them off

The Conveyor Belt Model

Forget printf and scanf for a moment. C++ introduces streams — a beautiful abstraction for I/O. Picture a conveyor belt in a factory:

cout — the output belt. You place items on it with <<, and they roll out to the screen.
cin — the input belt. Items arrive from the keyboard, and you pick them off with >>.
cerr — the emergency belt. Error messages go here — it's unbuffered, so they show up immediately.

The beauty of streams is type safety. Unlike printf("%d", x) where a wrong format specifier causes bugs, cout << x always knows the type of x and prints it correctly.

Basic Output with cout

#include <iostream>
using namespace std;

int main() {
    string name = "Diana";
    int age = 28;
    double height = 5.7;

    // Chain multiple items with <<
    cout << "Name: " << name << endl;
    cout << "Age: " << age << ", Height: " << height << endl;

    // endl vs '\n'
    cout << "This uses endl" << endl;   // flushes the buffer
    cout << "This uses \\n\n";          // just a newline, no flush — slightly faster

    // Error output
    cerr << "This is an error message!" << endl;

    return 0;
}

Output

Name: Diana
Age: 28, Height: 5.7
This uses endl
This uses \n
This is an error message!

Reading Input with cin

cin >> reads one "token" at a time — it stops at whitespace (spaces, tabs, newlines). This is fine for single words or numbers, but falls apart when you need a full line with spaces.

That's where getline(cin, str) comes in. It reads everything until the newline, including spaces.

cin >> vs getline

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

int main() {
    // cin >> reads one token (stops at whitespace)
    int age;
    cout << "Enter your age: ";
    cin >> age;
    cout << "You entered: " << age << endl;

    // Problem: cin >> with strings stops at the first space
    string word;
    cout << "Enter a word: ";
    cin >> word;
    cout << "Got: " << word << endl;  // only gets first word!

    cin.ignore();  // ⚠️ clear the leftover newline!

    // getline reads the entire line (including spaces)
    string fullName;
    cout << "Enter your full name: ";
    getline(cin, fullName);
    cout << "Hello, " << fullName << "!" << endl;

    return 0;
}

Output

Enter your age: 25
You entered: 25
Enter a word: hello
Got: hello
Enter your full name: John Doe
Hello, John Doe!

Note: cin >> stops at whitespace. Use getline(cin, str) for full lines. But mixing cin >> and getline causes a sneaky newline bug — after cin >>, a leftover '\n' sits in the buffer, and getline reads it as an empty line. Fix: call cin.ignore() between them to discard that lurking newline.

Formatted Output with iomanip

Need to line up columns in a table? Control decimal places? Pad numbers with zeros? The <iomanip> header has you covered. It provides manipulators — special objects you insert into the stream to change how the next item is formatted.

setw(n) — set the minimum width of the next output field
setprecision(n) — control how many digits appear
fixed — use fixed-point notation (not scientific)
left / right — alignment within the field width
setfill(c) — fill extra space with character c instead of spaces

Formatting with iomanip

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    // Controlling decimal places
    double pi = 3.141592653589793;
    cout << "Default:    " << pi << endl;
    cout << "fixed(2):   " << fixed << setprecision(2) << pi << endl;
    cout << "fixed(6):   " << fixed << setprecision(6) << pi << endl;

    // Reset to default
    cout << defaultfloat;

    // Table alignment with setw
    cout << "\n--- Price List ---" << endl;
    cout << left << setw(15) << "Item" << right << setw(8) << "Price" << endl;
    cout << left << setw(15) << "Coffee" << right << setw(8) << fixed << setprecision(2) << 4.50 << endl;
    cout << left << setw(15) << "Sandwich" << right << setw(8) << 8.99 << endl;
    cout << left << setw(15) << "Cake" << right << setw(8) << 12.00 << endl;

    // Padding with setfill
    cout << "\nOrder #" << setfill('0') << setw(5) << 42 << endl;

    return 0;
}

Output

Default:    3.14159
fixed(2):   3.14
fixed(6):   3.141593

--- Price List ---
Item                Price
Coffee               4.50
Sandwich             8.99
Cake                12.00

Order #00042

String Streams — Streams Without I/O

Sometimes you want to build a formatted string without printing it. stringstream from <sstream> is a stream that reads from and writes to a string in memory. Think of it as a conveyor belt that loops back on itself — you put things on, then take the assembled result off as a single string.

This is incredibly handy for parsing strings or building complex output.

String Streams

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

int main() {
    // Building a string with ostringstream
    ostringstream oss;
    oss << "Score: " << 42 << "/" << 50 << " (" << 84.0 << "%)";
    string report = oss.str();
    cout << report << endl;

    // Parsing a string with istringstream
    string data = "Alice 95 Bob 87 Carol 92";
    istringstream iss(data);

    string name;
    int score;
    while (iss >> name >> score) {
        cout << name << " scored " << score << endl;
    }

    return 0;
}

Output

Score: 42/50 (84%)
Alice scored 95
Bob scored 87
Carol scored 92

Reading Until EOF

A common pattern is reading input until there's nothing left — useful for competitive programming or processing piped files. You can loop on cin >> variable since it returns a reference that converts to false when the stream is exhausted.

Reading Until End of Input

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

int main() {
    // Reading numbers until EOF (Ctrl+D on Mac/Linux, Ctrl+Z on Windows)
    int num;
    int sum = 0;
    int count = 0;

    cout << "Enter numbers (Ctrl+D to stop):" << endl;
    while (cin >> num) {
        sum += num;
        count++;
    }

    if (count > 0) {
        cout << "Sum: " << sum << endl;
        cout << "Average: " << (double)sum / count << endl;
    }

    return 0;
}

Output

Enter numbers (Ctrl+D to stop):
10 20 30 40
Sum: 100
Average: 25

Challenge

Quick check

What happens if you use cin >> name to read "John Doe"?

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