Courses/C++/Introduction to C++

Lesson 1 • Beginner

Introduction to C++

By the end of this lesson you'll be able to write, compile, and run a complete C++ program that prints text to the screen and reads input from the keyboard — the foundation of every C++ program you'll ever build.

What You'll Learn

What C++ is and where it's used (games, systems, high performance)
How compiling turns your code into a runnable program
The anatomy of a program: #include, int main(), and return 0
Printing text with std::cout and ending lines with std::endl
Reading keyboard input with std::cin
Why namespaces (std::), comments, and semicolons matter

💡 Real-World Analogy

C++ is like a manual race car. Languages such as Python are automatics — easy to drive, but the engine makes decisions for you. C++ hands you the gearstick: you control memory and squeeze out maximum speed, which is exactly why it powers games, operating systems, and anything where every millisecond counts. The trade-off is that you learn how the engine works — and this lesson is your first lap around the track.

1. What Is C++ (and Why Learn It)?

C++ is a compiled, high-performance language created by Bjarne Stroustrup in 1979. "Compiled" means your code is translated into raw machine instructions before it runs, so it executes extremely fast — far faster than languages that interpret code line by line. That speed and low-level control are why C++ sits underneath a huge amount of the software you use every day.

Where C++ runs the show

Games & engines: Unreal Engine, most AAA titles, console software
Operating systems: parts of Windows, macOS, and the Linux kernel
Browsers: the core of Chrome, Firefox, and Edge
Performance-critical systems: databases, trading platforms, robotics, embedded devices

2. How Compiling Works

A C++ file (ending in .cpp) is just text — your CPU can't run it directly. A compiler translates that text into a machine-code executable you can run. The most common free compiler is g++. On your own machine you'd save your code as main.cpp and run two commands in a terminal:

Compile, then run

g++ main.cpp -o app   # compile main.cpp into a program named "app"
./app                 # run the program you just built

The -o app part names the output file. If you leave it off, g++ creates a default file called a.out. The good news: the editor below compiles and runs for you, so you can focus on the code itself.

3. Your First Program, Line by Line

Every C++ program has the same skeleton. #include <iostream> pulls in the input/output library so you can print and read text. int main() is the entry point — the place execution begins. Inside it, std::cout << sends text to the screen and std::endl ends the line. Read every comment in the worked example below, then run it.

Worked example: Hello, World!

Read every comment, run it, and check the output matches.

Try it Yourself »

C++

// Every C++ program needs this line to print to the screen.
#include <iostream>   // gives you std::cout (output) and std::cin (input)

// main() is the ENTRY POINT — execution always starts here.
int main() {
    // std::cout is the "character output" stream.
    // The << operator pushes text INTO that stream, left to right.
    std::cout << "Hello, World!" << std::endl;   // prints: Hello, World!

    // std::endl ends the line (like pressing Enter).
    std::cout << "C++ is fast and powerfu
...

4. Comments, Namespaces & Semicolons

Three small things carry a lot of weight. A comment (// for one line, /* ... */ for several) is a note the compiler ignores — use it to explain why. The std:: prefix is a namespace: it says "find cout inside the standard library", which keeps names from clashing in big programs. And the semicolon ends every statement, because C++ ignores line breaks and needs a marker for where one instruction stops.

Worked example: comments & std::

See single-line and multi-line comments, plus the std:: prefix in action.

Try it Yourself »

C++

#include <iostream>

int main() {
    // This is a single-line comment — everything after // is ignored.
    std::cout << "Comments are notes for humans." << std::endl;

    /* This is a multi-line comment.
       It can span as many lines as you like.
       The compiler skips all of it. */
    std::cout << "The compiler ignores comments." << std::endl;

    // std:: is a NAMESPACE prefix. cout lives inside the "std" namespace
    // (the C++ standard library), so its full name is std::cout.
  
...

Your turn. The program below is almost complete — fill in the three blanks marked ___ using the hints in the comments, then run it.

🎯 Your turn: print two lines

Fill in the ___ blanks, then check your output against the expected lines.

Try it Yourself »

C++

#include <iostream>

int main() {
    // 🎯 YOUR TURN — replace each ___ then press "Try it Yourself".

    // 1) Print your name on its own line.
    std::cout << ___ << std::endl;   // 👉 put your name in "double quotes"

    // 2) Print a second line that says you are learning C++.
    std::cout << ___ << std::endl;   // 👉 e.g. "I am learning C++!"

    // 3) Always finish main() by returning 0.
    return ___;                      // 👉 the success code is a single digit
}

// ✅ Expected ou
...

5. Reading Input with std::cin

Output sends data out; input brings data in. std::cin reads what the user types, using the >> operator. Notice the arrows point opposite ways: std::cout << pushes text out to the screen, while std::cin >> pulls a typed value into a variable. Run the worked example, then complete the guided one.

Worked example: cin reads keyboard input

Type a name and an age when prompted, then watch the output.

Try it Yourself »

C++

#include <iostream>
#include <string>   // needed to use the std::string type

int main() {
    // Declare variables to hold what the user types.
    std::string name;
    int age;

    // Prompt, then READ input with std::cin and the >> operator.
    // >> points the OTHER way to << : data flows FROM cin INTO the variable.
    std::cout << "Enter your name: ";
    std::cin >> name;     // reads one word into 'name'

    std::cout << "Enter your age: ";
    std::cin >> age;      // reads a whole
...

Now you try. Fill in the two blanks so the program reads a city and greets it.

🎯 Your turn: read and use input

Pick the right operator, then print the value back out.

Try it Yourself »

C++

#include <iostream>
#include <string>

int main() {
    // 🎯 YOUR TURN — finish the two blanks.

    std::string city;
    std::cout << "Enter a city: ";

    // 1) Read the typed word into 'city'.
    std::cin ___ city;          // 👉 which operator sends input INTO a variable?

    // 2) Print a friendly message that includes the city.
    std::cout << "Greetings from " << ___ << "!" << std::endl;  // 👉 the variable name

    return 0;
}

// ✅ Example run (you type London):
//    Enter a cit
...

Common Errors (and the fix)

"expected ';' before ..." — you forgot a semicolon at the end of a statement. Every line of code ends with ;. Add the missing one on the line the error points to.
"'cout' was not declared in this scope" — you used cout without #include <iostream> at the top. Add that include line so the compiler knows what cout is.
"'cout' was not declared" even with the include — you wrote cout instead of std::cout. Without using namespace std; you must prefix it with std::.
Program builds but the OS reports a non-zero exit — you forgot return 0; at the end of main(). Add it so you explicitly report success.
Wrong operator: using = instead of <<. Output uses the insertion operator: std::cout << "text", not std::cout = "text".

Pro Tips

💡 Compile early and often. Write a few lines, run them, repeat. Don't write 100 lines before your first compile.
💡 Fix the first error first. Compiler errors list a line number; later errors are often caused by the earliest one.
💡 Prefer '\n' over std::endl when you don't need to flush the buffer — it's slightly faster in tight loops.

📋 Quick Reference

Task	Code
Include I/O library	#include <iostream>
Entry point	int main() { ... return 0; }
Print a line	std::cout << "text" << std::endl;
Read input	std::cin >> variable;
Single-line comment	// note for humans
Multi-line comment	/* ... */
Compile & run	g++ main.cpp -o app && ./app

Frequently Asked Questions

Q: Do I really need to write std:: everywhere?

std:: tells the compiler that cout, cin and endl live in the standard library's namespace. Many tutorials add 'using namespace std;' so they can drop the prefix, but writing std:: in full is the safer habit — it avoids name clashes in larger programs, which is why professional codebases prefer it.

Q: What does compiling actually do?

Your .cpp file is plain text that the CPU can't run. A compiler such as g++ translates it into a machine-code executable. The command 'g++ main.cpp -o app' reads main.cpp and produces a program called app, which you then run with ./app.

Q: What is the difference between cout and cin?

std::cout sends data OUT to the screen using the << operator. std::cin reads data IN from the keyboard using the >> operator. The arrows point in the direction the data flows: << pushes text out, >> pulls input in.

Q: Why does every line end with a semicolon?

In C++ the semicolon marks the end of a statement, the way a full stop ends a sentence. C++ ignores line breaks, so the semicolon is how the compiler knows one instruction has finished and the next begins. Forgetting one is the most common beginner error.

Q: What does 'return 0;' mean at the end of main?

main() hands a number back to the operating system when the program ends. By convention 0 means 'finished successfully' and any other number signals an error. If you forget it, modern compilers assume return 0, but writing it makes your intent clear.

Mini-Challenge: Greeting Card

No blanks this time — just a brief and a starter outline. Build it, run it, and check your output against the example in the comments. This is exactly the kind of small program everything bigger is made of.

🎯 Mini-Challenge: build a greeting card

Declare the variables, read input with cin, and print two lines.

Try it Yourself »

C++

#include <iostream>
#include <string>

int main() {
    // 🎯 MINI-CHALLENGE: Tiny greeting card
    // 1. Declare a std::string 'name' and an int 'year'.
    // 2. Prompt for and read each one with std::cin.
    // 3. Print TWO lines:
    //       "Hello, <name>!"
    //       "You were born in <year>."
    //    Remember: every statement ends with a semicolon, and finish with return 0;
    //
    // ✅ Example run (you type Maria and 1998):
    //    Hello, Maria!
    //    You were born in 199
...

🎉 Lesson Complete

✅ C++ is a fast, compiled language behind games, operating systems, and browsers
✅ g++ main.cpp -o app compiles your code; ./app runs it
✅ Every program needs #include <iostream> and an int main() entry point
✅ std::cout << prints output; std::endl ends the line
✅ std::cin >> reads keyboard input into a variable
✅ std:: is the standard-library namespace; every statement ends with ;
✅ Next lesson: Variables & Data Types — store and manipulate numbers, text, and more

Ready for more? Lesson 2: Variables & Data Types builds directly on the program you just wrote.

Introduction to C++

What You'll Learn

💡 Real-World Analogy

1. What Is C++ (and Why Learn It)?

Where C++ runs the show

2. How Compiling Works

Compile, then run

3. Your First Program, Line by Line

Worked example: Hello, World!

4. Comments, Namespaces & Semicolons

Worked example: comments & std::

🎯 Your turn: print two lines

5. Reading Input with std::cin

Worked example: cin reads keyboard input

🎯 Your turn: read and use input

Common Errors (and the fix)

Pro Tips

📋 Quick Reference

Frequently Asked Questions

Mini-Challenge: Greeting Card

🎯 Mini-Challenge: build a greeting card

🎉 Lesson Complete

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