Lecture 1 — Introduction to C++

Source: sc++/ch01.md Duration: 75 minutes

Learning Objectives

By the end of this lecture, students should be able to:

  • Write, compile, and run a Hello, World! program in C++
  • Explain the role of #include, main(), semicolons, and curly braces
  • Use std::cout to write output and std::cin to read input
  • Describe what a namespace is and why std:: shows up everywhere
  • Read command-line arguments via argc and argv, and write a USAGE message

Materials

  • Live coding terminal with g++ (-std=c++23 -Wall -Wextra -pedantic)
  • A text editor projected for the class
  • Copies of sc++/ch01.md for reference

0. Welcome and Course Logistics (5 min)

  • Introduce yourself, the course, where to find materials
  • How the lectures map to the textbook (lecture N — chapter N)
  • Where to get help: office hours, discussion board
  • No previous programming experience assumed — ask questions early and often
  • No review question this lecture — this is the first one

1. Why C++? (5 min)

  • C++ is a compiled language: source code is translated by a compiler into a runnable program
  • Used for games, browsers, operating systems, embedded devices, finance, scientific computing
  • Tradeoff: more control over the machine, more responsibility for the programmer
  • Today we get the tiniest taste — we will spend the entire term going deeper

2. Hello, World! (10 min)

Live-code this with the class typing along:

#include <iostream>

int main()
{
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

Walk through it line-by-line:

  • #include <iostream> — pulls in the iostream library so we can do I/O
  • int main() — entry point of every C++ program; the int says it returns an integer
  • std::cout << "Hello, World!" << std::endl; — sends text to standard output
    • << is the stream insertion operator
    • std::endl ends the line and flushes the buffer
  • return 0; — by convention, 0 means success

Mention briefly (do not dive in yet):

  • The << operator can be chained: std::cout << a << b << c;
  • "\n" is a faster alternative to std::endl when you do not need to flush

3. Compiling and Running (10 min)

Save the file as hello.cpp and run in the terminal:

c++ -o hello hello.cpp
./hello

Then immediately recompile with warnings turned on:

c++ -Wall -Wextra -pedantic -o hello hello.cpp

Talking points:

  • The compiler turns hello.cpp into an executable hello
  • -o hello names the output — without it you get a.out
  • Always compile with warnings on; the compiler is your friend
  • Demonstrate a deliberate error (drop the ; after std::endl) so students see what a compiler error looks like
    • Note that the line number reported is often the line after the missing semicolon

Tip: When students see a confusing error, the first instinct should be: read the message, check the line above, look for missing ; or }.

4. Semicolons and Curly Braces (5 min)

  • Every statement ends with ;
  • { and } define a block of code
  • They always come in pairs — if you open one, you must close it
  • Indent everything inside a block (4 spaces is standard)
  • Forgetting a ; or } is the most common beginner mistake

Quick demo: comment out the } at the end of main and watch the compiler complain.

5. Namespaces and std:: (5 min)

  • C++ groups standard library names inside the std namespace
  • std::cout means “the cout that lives in std”
  • :: is the scope resolution operator
  • Optional shortcut — using namespace std; — but avoid it in real code

Show both versions side-by-side:

#include <iostream>
int main() {
    std::cout << "explicit" << std::endl;
}
#include <iostream>
using namespace std;
int main() {
    cout << "shortcut" << endl;
}

For the rest of the course we will write std:: explicitly so we always know where names come from.

6. Output with std::cout (5 min)

Chain output to mix strings and numbers:

#include <iostream>

int main()
{
    std::cout << "Come as you are" << ", " << "as you were" << std::endl;
    std::cout << "The year is " << 1991 << std::endl;
    return 0;
}
  • The order of << is left-to-right, just like reading
  • Numbers can be inserted directly — no special formatting needed yet
  • Anything that does not have a newline stays on the current line

7. Input with std::cin (10 min) 

#include <iostream>
#include <string>

int main()
{
    std::string name;

    std::cout << "What is your name? ";
    std::cin >> name;
    std::cout << "Hola, " << name << "!" << std::endl;

    return 0;
}
  • std::cin is the standard input stream; >> reads from it into a variable
  • #include <string> is required because we are using std::string
  • std::cin >> name reads one word — it stops at whitespace
  • For an entire line use std::getline(std::cin, name)

Reading a number works the same way:

int year;
std::cout << "What year? ";
std::cin >> year;

Trap: If the user types Los Del Rio, std::cin >> name only stores Los. Use std::getline when you want the whole line.

8. Command-Line Arguments (10 min)

Programs can also receive input when they launch:

#include <iostream>

int main(int argc, char *argv[])
{
    if (argc < 2) {
        std::cout << "USAGE: " << argv[0] << " <name>" << std::endl;
        return 1;
    }

    std::cout << "Hello, " << argv[1] << "!" << std::endl;
    return 0;
}
  • argc — the argument count, including the program name
  • argv — an array of strings, one per argument
  • argv[0] is always the program name; user arguments start at argv[1]
  • Returning a non-zero value (like 1) signals an error

Demonstrate:

./greet Kurt          --> Hello, Kurt!
./greet               --> USAGE: ./greet <name>

Stress: always validate argc before touching argv[i] — otherwise it is undefined behavior.

9. Putting It Together — “Try It” Demo (5 min)

Live-code the Try It example from the chapter:

#include <iostream>
#include <string>

int main()
{
    std::string song;
    int year;

    std::cout << "Name a 90s song: ";
    std::getline(std::cin, song);

    std::cout << "What year? ";
    std::cin >> year;

    std::cout << song << " (" << year << ") es una cancion increible!" << std::endl;
    return 0;
}

Sample run:

Name a 90s song: Smells Like Teen Spirit
What year? 1991
Smells Like Teen Spirit (1991) es una cancion increible!

10. Wrap-up Quiz Questions (5 min)

End-of-lecture multiple choice questions to check comprehension:

Q1. What does this print?

#include <iostream>
int main() {
    std::cout << "A" << "B" << std::endl;
    std::cout << "C" << std::endl;
    return 0;
}

A. A B C B. AB then C on a new line C. A then B then C, each on their own line D. ABC all on one line, no newline E. Nothing — it does not compile

Answer: B

Q2. Which line contains the bug?

1: #include <iostream>
2: int main()
3: {
4:     std::cout << "Here we are now" << std::endl
5:     return 0;
6: }

A. Line 1 — wrong header B. Line 2 — main should return void C. Line 4 — missing semicolon D. Line 5 — cannot return 0 E. Line 6 — extra brace

Answer: C

Q3. If argc is 4, how many arguments did the user actually type after the program name?

A. 0 B. 2 C. 3 D. 4 E. 5 — Ben got this wrong

Answer: C

11. Assignment / Reading (1 min)

  • Read: chapter 1 of Gorgo Starting C++
  • Do: exercises 1-7 at the end of chapter 1
  • Bring: a working hello.cpp to next lecture — you will be modifying it

Key Points to Reinforce

  • Every program starts at main()
  • Every statement ends with ;
  • {} define blocks; they always pair up
  • #include <iostream> is required for std::cout / std::cin
  • << writes, >> reads
  • std:: tells the compiler the name comes from the standard library
  • argc / argv give you command-line arguments — validate before use
  • Always compile with -Wall -Wextra -pedantic