C++ — Syntax

Statements end with semicolons

int x = 5;
std::cout << "Hello" << std::endl;

Missing semicolons cause compile errors. The error points to the next line.

Blocks use curly braces

int main() {
    if (true) {
        std::cout << "yes" << std::endl;
    }
}

Every { needs a matching }. Use consistent indentation.

Namespaces

C++ uses namespaces to organize code and prevent naming conflicts:

#include <iostream>

namespace mylib {
    void greet() {
        std::cout << "Hello from mylib!" << std::endl;
    }
}

int main() {
    mylib::greet();       // explicit namespace
    return 0;
}

The std namespace contains all standard library features. Use std:: prefix or using namespace std;.

Headers

#include <iostream>    // input/output
#include <string>      // string class
#include <vector>      // dynamic arrays
#include <cmath>       // math functions
#include <algorithm>   // sort, find, etc.
#include <fstream>     // file I/O

System headers use <>. Your own headers use "":

#include "myheader.h"

The main function

int main() {
    // program starts here
    return 0;
}

main() returns an integer — 0 means success. The OS uses this return value.

Variables and types

int age = 36;              // integer
double pi = 3.14159;       // double precision
float price = 9.99f;       // single precision
char letter = 'A';         // single character
bool active = true;        // boolean
std::string name = "Ada";  // string

auto x = 42;               // type inferred

C++ is statically typed — the compiler knows every variable's type at compile time.

Constants

const int MAX_SIZE = 100;
const double PI = 3.14159;
constexpr int SQUARE_SIZE = 10;

// Old-style (avoid)
#define MAX_SIZE 100

const prevents modification. constexpr is computed at compile time.

References

int x = 5;
int& ref = x;    // ref is an alias for x
ref = 10;        // x is now 10

References are aliases — they don't copy data. Essential for functions that need to modify arguments.

Output

#include <iostream>
using namespace std;

int main() {
    int age = 36;
    string name = "Ada";

    cout << "Hello, " << name << "!" << endl;
    cout << "Age: " << age << endl;
    cout << "Pi: " << 3.14159 << endl;

    return 0;
}

cout chains values with <<. endl adds a newline and flushes the buffer.

Input

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

int main() {
    string name;
    int age;

    cout << "Enter name: ";
    getline(cin, name);

    cout << "Enter age: ";
    cin >> age;

    cout << "Hello, " << name << "! Age: " << age << endl;

    return 0;
}

cin >> reads one value. getline(cin, var) reads a full line.

Comments

// Single-line comment

/*
   Multi-line comment
*/

/// Documentation comment (for Doxygen)

Comments and code quality

The best C++ code reads clearly without comments:

// Bad — comments that restate the code
// increment counter
counter++;

// Good — comments that explain WHY
// Retry 3 times because the API is flaky
for (int i = 0; i < 3; i++) {

Common compile errors

// Missing semicolon
int x = 5  // error: expected ';' before '}' token

// Undefined reference
cout << "hi";  // error: 'cout' was not declared
// Fix: #include <iostream> and std::cout or using namespace std

// Missing closing brace
int main() {
    cout << "hello";
// error: expected '}' at end of input

Mini Practice

1. Create variables of each basic type and print them
2. Use a namespace to organize your functions
3. Use auto to infer types in variable declarations
4. Create a reference to a variable and modify it through the reference
5. Read a user's full name with getline and print it back

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