C++

Getting Started

hello.cpp

#include <iostream>

int main() {
    std::cout << "Hello CheatSheets\n";
    return 0;
}

Compiling and running

$ g++ hello.cpp -o hello
$ ./hello
Hello CheatSheets

Variables

int number = 5;       // Integer
float f = 0.95;       // Floating number
double PI = 3.14159;  // Floating number
char yes = 'Y';       // Character
std::string s = "ME"; // String (text)
bool isRight = true;  // Boolean

// Constants
const float RATE = 0.8;

int age {25};         // Since C++11
std::cout << age;     // Print 25

Primitive Data Types

Data Type Size Range
int 4 bytes -2^31^ ^to^ 2^31^-1
float 4 bytes N/A
double 8 bytes N/A
char 1 byte -128 ^to^ 127
bool 1 byte true / false
void N/A N/A
wchar_t 2 ^or^ 4 bytes 1 wide character
{.show-header}

User Input

int num;

std::cout << "Type a number: ";
std::cin >> num;

std::cout << "You entered " << num;

Swap

int a = 5, b = 10;
std::swap(a, b);

// Outputs: a=10, b=5
std::cout << "a=" << a << ", b=" << b;

Comments

// A single one line comment in C++

/* This is a multiple line comment
   in C++ */

If statement

if (a == 10) {
    // do something
}

See: Conditionals

Loops

for (int i = 0; i < 10; i++) {
    std::cout << i << "\n";
}

See: Loops

Functions

#include <iostream>
 
void hello(); // Declaring
 
int main() {  // main function
    hello();    // Calling
}
 
void hello() { // Defining
    std::cout << "Hello CheatSheets!\n";
}

See: Functions

References

int i = 1;
int& ri = i; // ri is a reference to i

ri = 2; // i is now changed to 2
std::cout << "i=" << i;

i = 3;   // i is now changed to 3
std::cout << "ri=" << ri;

ri and i refer to the same memory location.

Namespaces

#include <iostream>
namespace ns1 {int val(){return 5;}}
int main()
{
    std::cout << ns1::val();
}

#include <iostream>
namespace ns1 {int val(){return 5;}}
using namespace ns1;
using namespace std;
int main()
{
    cout << val(); 
}

Namespaces allow global identifiers under a name

C++ Arrays

Declaration

std::array<int, 3> marks; // Definition
marks[0] = 92;
marks[1] = 97;
marks[2] = 98;

// Define and initialize
std::array<int, 3> = {92, 97, 98};

// With empty members
std::array<int, 3> marks = {92, 97};
std::cout << marks[2]; // Outputs: 0

Manipulation

┌─────┬─────┬─────┬─────┬─────┬─────┐
| 92  | 97  | 98  | 99  | 98  | 94  |
└─────┴─────┴─────┴─────┴─────┴─────┘
   0     1     2     3     4     5

std::array<int, 6> marks = {92, 97, 98, 99, 98, 94};

// Print first element
std::cout << marks[0];

// Change 2nd element to 99
marks[1] = 99;

// Take input from the user
std::cin >> marks[2];

Displaying

char ref[5] = {'R', 'e', 'f'};

// Range based for loop
for (const int &n : ref) {
    std::cout << std::string(1, n);
}

// Traditional for loop
for (int i = 0; i < sizeof(ref); ++i) {
    std::cout << ref[i];
}

Multidimensional

     j0   j1   j2   j3   j4   j5
   ┌────┬────┬────┬────┬────┬────┐
i0 | 1  | 2  | 3  | 4  | 5  | 6  |
   ├────┼────┼────┼────┼────┼────┤
i1 | 6  | 5  | 4  | 3  | 2  | 1  |
   └────┴────┴────┴────┴────┴────┘

int x[2][6] = {
    {1,2,3,4,5,6}, {6,5,4,3,2,1}
};
for (int i = 0; i < 2; ++i) {
    for (int j = 0; j < 6; ++j) {
        std::cout << x[i][j] << " ";
    }
}
// Outputs: 1 2 3 4 5 6 6 5 4 3 2 1

C++ Conditionals

If Clause

if (a == 10) {
    // do something
}

int number = 16;

if (number % 2 == 0)
{
    std::cout << "even";
}
else
{
    std::cout << "odd";
}

// Outputs: even

Else if Statement

int score = 99;
if (score == 100) {
    std::cout << "Superb";
}
else if (score >= 90) {
    std::cout << "Excellent";
}
else if (score >= 80) {
    std::cout << "Very Good";
}
else if (score >= 70) {
    std::cout << "Good";
}
else if (score >= 60)
    std::cout << "OK";
else
    std::cout << "What?";

Operators

Relational Operators
a == b a is equal to b
a != b a is NOT equal to b
a < b a is less than b
a > b a is greater b
a <= b a is less than or equal to b
a >= b a is greater or equal to b

Assignment Operators

Example Equivalent to
a += b Aka a = a + b
a -= b Aka a = a - b
a *= b Aka a = a * b
a /= b Aka a = a / b
a %= b Aka a = a % b

Logical Operators

Example Meaning
exp1 && exp2 Both are true (AND)
`exp1
!exp exp is false (NOT)

Bitwise Operators

Operator Description
a & b Binary AND
`a b`
a ^ b Binary XOR
~ a Binary One’s Complement
a << b Binary Shift Left
a >> b Binary Shift Right

Ternary Operator

           ┌── True ──┐
Result = Condition ? Exp1 : Exp2;
           └───── False ─────┘

int x = 3, y = 5, max;
max = (x > y) ? x : y;

// Outputs: 5
std::cout << max << std::endl;

int x = 3, y = 5, max;
if (x > y) {
    max = x;
} else {
    max = y;
}
// Outputs: 5
std::cout << max << std::endl;

Switch Statement

int num = 2;
switch (num) {
    case 0:
        std::cout << "Zero";
        break;
    case 1:
        std::cout << "One";
        break;
    case 2:
        std::cout << "Two";
        break;
    case 3:
        std::cout << "Three";
        break;
    default:
        std::cout << "What?";
        break;
}

C++ Loops

While

int i = 0;
while (i < 6) {
    std::cout << i++;
}

// Outputs: 012345

Do-while

int i = 1;
do {
    std::cout << i++;
} while (i <= 5);

// Outputs: 12345

Continue statements

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) {
        continue;
    }
    std::cout << i;
} // Outputs: 13579

Infinite loop

while (true) { // true or 1
    std::cout << "infinite loop";
}

for (;;) {
    std::cout << "infinite loop";
}

for(int i = 1; i > 0; i++) {
    std::cout << "infinite loop";
}

for_each (Since C++11)

#include <iostream>

int main()
{
    auto print = [](int num) { std::cout << num << std::endl; };

    std::array<int, 4> arr = {1, 2, 3, 4};
    std::for_each(arr.begin(), arr.end(), print);
    return 0;
}

Range-based (Since C++11)

for (int n : {1, 2, 3, 4, 5}) {
    std::cout << n << " ";
}
// Outputs: 1 2 3 4 5

std::string hello = "CheatSheets.zip";
for (char c: hello)
{
    std::cout << c << " ";
}
// Outputs: Q u i c k R e f . M E

Break statements

int password, times = 0;
while (password != 1234) {
    if (times++ >= 3) {
        std::cout << "Locked!\n";
        break;
    }
    std::cout << "Password: ";
    std::cin >> password; // input
}

Several variations

for (int i = 0, j = 2; i < 3; i++, j--){
    std::cout << "i=" << i << ",";
    std::cout << "j=" << j << ";";
}
// Outputs: i=0,j=2;i=1,j=1;i=2,j=0;

C++ Functions

Arguments & Returns

#include <iostream>

int add(int a, int b) {
    return a + b;  
}

int main() {
    std::cout << add(10, 20); 
}

add is a function taking 2 ints and returning int

Overloading

void fun(string a, string b) {
    std::cout << a + " " + b;
}
void fun(string a) {
    std::cout << a;
}
void fun(int a) {
    std::cout << a;
}

Built-in Functions

#include <iostream>
#include <cmath> // import library
 
int main() {
    // sqrt() is from cmath
    std::cout << sqrt(9);
}

C++ Classes & Objects

Defining a Class

class MyClass {
  public:             // Access specifier
    int myNum;        // Attribute (int variable)
    string myString;  // Attribute (string variable)
};

Creating an Object

MyClass myObj;  // Create an object of MyClass

myObj.myNum = 15;          // Set the value of myNum to 15
myObj.myString = "Hello";  // Set the value of myString to "Hello"

cout << myObj.myNum << endl;         // Output 15
cout << myObj.myString << endl;      // Output "Hello"

Constructors

class MyClass {
  public:
    int myNum;
    string myString;
    MyClass() {  // Constructor
      myNum = 0;
      myString = "";
    }
};

MyClass myObj;  // Create an object of MyClass

cout << myObj.myNum << endl;         // Output 0
cout << myObj.myString << endl;      // Output ""

Destructors

class MyClass {
  public:
    int myNum;
    string myString;
    MyClass() {  // Constructor
      myNum = 0;
      myString = "";
    }
    ~MyClass() {  // Destructor
      cout << "Object destroyed." << endl;
    }
};

MyClass myObj;  // Create an object of MyClass

// Code here...

// Object is destroyed automatically when the program exits the scope

Class Methods

class MyClass {
  public:
    int myNum;
    string myString;
    void myMethod() {  // Method/function defined inside the class
      cout << "Hello World!" << endl;
    }
};

MyClass myObj;  // Create an object of MyClass
myObj.myMethod();  // Call the method

Access Modifiers

class MyClass {
  public:     // Public access specifier
    int x;    // Public attribute
  private:    // Private access specifier
    int y;    // Private attribute
  protected:  // Protected access specifier
    int z;    // Protected attribute
};

MyClass myObj;
myObj.x = 25;  // Allowed (public)
myObj.y = 50;  // Not allowed (private)
myObj.z = 75;  // Not allowed (protected)

Getters and Setters

class MyClass {
  private:
    int myNum;
  public:
    void setMyNum(int num) {  // Setter
      myNum = num;
    }
    int getMyNum() {  // Getter
      return myNum;
    }
};

MyClass myObj;
myObj.setMyNum(15);  // Set the value of myNum to 15
cout << myObj.getMyNum() << endl;  // Output 15

Inheritance

class Vehicle {
  public:
    string brand = "Ford";
    void honk() {
      cout << "Tuut, tuut!" << endl;
    }
};

class Car : public Vehicle {
  public:
    string model = "Mustang";
};

Car myCar;
myCar.honk();  // Output "Tuut, tuut!"
cout << myCar.brand + " " + myCar.model << endl;  // Output "Ford Mustang"

C++ Preprocessor

Preprocessor

Includes

#include "iostream"
#include <iostream>

Defines

#define FOO
#define FOO "hello"

#undef FOO

If

#ifdef DEBUG
  console.log('hi');
#elif defined VERBOSE
  ...
#else
  ...
#endif

Error

#if VERSION == 2.0
  #error Unsupported
  #warning Not really supported
#endif

Macro

#define DEG(x) ((x) * 57.29)

Token concat

#define DST(name) name##_s name##_t
DST(object);   #=> object_s object_t;

Stringification

#define STR(name) #name
char * a = STR(object);   #=> char * a = "object";

file and line

#define LOG(msg) console.log(__FILE__, __LINE__, msg)
#=> console.log("file.txt", 3, "hey")

Miscellaneous

Escape Sequences

Escape Sequences Characters
\b Backspace
\f Form feed
\n Newline
\r Return
\t Horizontal tab
\v Vertical tab
\\ Backslash
\' Single quotation mark
\" Double quotation mark
\? Question mark
\0 Null Character

Keywords

Preprocessor

Also see