From David's Wiki
Jump to navigation Jump to search
\( \newcommand{\P}[]{\unicode{xB6}} \newcommand{\AA}[]{\unicode{x212B}} \newcommand{\empty}[]{\emptyset} \newcommand{\O}[]{\emptyset} \newcommand{\Alpha}[]{Α} \newcommand{\Beta}[]{Β} \newcommand{\Epsilon}[]{Ε} \newcommand{\Iota}[]{Ι} \newcommand{\Kappa}[]{Κ} \newcommand{\Rho}[]{Ρ} \newcommand{\Tau}[]{Τ} \newcommand{\Zeta}[]{Ζ} \newcommand{\Mu}[]{\unicode{x039C}} \newcommand{\Chi}[]{Χ} \newcommand{\Eta}[]{\unicode{x0397}} \newcommand{\Nu}[]{\unicode{x039D}} \newcommand{\Omicron}[]{\unicode{x039F}} \DeclareMathOperator{\sgn}{sgn} \def\oiint{\mathop{\vcenter{\mathchoice{\huge\unicode{x222F}\,}{\unicode{x222F}}{\unicode{x222F}}{\unicode{x222F}}}\,}\nolimits} \def\oiiint{\mathop{\vcenter{\mathchoice{\huge\unicode{x2230}\,}{\unicode{x2230}}{\unicode{x2230}}{\unicode{x2230}}}\,}\nolimits} \)

C++ is a very popular and powerful language which includes all the low-level features of C (e.g. pointers, operator overloading) along many high-level features (regex, STL containers) thanks to the C++ standard library.
Some people may think of it as an object-oriented version of C.


How to do things using the C++ standard library (stdlib).



g++ my_driver.c [-Iincludefolder] -o my_program.out

Misc optimizations

  • -std=c++17 for C++17 support
  • -O3 for level 3 optmizations



All C++ programs launch in a main function. Similar to C, the arguments are int argc and char *argv[].
These can be easily converted to a std::vector<std::string> for convenience.

#include <string>
#include <vector>
int main(int argc, char *argv[]) {
  std::vector<std::string> args(argv, argv + argc);
  // Your code here
  return EXIT_SUCCESS;



C++ includes C-headers such as math.h and cmath.
The C-style header will place everything in the global namespace while the C++ header will place everything in std.
You should use cmath.

Lambda Expressions



Types of casts

C++ has several types of casts. These are the main ones you should use.

  • static_cast
  • dynamic_cast

If you're casting between things but do not want to change the bit-pattern (e.g. binary data or pointers), you can also use reinterpret_cast.


#include <string>

// c-str to string
char *old_string = "my c-style string";
string cpp_string(old_string);

// string to c-str

// char to string
char my_char = 'a';
string my_str(1, my_char);

String Interpolation


#include <iostream>
#include <sstream>
#include <string>

int main() {
    std::string a = "a", b = "b", c = "c";
    // apply formatting
    std::stringstream s;
    s << a << " " << b << " > " << c;
    // assign to std::string
    std::string str = s.str();
    std::cout << str << "\n";

Buildings Strings

The Complete Guide to Building Strings In C++
There are multiple ways of buildings strings in C++.
Strings are mutable in C++.
I typically use + or ostringstream to build strings.


#include <filesystem>
Convenient functions for filesystem. Added since C++17.



Note if you use g++ <= version 9, you will need to add the flag -lstdc++fs.

using std::filesystem::path;

// Initialization
path my_path = "my_dir/my_file";
// or my_path = path("my_dir") / "my_file";

// Append to path
path("foo") / "bar"; // path("foo/bar")
path("foo") / "/bar"; // path("/bar")

// Print
std::cout << my_path << std::endl; // prints "my_dir/my_file" with quotes
std::cout << my_path.string() << std::endl; // prints my_dir/my_file without quotes
  • path supports implicit conversion to string


  • create_directory requires that the parent directory already exists
    • If not, use create_directories instead


#include <fstream>
Used for input/output of files

Reading and Writing

Reading and writing is done using fstream.
If you don't need r/w, use istream for reading or ostream for writing.

#include <iostream>
#include <fstream>

int main() {
  std::istream my_file("my_file.txt");
  std::string line;
  // Read line by line
  // You can also read using <<
  while (getline(my_file, line)) {
    std::cout << line << std::endl;
  return 0;

Reading a whole file

Reference and comparison of different methods

#include <fstream>
#include <string>
#include <cerrno>

std::string get_file_contents(const std::string &filename)
  std::ifstream in(filename, std::ios::in | std::ios::binary);
  if (in.good())
    std::string contents;
    in.seekg(0, std::ios::end);
    contents.resize(static_cast<unsigned int>(in.tellg()));
    in.seekg(0, std::ios::beg);
    in.read(&contents[0], contents.size());
    return contents;
  std::cerr << "Failed to open file: " << filename << std::endl;

Regular Expressions

#include <regex>



#include <thread>
std::thread reference

Basic Usage:

std::thread my_thread(thread_function);
// Calling methods
// You can also pass in parameters as usual
std::thread my_thread(&Class::method, this));
// Lambda functions
std::thread my_thread([&]() {
 // do something

// Wait for thread to finish

// get id of thread
std::thread::id my_id = my_thread.get_id();

// get id of this thread
std::thread::id my_id = std::this_thread::get_id();



Parallel For



#include <memory>

Smart Pointers

Smart Pointers
Smart pointers were added in C++11.
There are 4 types of smart pointers:

  • auto_ptr which is deprecated
  • unique_ptr
  • shared_ptr
  • weak_ptr

Use unique_ptr for ownership models.
Use shared_ptr when multiple objects need to reference the same thing.
Use weak_ptr to avoid cyclic dependencies which cause issues with reference counting.
If you are using C++14 or newer, you should use make_unique or make_shared which will only make one memory allocation for both the object and the pointer rather than two memory allocations.
Alternatively if you already have a smart pointer, you can call my_ptr.reset(new Car()) to change the pointer or my_ptr.reset() to deallocate the object referenced by the pointer. Example:

// Block-scope car
Car my_car;

// Old C++
// Must call delete my_car; to avoid memory leaks.
Car *my_car = new Car();

// Using unique ptr
std::unique_ptr<Car> my_car(new Car());

// Or starting from C++14
auto my_car = std::make_unique<Car>();
  • If the object you need is not very large, you can consider just including it as part of your class (or leaving it on the stack) rather than use pointers.
  • If you want to get a copy of the smart pointer to the current object, the object must publically inherit std::enable_shared_from_this<T>
    • Then you can call shared_from_this() from within any method (not the constructor).
    • May throw bad_weak_ptr if you call shared_from_this() without make_shared or if you do not publically inherit std::enable_shared_from_this<T>

Garbage Collection

Starting from C++11, you should use smart pointers such as shared_ptr which have automatic garbage collection.

Traditional C++ does not have garbage collection.
After using new to allocate an object, use delete to deallocate it.
You can also use C allocation with malloc, calloc, alloca, and free, though it is not recommended since these are not type-safe.

Custom Deleter

Custom Deleters
When using smart pointers, the default deleter is the delete function but you can also specify your own deleter.

# Using a functor
struct AVFrameDeleter {
  void operator()(AVFrame *p) { av_frame_free(&p); }
std::unique_ptr<AVFrame, AVFrameDeleter> rgb_frame(av_frame_alloc());

# Using free
std::unique_ptr<void *, decltype(std::free) *> my_buffer(std::malloc(10), std::free);


Normally, containers such as std::vector will automatically deallocate memory from the heap when the destructor is called. However, occationally you may want to coerse this deallocation yourself.
There are a few ways to do this:

  • Use smart pointers
  • Swap
  • Call a clear/shrink/deallocate function

Example Reference:

// Using smart pointers
std::unique_ptr<std::vector<float>> my_vector = make_unique<std::vector<float>>(99);

// Swap
std::vector<float> my_vector(99);
my_vector = std::vector<float>;
// Or alternatively
// std::vector<float>().swap(my_vector);
// std::swap(my_vector, std::vector<float>);

// Swap for cl::Buffer
cl::Buffer my_buf(context, CL_MEM_READ_WRITE, size);
my_buf = cl::Buffer();

// Clear and shrink
// Specific to std::vector
std::vector<float> my_vector(99);


#include <limits>
C++ has standard macros such as INT_MAX.
The limits header adds these limits for every type.

// Equivalent to FLT_MAX


#include <utility>


Use std::move to move containers.





Allows you to fill a container using a function call

#include <random>
#include <iostream>
#include <algorithm>

int main()
    std::random_device rd;
    std::mt19937 gen(rd());
    # Fill with integers in [0, 10]
    std::uniform_int_distribution<> dis(0, 10);

    std::vector<int> my_vec(10, 0);
    std::generate(my_vec.begin(), my_vec.end(), [&](){return dis(gen);});
  <br />
    for (int v : my_vec) {
        std::cout << v << " ";
    std::cout << std::endl;

    return 0;



Fills an array or vector with increasing values. Can pass in a starting number.

std::vector<int> v(60);
std::iota(v.begin(), v.end(), 0);


Adds up numbers. Can pass in a starting number.

std::vector<int> v(60);
std::iota(v.begin(), v.end(), 0);
std::accumulate(v.begin(), v.end(), 0);


#include <chrono>
Lots of useful time stuff. Good for timing your code.

auto start = std::chrono::high_resolution_clock::now();
// do something
auto end = std::chrono::high_resolution_clock::now();
std::cout << "Time elapsed: " 
          << std::chrono::duration_case<std::chrono::milliseconds>(end - start).count() 
          << " ms" << std::endl;


#include <execution>
The execution header gives you tools for parallel execution.
See execution_policy_tag.
C++17 Parallel Algorithms blog.

Parallel Sorting Example
std::sort(std::execution::par_unseq, sorted.begin(), sorted.end());


#include <random>

std::random_device rd;  //Will be used to obtain a seed for the random number engine
std::mt19937 gen(rd()); //Standard mersenne_twister_engine seeded with rd()
std::uniform_int_distribution<> dis(1, 6);
for (int n=0; n<10; ++n)
   //Use dis to transform the random unsigned int generated by gen into an int in [1, 6]
   std::cout << dis(gen) << ' ';
std::cout << '\n';


STL is the Standard Template Library originally implemented in 1994 by Stepanov and Lee from HP.
STL consists of a general set of algorithms, containers, functions, and iterators, many of which are now built into the standard library (std) of C++.
This section focuses only on the portions of STL which have been incorporated into the C++ standard library.

Simple Containers




#include <array>
In C++, you can use std::vector which gives you a resizable array. This will allocate an array in the heap.

array vs vector
If you need a statically allocated array, you can use std::array in the array header.
This wrapper around C-style arrays gives us size information and allows the array to be passed around by reference while keeping the array on the stack unlike std::vector.
If you want to manually allocate an array on the heap, you can do so as follows:

auto my_arr = std::make_shared<std::array<char,64>>();


Use vector for almost everything...
It is an ArrayList.
Note that vector<bool> is not an array of bools.
This has several nuances so you should use vector<char> instead.

// Basics
vector my_vec;
// Vector with size 5
vector my_vec(5);
// Vector with size 5 initialized to 1
vector my_vec(5, 1);

// Length of vector

// Equivalent to size()==0

// Equivalent to my_vec[0];
// Undefined on empty vectors

// Equivalent to my_vec[my_vec.size()-1];
// Undefined on empty vectors


Double-ended queue


This is a doubly linked list. You can delete elements from the middle of the list if you know have an iterator.

list<int> m_list;

list<int>::iterator m_it = m_list.insert(5);

// Remove the element

Container adaptors





std::stack<char> my_stack;

// Push to stack
// You can also use emplace
// Returns void

// Peek
// Always make sure stack is not empty
char top = my_stack.top('a');

// Pop
// Note: returns void
// Always make sure stack is not empty

Associative Containers

Also known as maps or associative arrays.


#include <unordered_set>
This is a hashset.

std::unordered_set<int> my_set;
// add things to myset
// Check contains
my_set.find(5) != my_set.end();


Custom Keys

How to use a rational number as a key in C++

struct Fraction
    int num;
    int den;

    bool operator==(const Fraction &other) const { 
        return num*other.den == den * other.num;

    Fraction(int a, int b) : num(a), den(b) {}


Programming Styles

Modern C++

List of resources

  • Use RAII principles.
  • Use smart pointers instead of new and delete.
  • Use clang-format.

Orthodox C++

Somewhat opposite of modern C++.
Also known as "C with Classes"
Basically only use C++ for its classes. Do everything else C-style.
The main benefit is compatibility with older compilers/libraries and easier understanding for people less familiar with newer C++ features.

  • Don't use C++ runtime wrapper for C runtime includes (<cstdio>, <cmath>, etc.), use C runtime instead (<stdio.h>, <math.h>, etc.)
  • Don't use stream (<iostream>, <stringstream>, etc.), use printf style functions instead.
  • Don't use anything from STL that allocates memory, unless you don't care about memory management.
  • Don't use exceptions.
  • Don't use RTTI.


cppreference raii
Resource Acquisition Is Initialization - binds the life cycle of a resource to the lifetime of an object.
For instance, the resource for a vector is an allocated amount of memory. Once the vector is destroyed (destructor called), the resource is released.
In general, each RAII object should have all of the following:

  • Constructor acquiring resources
  • Copy Constructor
  • Assignment operator
  • Destructor releasing resources
  • Swap function (for std::swap)
  • Move constructor (since C++11, for std::move)
Example RAII Class

Copied from stack overflow

#include <algorithm> // std::copy
#include <cstddef> // std::size_t

class dumb_array
    // (default) constructor
    dumb_array(std::size_t size = 0)
        : mSize(size),
          mArray(mSize ? new int[mSize]() : nullptr)

    // copy-constructor
    dumb_array(const dumb_array& other)
        : mSize(other.mSize),
          mArray(mSize ? new int[mSize] : nullptr),
        // note that this is non-throwing, because of the data
        // types being used; more attention to detail with regards
        // to exceptions must be given in a more general case, however
        std::copy(other.mArray, other.mArray + mSize, mArray);

    // destructor
        delete [] mArray;

    friend void swap(dumb_array& first, dumb_array& second) // nothrow
        // enable ADL (not necessary in our case, but good practice)
        using std::swap;

        // by swapping the members of two objects,
        // the two objects are effectively swapped
        swap(first.mSize, second.mSize);
        swap(first.mArray, second.mArray);

    dumb_array& operator=(dumb_array other) // (1)
        swap(*this, other); // (2)

        return *this;

    dumb_array(dumb_array&& other) noexcept ††
        : dumb_array() // initialize via default constructor, C++11 only
        swap(*this, other);

    std::size_t mSize;
    int* mArray;

Useful Libraries

A list of useful libraries


A header-only C++ argument parser.
Note that if you already use Boost, you can use Boost::Program_options instead.


A header-only C++ linear algebra library.