C++

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.

Usage

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

Compilation

See also: CMake and Makefile

g++

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

Misc optimizations

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

Syntax

Main

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;
}

Headers

Reference

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

Reference

Casting

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.

String

#include <string>

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

// string to c-str
cpp_string.c_str();

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

String Interpolation

Reference

#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.

Filesystem

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

Path

cppreference

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

Notes

• path supports implicit conversion to string

Directories

• create_directory(path) and create_directories(path)
• directory_iterator(path)

Notes

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

Fstream

#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;
  throw(errno);
}

Regular Expressions

#include <regex>

Reference

Thread

#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
my_thread.join();

// 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();

Sleep

std::this_thread::sleep_for(std::chrono::milliseconds(1));

Parallel For

Reference

Memory

#include <memory>

Smart Pointers

Smart Pointers
Smart pointers were added in C++11.
There are 3 types of smart pointers:

• unique_ptr
• shared_ptr
• weak_ptr

Use unique_ptr when one piece of code owns the memory at any given time.
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>();

Notes

• 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>
• When writing functions when do not operate on pointers and do not claim ownership of objects, you should just take a reference to the object as the argument.

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);

Deallocate

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);
my_vector.reset();

// 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);
my_vector.clear();
my_vector.shrink_to_fit();

Limits

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

// Equivalent to FLT_MAX
std::numeric_limits<float>::max();

Utility

#include <utility>

std::move

Ref
Use std::move to move containers.

Algorithm

std::find

Reference

std::generate

cppreference
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;
}

Numeric

std::iota

Reference
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);

std::accumulate

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);

Chrono

#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;

Execution

#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());

Random

#include <random>
cppreference.com

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

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.

Sequences

std::array

#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>>();

std::vector

Reference
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
my_vec.size();

// Equivalent to size()==0
my_vec.empty();

// Equivalent to my_vec[0];
// Undefined on empty vectors
my_vec.front();

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

std::deque

Double-ended queue

std::list

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
m_list.erase(m_it);

Container adaptors

std::queue

Reference

std::stack

cppreference

std::stack<char> my_stack;

// Push to stack
// You can also use emplace
// Returns void
my_stack.push('a');

// 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
my_stack.pop();

Associative Containers

Also known as maps or associative arrays.

std::unordered_set

#include <unordered_set>
This is a hashset.

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

std::unordered_map

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.
  • Namely, std::make_unique and std::shared_ptr.
  • Or std::vector<char> if you just need some memory in the heap.
• Use clang-format.

Resources

• CppCoreGuidelines
• Google C++ Style Guide - note that some people dislike this since it is focused on interoperability and suggests avoiding exceptions.

Orthodox C++

Reference
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.

RAII

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
{
public:
    // (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
    ~dumb_array()
    {
        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);
    }

private:
    std::size_t mSize;
    int* mArray;
};

Useful Libraries

A list of useful libraries

cxxopts

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

Eigen

Main article: Eigen (C++ library)

A header-only C++ linear algebra library.