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

Lambda Expressions

Reference

Arrays

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 static sized array, you can use std::array.
This wrapper around C-style arrays gives us size information and allows the array to be passed around while keeping the array on the stack unlike std::vector If you want to allocate a static array on the heap, you can do so as follows:

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

Strings

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";
}

Filesystem

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 char *filename)
{
  std::ifstream in(filename, std::ios::in | std::ios::binary);
  if (in)
  {
    std::string contents;
    in.seekg(0, std::ios::end);
    contents.resize(in.tellg());
    in.seekg(0, std::ios::beg);
    in.read(&contents[0], contents.size());
    in.close();
    return(contents);
  }
  throw(errno);
}

Regular Expressions

Reference

Thread

Sleep

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

Parallel For

Reference

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.
Example:

std::unique_ptr<Car> my_car(new Car());
// Or starting from C++14
auto my_car = std::make_unique<Car>();

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
• Copy-and-swap idiom
• 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();

// Copy-and-swap idiom
std::vector<float> my_vector(99);
std::vector<float>().swap(my_vector);

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

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.

Programming Styles

Modern C++

List of resources
Prefer newer std functions available in C++17.
Use shared pointers instead of new and delete.

• Use clang-format.

Orthodox C++

Reference
Somewhat opposite of modern C++.
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.