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A brief introduction to C++

Rupert Nash

[email protected]

Introduction

References

Bjarne Stroustrup, "Programming: Principles and Practice Using C++" (2nd Ed.). Assumes very little but it's long
Bjarne Stroustrup, "A Tour of C++". Assumes you're an experience programmer and is quite brief.
Best online reference I've found is http://en.cppreference.com/ (Comes other human languages too!)
Scott Meyers, "Effective Modern C++", 2014. This is the book to get once you know your way around C++, but you want to improve. Teaches lots of techniques and rules of thumb for writing correct, idiomatic, maintainable code.
stackoverflow.com has a lot of good questions about C++ (look for ones with at least 100 up-votes).

The misunderstood monster

By Universal Studios - Dr. Macro, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3558176

The misunderstood monster

Large: the C++11 standard is about 1300 pages
Composed of many parts: C, classes, generics, functional programming, exceptions, the vast library, ...
Inspires dread in those do not understand it
Dangerous:

"C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do it blows your whole leg off." - Bjarne Stroustrup

"Expert friendly"

#Octopus vs Swiss Army knife

"C++ is an octopus made by nailing extra legs onto a dog" - Steve Taylor

But you can cut off some extra legs to get the right dog for your program!

The philosophy of C++

General purpose
Flexible by allowing developers to build abstractions (and provides a large number through the library)
Performance and efficiency always targeted "You only pay for what you use"
Use the powerful type system to express intent.

These few lectures

We could spend all semester going in depth on C++, but we've got three lectures plus a "drop in".

So I've picked a handful of features to cover today that you really need to write C++ for HPC:

References and memory
Overloading
Classes
Templates

Hello!

#include <iostream>

int main(int argc, char* argv[]) {
  std::cout << "Hello, world!" << std::endl;
}

$ g++ --std=c++11 hello.cpp -o hello
$ ./hello 
Hello, world!

Include the iostream standard library header which gives us a way to communicate with the file system

std is the standard library namespace.

A namespace allows scoping of names (much like a filesystem has directories).

The scope resolution operator :: lets us access a declaration from inside a namespace.

cout represents console output (i.e. stdout)

The standard library uses the bitwise left shift operator (<<) to mean stream insertion

I.e. output the right hand side to the left.

(Similarly, the right shift (>>) operator is used for extraction, i.e. input.)

Memory and references

Don't use malloc again

While you can use malloc and free in C++, you should not.

Instead, if you need to directly allocate memory use new and delete.

int* x = nullptr;
x = new int;
*x = 42;

std::cout << "The answer is "<< *x <<std::endl;
// The answer is 42

delete x;

Slightly different for arrays

int* squares = nullptr;
squares = new int[5];
for (auto i=0; i<5; ++i) {
  squares[i] = i*i;
}

// Do something

delete[] squares;

Note the square brackets!

References

As well as pointers and values, C++ has the concept of a reference.

They are like pointers in that they don't copy the thing-that-is-referred-to, but syntactically they are just like a value.

double pi = 3.14;
double& pr = pi;
std::cout << pr << std::endl;

void iLoveIntegers(double& x) {
x = 3;
}

iLoveIntegers(pi);

\\ prints 3
std::cout << pi << std::endl;

References: whys and whens

References have simpler syntax.

References are safer than pointers: a reference cannot be null and cannot be reseated (must be bound when it's defined)

double twopi = 6.24;
pr = twopi; \\ Error will not compile

And you can rely on it being valid, unless there is an evil coder around:

char& get() {
  char x = '!';
  return x;
  // Many compilers will warn about this
}

Use a reference by default, until you need a pointer. (E.g. need to reseat or interface with C.)

Overloading

Function overloading

You can have multiple functions with the same name but different arguments.

int sum(int a, int b) {
  return a + b;
}
double sum(double a, double b) {
  return a + b;
}

When you call sum, the compiler knows the types of the arguments and will try to find the best match from all the candidates with the name.

The compiler will also try to use any built-in or user-defined conversion rules.

What happens here?

int i1 = 1;
int i2 = 2;
double d1 = 1.0;
double d2 = 2.0;
unsigned u42 = 42;

std::cout << sum(i1, i2) << std::endl;
std::cout << sum(3, 72) << std::endl;
std::cout << sum(i1, u42) << std::endl;
std::cout << sum(d2, d1) << std::endl;
std::cout << sum(d2, i1) << std::endl;
std::cout << sum(d2, 1.0f) << std::endl;

Operators are functions

C++ operators, for the non-built in types, are just functions with odd names, e.g.:

Vector operator+(const Vector& a, const Vector& b);

You can then use the natural syntax when manipulating these in other code:

Vector c = a + b;

We'll come back to this.

Classes

Objects in C++

You define a class just like a struct, but using the keyword class instead.[^1]

As well as data members, instantiations of classes (i.e. objects) can have member functions.

class Complex {
    public:
      Complex();
      Complex(float re, float im);
      Complex(const Complex&) = default;
      float magnitude() const;
    private:
      float real;
      float imag;
};

Creating complexes

When you create an instance of your class, you usually need to provide a constructor.

Note we provide two overloads - a default one needing no arguments and one that initialises with a value.

We also tell the compiler to create a default copy constructor for us.

Complex::Complex() : real(0), imag(0) {
}

Complex::Complex(float re, float im) : real(re), imag(im) {
}

// This is roughly what the compiler will create
// Complex::Complex(const Complex& c) :
//   real(c.real), imag(c.imag) {
// }

#Creating complexes We can now create these numbers

Complex zero;
Complex imaginary_unit(0, 1);
Complex copy_of_i(imaginary_unit);

What about actually doing something?

Using complexes

float Complex::magnitude() const {
  return sqrt(real*real + imag*imag);
}

The const at the end of the declaration says that this function will not alter the instance it is working on. Add this whenever possible!
We can access the members by just giving their names.
The instance that the function is working on is also available as a pointer, called this, so could rewrite as:

float Complex::magnitude() const {
return sqrt(this->real*this->real + this->imag*this->imag);
}

Using complexes

Complex numbers have the usual arithmetic operations: ($+-\times\div$)

We have to provide operator overloads, like:

Complex operator+(const Complex& a, const Complex& b) {
  return Complex{a.real+b.real, a.imag+b.imag};
}

This is just a function (with an unusual name) that takes two complex numbers and returns one.

To let this function touch the private members of the class, we must declare that it is a friend:

    class Complex {
      // ...
      friend Complex operator+(const Complex&, const Complex&);
};

Templates

Templates 101

Templates are a method of doing metaprogramming: a program that writes a program.

An easy example:

int sum(int a, int b) {
  return a+b;
}
double sum(double a, double b) {
  return a+b;
}

What if we need this for float and unsigned?

Going to get boring and hard to maintain quickly!

Templates 101

template<typename T>
T sum(T a, T b) {
  return a+b;
}

When you use it later, the compiler will substitute the types you supply for T and then try to compile the template.

cout <<"add unsigned=" << sum(1U, 4U) << endl;
cout <<"add floats=" << sum(1.0f, 4e-2f) << endl;

Template classes

You can define a template class - i.e. a template that will produce a class when you instatiate it.

Let's build something useful, like a simple array class.

template<class T>
class Array {
  unsigned _size;
  T* _data;
public:
  Array();
  Array(unsigned n);
  ~Array();
  unsigned size() const;
  const T& operator[](unsigned i) const;
  T& operator[](unsigned i);
};

Where to put your implementation?

Templates are not executable code - they tell the compiler how to create it.

So the definition must be available to the user of your template - i.e. typically in a header file.

You can define the functions in place like:

template<class T>
class Array {
public:
  Array() : _size(0), _data(nullptr) {}
};

Or at the end of the header (or equivalently in another file that you include at the end of your header)

template<class T>
Array<T>::Array(unsigned n) : _size(n) {
  _data = new T[n];
}

How to release that memory?

We have acquired some memory in the constructor and at the moment we will leak this.

Typically a class's destructor will do this.

The name of this function is ~Array

template<class T>
Array<T>::~Array() {
  delete[] _data;
}

It's important to note that you should never call this directly - the compiler will call it for you when your Array objects:

go out of scope (i.e. local variables)
are deleted
belong to another object and that object is destructed

#RAII / CADRe

A very important pattern in C++ is RAII: resource allocation is instantiation. Also known as constructor acquires, destructor releases.

This odd name is trying to communicate that any resource you have (heap memory in this case) should be tied to the lifetime of an object.

So the when the compiler destroys your object it will release the resource (e.g. memory).

void do_simulation(Parameters& p) {
  Array<float> work_array(p.problem_size);
  initial_condition(work_array);
  for(int i=0; i<p.timesteps; ++i) {
    do_timestep(work_array);
  }
  write_output(p.outfile, work_array);
}

Copying

One thing we've not discussed about this array is what do we want to do about copying?

We could create a shallow copy, using a simple pointer copy, but then which instance owns the data?
We could instead do a deep copy of the data each time, but that might be expensive. Maybe we want to disallow implicit copying but allow a user to explicitly copy with a special copy() method?

These design decisions should be considered!

Returning an Array

What happens if we compute an array in a function and return it?

Array<int> load(const string& fn) {
  auto n = getsize(fn);

  Array<int> ans(n);
  for (auto i=0; i<n; ++i)
    ans[i] = read(fn, i);
  return ans;
}

void user() {

  auto data = load(fn);

}

Returning an Array

People assume this is what happens:

Array<int> load(const string& fn) {
  auto n = getsize(fn);

  Array<int> ans(n); // allocate
  for (auto i=0; i<n; ++i)
    ans[i] = read(fn, i);
  return ans; // copy to a temporary
}

void user() {
  // Call default c'tor
  auto data = load(fn); // copy tmp->data
                        // destroy tmp
}

Returning an Array

So they try alternatives, e.g return a pointer:

Array<int>* load(const string& fn) {
  auto n = getsize(fn);
  // OMG - new!
  auto ans = new Array<int>(n); //allocate twice
  for (auto i=0; i<n; ++i)
    (*ans)[i] = read(fn, i); // UGLY!
  return ans;
}

void user() {

  auto data = load(fn);
 // Better remember to delete this!
}

Returning an Array

Or pass an output argument by reference

void load(const string& fn, Array<int>& ans) {
  auto n = getsize(fn);

  ans.resize(n); // probably allocate
  for (auto i=0; i<n; ++i)
    ans[i] = read(fn, i);
  // no return :)
}

void user() {
  Array<int> data; // Have to declare :(
  load(fn, data); // No copy :)
  // Regressing to assembly :(
}

exclude:true

Returning an Array

How about not copying - move intead

Need to define a new move constructor and assignment

template<class T>
class Array {
public:
  Array(Array&& other) :
    _size(other._size), _data(other._data) {
    other._data = nullptr;
    other._size = 0;
  }
};

The double ampersand indicates an "r-value reference".\

The compiler will only use this when the argument is a temporary value that is going to be destructed - you can safely steal its resources.

Returning an Array

How about not copying - move intead

Array<int> load(const string& fn) {
  auto n = getsize(fn);

  Array<int> ans(n); // Construct
  for (auto i=0; i<n; ++i)
    ans[i] = read(fn, i);
  return ans; // This moves to the temporary
}

void user() {

  Array<int> data = load(fn); // temporary moved to data
                              // temporary destructed
}

Returning an Array

Compilers are allowed to do copy elision

// Compiler adds secret arg 
Array<int> load(const string& fn) {
  auto n = getsize(fn);

  Array<int> ans(n); // Alias ans to secret
  for (auto i=0; i<n; ++i)
    ans[i] = read(fn, i);
  return ans; // No-op
}

void user() {
  // Create empty space for data
  Array<int> data = load(fn); // pass data as secret arg          
}

Compilers can do this even when this may have side effects!

They directly construct the return value in its destination

In C++17 this is required

Practical

The practical will have you rewriting a simple linked-list example with C++ and improving this Array template.

You will then try using the STL: the standard template library instead.

See the repository https://github.com/rupertnash/cpp4computsci

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parent directory

README.md

A brief introduction to C++

Rupert Nash

[email protected]

Introduction

References

The misunderstood monster

The misunderstood monster

The philosophy of C++

These few lectures

Hello!

Memory and references

Don't use malloc again

Slightly different for arrays

References

References: whys and whens

Overloading

Function overloading

What happens here?

Operators are functions

Classes

Objects in C++

Creating complexes

Using complexes

Using complexes

Templates

Templates 101

Templates 101

Template classes

Where to put your implementation?

How to release that memory?

Copying

Returning an Array

Returning an Array

Returning an Array

Returning an Array

Returning an Array

The compiler will only use this when the argument is a temporary value that is going to be destructed - you can safely steal its resources.

Returning an Array

Returning an Array

Practical

Practical