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Showing content from http://www.cplusplus.com/doc/tutorial/classes2/ below:

Special member functions are member functions that are implicitly defined as member of classes under certain circumstances. There are six:

If a class definition has no constructors, the compiler assumes the class to have an implicitly defined default constructor. Therefore, after declaring a class like this:

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class Example {
  public:
    int total;
    void accumulate (int x) { total += x; }
};

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class Example2 {
  public:
    int total;
    Example2 (int initial_value) : total(initial_value) { };
    void accumulate (int x) { total += x; };
};

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Example2 ex (100);   // ok: calls constructor 

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Example2 ex;         // not valid: no default constructor 

Therefore, if objects of this class need to be constructed without arguments, the proper default constructor shall also be declared in the class. For example:

// classes and default constructors
#include <iostream>
#include <string>
using namespace std;

class Example3 {
    string data;
  public:
    Example3 (const string& str) : data(str) {}
    Example3() {}
    const string& content() const {return data;}
};

int main () {
  Example3 foo;
  Example3 bar ("Example");

  cout << "bar's content: " << bar.content() << '\n';
  return 0;
}

bar's content: Example

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Example3 (const string& str);

But now, let's imagine that the class in the last example allocates dynamic memory to store the string it had as data member; in this case, it would be very useful to have a function called automatically at the end of the object's life in charge of releasing this memory. To do this, we use a destructor. A destructor is a member function very similar to a default constructor: it takes no arguments and returns nothing, not even void. It also uses the class name as its own name, but preceded with a tilde sign (~):

// destructors
#include <iostream>
#include <string>
using namespace std;

class Example4 {
    string* ptr;
  public:
    // constructors:
    Example4() : ptr(new string) {}
    Example4 (const string& str) : ptr(new string(str)) {}
    // destructor:
    ~Example4 () {delete ptr;}
    // access content:
    const string& content() const {return *ptr;}
};

int main () {
  Example4 foo;
  Example4 bar ("Example");

  cout << "bar's content: " << bar.content() << '\n';
  return 0;
}

bar's content: Example

The destructor for an object is called at the end of its lifetime; in the case of foo and bar this happens at the end of function main.

A copy constructor is a constructor whose first parameter is of type reference to the class itself (possibly const qualified) and which can be invoked with a single argument of this type. For example, for a class MyClass, the copy constructor may have the following signature:

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MyClass::MyClass (const MyClass&);

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class MyClass {
  public:
    int a, b; string c;
};

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MyClass::MyClass(const MyClass& x) : a(x.a), b(x.b), c(x.c) {}

// copy constructor: deep copy
#include <iostream>
#include <string>
using namespace std;

class Example5 {
    string* ptr;
  public:
    Example5 (const string& str) : ptr(new string(str)) {}
    ~Example5 () {delete ptr;}
    // copy constructor:
    Example5 (const Example5& x) : ptr(new string(x.content())) {}
    // access content:
    const string& content() const {return *ptr;}
};

int main () {
  Example5 foo ("Example");
  Example5 bar = foo;

  cout << "bar's content: " << bar.content() << '\n';
  return 0;
}

bar's content: Example

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MyClass foo;
MyClass bar (foo);       // object initialization: copy constructor called
MyClass baz = foo;       // object initialization: copy constructor called
foo = bar;               // object already initialized: copy assignment called 

The assignment on foo is an assignment operation. No object is being declared here, but an operation is being performed on an existing object; foo.

The copy assignment operator is an overload of operator= which takes a value or reference of the class itself as parameter. The return value is generally a reference to *this (although this is not required). For example, for a class MyClass, the copy assignment may have the following signature:

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MyClass& operator= (const MyClass&);

But again, the implicit version performs a shallow copy which is suitable for many classes, but not for classes with pointers to objects they handle its storage, as is the case in Example5. In this case, not only the class incurs the risk of deleting the pointed object twice, but the assignment creates memory leaks by not deleting the object pointed by the object before the assignment. These issues could be solved with a copy assignment that deletes the previous object and performs a deep copy:

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Example5& operator= (const Example5& x) {
  delete ptr;                      // delete currently pointed string
  ptr = new string (x.content());  // allocate space for new string, and copy
  return *this;
}

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Example5& operator= (const Example5& x) {
  *ptr = x.content();
  return *this;
}

Unnamed objects are objects that are temporary in nature, and thus haven't even been given a name. Typical examples of unnamed objects are return values of functions or type-casts.

Using the value of a temporary object such as these to initialize another object or to assign its value, does not really require a copy: the object is never going to be used for anything else, and thus, its value can be moved into the destination object. These cases trigger the move constructor and move assignments:

The move constructor is called when an object is initialized on construction using an unnamed temporary. Likewise, the move assignment is called when an object is assigned the value of an unnamed temporary:

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MyClass fn();            // function returning a MyClass object
MyClass foo;             // default constructor
MyClass bar = foo;       // copy constructor
MyClass baz = fn();      // move constructor
foo = bar;               // copy assignment
baz = MyClass();         // move assignment 

The move constructor and move assignment are members that take a parameter of type rvalue reference to the class itself:

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MyClass (MyClass&&);             // move-constructor
MyClass& operator= (MyClass&&);  // move-assignment 

The concept of moving is most useful for objects that manage the storage they use, such as objects that allocate storage with new and delete. In such objects, copying and moving are really different operations:

For example:

// move constructor/assignment
#include <iostream>
#include <string>
using namespace std;

class Example6 {
    string* ptr;
  public:
    Example6 (const string& str) : ptr(new string(str)) {}
    ~Example6 () {delete ptr;}
    // move constructor
    Example6 (Example6&& x) : ptr(x.ptr) {x.ptr=nullptr;}
    // move assignment
    Example6& operator= (Example6&& x) {
      delete ptr; 
      ptr = x.ptr;
      x.ptr=nullptr;
      return *this;
    }
    // access content:
    const string& content() const {return *ptr;}
    // addition:
    Example6 operator+(const Example6& rhs) {
      return Example6(content()+rhs.content());
    }
};


int main () {
  Example6 foo ("Exam");
  Example6 bar = Example6("ple");   // move-construction
  
  foo = foo + bar;                  // move-assignment

  cout << "foo's content: " << foo.content() << '\n';
  return 0;
}

foo's content: Example

Note that even though rvalue references can be used for the type of any function parameter, it is seldom useful for uses other than the move constructor. Rvalue references are tricky, and unnecessary uses may be the source of errors quite difficult to track.

function_declaration = default;
function_declaration = delete;

For example:

// default and delete implicit members
#include <iostream>
using namespace std;

class Rectangle {
    int width, height;
  public:
    Rectangle (int x, int y) : width(x), height(y) {}
    Rectangle() = default;
    Rectangle (const Rectangle& other) = delete;
    int area() {return width*height;}
};

int main () {
  Rectangle foo;
  Rectangle bar (10,20);

  cout << "bar's area: " << bar.area() << '\n';
  return 0;
}

bar's area: 200

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Rectangle::Rectangle (const Rectangle& other) = default;

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Rectangle::Rectangle (const Rectangle& other) : width(other.width), height(other.height) {}

In general, and for future compatibility, classes that explicitly define one copy/move constructor or one copy/move assignment but not both, are encouraged to specify either delete or default on the other special member functions they don't explicitly define.

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