C++ rvalue references and move semantics for beginners

The logic behind rvalues is that in C++ you will find such temporary, short-lived values that you cannot alter in any way.

int x = 666;                    // (1)
int y = x + 5;                  // (2)

std::string s1 = "hello ";
std::string s2 = "world";
std::string s3 = s1 + s2;       // (3)

std::string getString() {
  return "hello world";
}
std::string s4 = getString();   // (4)

Introducing the magic of rvalue references

The traditional C++ rules say that you are allowed to take the address of an rvalue only if you store it in a const variable. More technically, you are allowed to bind a const lvalue to an rvalue.

int& x = 666;       // Error
const int& x = 666; // OK

C++0x has introduced a new type called rvalue reference, denoted by placing a double ampersand && after some type. Such rvalue reference lets you modify the value of a temporary object: it’s like removing the const attribute.

std::string   s1     = "Hello ";
std::string   s2     = "world";
std::string&& s_rref = s1 + s2;    // the result of s1 + s2 is an rvalue
s_rref += ", my friend";           // I can change the temporary string!
std::cout << s_rref << '\n';       // prints "Hello world, my friend"

Move semantics

Move semantics is a new way of moving resources around in an optimal way by avoid unnecessary copies of temporary objects, based on rvalue references.

class Holder
{
  public:

    Holder(int size)         // Constructor
    {
      m_data = new int[size];
      m_size = size;
    }

    ~Holder()                // Destructor
    {
      delete[] m_data;
    }

  private:

    int*   m_data;
    size_t m_size;
};

Rule of Three

If your class defines one or more of the following methods, it should probably explicitly define all three

destructor
copy constructor
copy assignment operator

Implementing the copy constructor

The copy constructor is used to create a new object from another existing object.

Holder h1(10000); // regular constructor
Holder h2 = h1;   // copy constructor
Holder h3(h1);    // copy constructor (alternate syntax)

Holder(const Holder& other)
{
  m_data = new int[other.m_size];  // (1)
  std::copy(other.m_data, other.m_data + other.m_size, m_data);  // (2)
  m_size = other.m_size;
}

Implementing the assignment operator

Holder h1(10000);  // regular constructor
Holder h2(60000);  // regular constructor
h1 = h2;           // assignment operator

Holder& operator=(const Holder& other) 
{
  if(this == &other) return *this;  // (1)
  delete[] m_data;  // (2)
  m_data = new int[other.m_size];
  std::copy(other.m_data, other.m_data + other.m_size, m_data);
  m_size = other.m_size;
  return *this;  // (3)
}

The limitations of our current class design

Holder createHolder(int size)
{
  return Holder(size);
}

It returns a Holder object by value. We know that when a function returns an object by value, the compiler has to create a temporary object (rvalue). Our Holder is a heavy-weight object due to its internal memory allocation, which is a very expensive task: returning such things by value with our current class design would trigger multiple expensive memory allocations.

int main()
{
  Holder h = createHolder(1000); // Copy constructor
  h = createHolder(500);         // Assignment operator
}

Implementing move semantics with rvalue references

The idea of move semantics is to add new versions of the copy constructor and assignment operator so that they can take a temporary object in input to steal data from.

Rule of Five

Any class for which move semantics are desirable, has to declare two additional member functions

the move constructor - to constructor new objects by stealing data from temporaries
the move assignment operator - to replace existing objects by stealing data from temporaries

Implementing the move constructor

Holder(Holder&& other)     // <-- rvalue reference in input
{
  m_data = other.m_data;   // (1)
  m_size = other.m_size;
  other.m_data = nullptr;  // (2)
  other.m_size = 0;
}

Implementing the move assignment operator

Holder& operator=(Holder&& other)     // <-- rvalue reference in input  
{  
  if (this == &other) return *this;

  delete[] m_data;         // (1)

  m_data = other.m_data;   // (2)
  m_size = other.m_size;

  other.m_data = nullptr;  // (3)
  other.m_size = 0;

  return *this;
}

int main()
{
  Holder h1(1000);                // regular constructor
  Holder h2(h1);                  // copy constructor (lvalue in input)
  Holder h3 = createHolder(2000); // move constructor (rvalue in input) (1) 

  h2 = h3;                        // assignment operator (lvalue in input)
  h2 = createHolder(500);         // move assignment operator (rvalue in input)
}

Can I move lvalues?

int main()
{
  Holder h1(1000);     // h1 is an lvalue
  Holder h2(h1);       // copy-constructor invoked (because of lvalue in input)
}

int main()
{
  Holder h1(1000);           // h1 is an lvalue
  Holder h2(std::move(h1));  // move-constructor invoked (because of rvalue in input)
}

References:

https://www.internalpointers.com/post/c-rvalue-references-and-move-semantics-beginners

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