Left Value & Right Value

Left Value or lvalue refers to the value having an address can reference to. From the code aspect, it usually means a value with a name associated.

Right Value or rvalue refers to a temporary value that cannot reference using an address. From coding perspective, it is usually a literal value we put on the right side of the = or directly passed into function.

Right value can be bound as const left value, but not left value. To convert left value to right value, std::move has to be called.

struct Dummy {};
class MyClass
{
public:
    void func1(Dummy&);
    void func2(const Dummy&);
    void func3(Dummy&&);
};

int main()
{
    MyClass obj;
    Dummy dummy;

    obj.func1({}); // Error: Cannot bind lvalue to rvalue reference
    obj.func1(dummy);

    obj.func2({});
    obj.func2(dummy);
   
    obj.func3({});
    obj.func3(dummy); // Error: Cannot pass in a lvalue as a rvalue
    obj.func3(std::move(dummy));
}

Copy

Copy operation happens when creating data from left value to left value.Few examples:

• Passing value to a function which take left value as parameter - void func(MyClass obj).
• Constructing an object from another left value object - MyClass obj(another);.
• Capturing value returned by a function return left value - return obj;.
• Assigning non-right-value variable - MyClass obj = other;.

Copy Constructor

A user defined Copy Constructor can customize constructing new object through copying.

class MyClass
{
public:
	// Any One of below;
	MyClass(MyClass);
	MyClass(MyClass&);
	MyClass(const MyClass&);
}

Copy Assignment

A user defined Copy Assignment Operator can customize assigning object through copying.

class MyClass
{
public:
	// Any One of below;
	MyClass& operator=(MyClass);
	MyClass& operator=(MyClass&);
	MyClass& operator=(const MyClass&);
}

Move

Move operation happens when creating data from right value to left value. Few examples:

• Passing right value to a function which take left value as parameter - void func(MyClass obj) where a.func({...}).
• Constructing an object from a temporary object - MyClass obj({...});.
• Assigning from right value - MyClass obj = std::move(other);.

Move Constructor

Similar to Copy Constructor.

class MyClass
{
public:
	// Any One of below;
	MyClass(MyClass&&);
	MyClass(const MyClass&&);
}

Move Assignment

Similar Copy Assignment.

class MyClass
{
public:
	// Any One of below;
	MyClass& operator=(MyClass&&);
	MyClass& operator=(const MyClass&&);
}