lvalue, rvalue

lvalue(locator value): Anything that can hold a address. Appear on the left-hand side of an assignment.
rvalue(read value): temporary value that does not occupy a persistent memory location. Appears on the right-hand side of an assignment

int a = 10;                 // a is l value
                            // 10 is rvalue
int x = a+b;                // x is l value
                            // a+b is rvalue

lvalue reference or Reference Variable (&)

Reference to bind to lvalue. This is same as reference variable
Usage: Pass large objects without copying.

int a = 10;
int &b = a;         // lvalue reference
b = 11;
cout << a;    // 11

Example: Copy Constructor is lvalue reference

rvalue reference (&&)

Reference that binds to an rvalue(temporary value)

int a = 10;            //10 is rvalue
int &&b = 10;            // b is rvalue reference

Usage of rvalue reference:
  rvalues do occupy space in RAM ie they are temporaries on the stack or directly inside a CPU register.
  When we write int &&b = 10;, two things happen:
  The temporary value 10 is placed into stack.
  The rvalue reference b extends the lifetime of that temporary so it stays alive as long as b is in scope.
  -> The biggest use case of rvalue references is moving data instead of copying it (Move Semantics).
  -> Imagine you have a class that manages a large block of heap memory (like std::string or std::vector). Traditional copy would allocate new memory and copy every single element over.
  -> An rvalue reference tells the compiler: "This temporary object is about to die anyway, so don't copy its data—just steal its internal pointer."

XValue / Expiring Value

(b+c) is called xvalue/temporary value which is ready to expire

a = b+c;  
        

PRValue / Pure R Value

Literal value is called pure R value

a = 42;