12. lvalue explained

• lvalue is an object that refers to a region of storage in memory that can be both examined and stored into.

• However, lvalue does not necessarily permit modification of the object it designates.

• Thus, lvalue can be

  • modifiable lvalue

  • non-modifiable lvalue

• For example, a function call that returns a reference is an lvalue.

• Certain operators require lvalues:

  
      &X  // Unary address-of operator requires that operand X must be an lvalue.
      X++ --X  // Operand X must be an lvalue. This applies to both prefix and postfix forms.  
      = += -= *= %= <<= >>= &= ^= |=  // Left operand must be an lvalue.  
  
  

• The term rvalue refers to a data value that is stored at some address in memory.

• An rvalue cannot have a new value assigned to it, for example,

  • literal constant "Hello" or a const variable.

• When necessary, an lvalue is implicitly converted to an rvalue, but the reverse, however, is not true: an rvalue cannot be converted to an lvalue.