Operator Overloading


Changing the way user defined types work with built-in operators.

Overview
Global Operators
Member Operators
Special Cases of Operators: '.' (Member access), '@' (Address of), '->' (Pointer to member access), and '*' (Value of)

Overview
Simply, operators are procedures, and their arguments are called operands. Operators that take one operand (Operator Not) are called unary operators, operators that take two operands (Operator +) are called binary operators and operators taking three operands (Operator Iif) are called ternary operators.

Most operators are not called like procedures. Instead, their operator symbol is placed next to their operands. For unary operators, their sole operand is placed to the right of the symbol. For binary operators, their operands - referred to as the left and right-hand side operands - are placed to the left and right of the operator symbol. FreeBASIC has one ternary operator, Operator Iif, and it is called like a procedure, with its operands comma-separated surrounded by parenthesis.
For example, the following code calls Operator Iif to determine if a pointer is valid. If it is, Operator * (Value of) is called to dereference the pointer, and if not, Operator / (Divide) is called to find the value of twenty divided by four:
Dim i As Integer = 420
Dim p As Integer Ptr = @i

Dim result As Integer = IIf( p, *p, CInt( 20 / 4 ) )

Notice the call to Operator Iif is similar to a procedure call, while the calls to Operator * (Value of) and Operator / (Divide) are not. In the example, p is the operand to Operator * (Value of), and 20 and 4 are the left and right-hand side operands of Operator / (Divide), respectively.

All operators in FreeBASIC are predefined to take operands of standard data types, like Integer and Single, but they may also be overloaded for user-defined types; that is, they can be defined to accept operands that are objects as well. There are two types of operators that can be overloaded, global operators and member operators.

Global Operators
Global operators are those that are declared in module-level scope (globally). These are the operators - (Negate), Not (Bitwise not), -> (Pointer to member access), * (Value of), + (Add), - (Subtract), * (Multiply), / (Divide), \ (Integer divide), & (Concatenate), Mod (Modulus), Shl (Shift left), Shr (Shift right), And (Bitwise and), Or (Bitwise or), Xor (Bitwise xor), Imp (Bitwise imp), Eqv (Bitwise eqv), ^ (Exponentiate), = (Equal), <> (Not equal), < (Less than), > (Greater than), <= (Less than or equal), >= (Greater than or equal), Abs, Sgn, Fix, Frac, Int, Exp, Log, Sin, Asin, Cos, Acos, Tan, Atan, Len, and Sqr.

Declaring a custom global operator is similar to declaring a procedure. The Declare keyword is used with the Operator keyword. The operator symbol is placed next followed by the comma-separated list of parameters surrounded in parenthesis that will represent the operands passed to the operator. Unlike procedures, operators can be overloaded by default, so the Overload keyword is not necessary when declaring custom operators. At least one of the operator's parameters must be of user-defined type (after all, operators with built-in type parameters are already defined).

The following example declares the global operators - (Negate) and + (Multiply) to accept operands of a user-defined type:
Type Rational
    As Integer numerator, denominator
End Type

Operator - (ByRef rhs As Rational) As Rational
    Return Type(-rhs.numerator, rhs.denominator)
End Operator

Operator * (ByRef lhs As Rational, ByRef rhs As Rational) As Rational
    Return Type(lhs.numerator * rhs.numerator, _
        lhs.denominator * rhs.denominator)
End Operator

Dim As Rational r1 = (2, 3), r2 = (3, 4)
Dim As Rational r3 = -(r1 * r2)
Print r3.numerator & "/" & r3.denominator

Here the global operators are defined for type Rational, and are used in the initialization expression for r3. The output is -6/12.

Member Operators
Member operators are declared inside a Type or Class definition, like member procedures, and they are the cast and assignment operators Operator Cast (Cast), Operator @ (Address of), Operator [] (Pointer index), Operator New Overload, Operator Delete Overload, Operator For (iteration), Operator Step (Iteration), Operator Next (Iteration), Let (Assign), += (Add and assign), -= (Subtract and assign), *= (Multiply and assign), /= (Divide and assign), \= (Integer divide and assign), ^= (Exponentiate and assign), &= (Concat and assign), Mod= (Modulus and assign), Shl= (Shift left and assign), Shr= (Shift right and assign), And= (Conjunction and assign), Or= (Inclusive disjunction and assign), Xor= (Exclusive disjunction and assign), Imp= (Implication and assign) and Eqv= (Equivalence and assign).

When declaring member operators, the Declare and Operator keywords are used followed by the operator symbol and its parameter list. Like member procedures, member operators are defined outside the Type or Class definition, and the symbol name is prefixed with the name of the Type or Class name.

The following example overloads the member operators Operator Cast (Cast) and *= (Multiply and assign) for objects of a user-defined type:
Type Rational
    As Integer numerator, denominator
   
    Declare Operator Cast () As Double
    Declare Operator Cast () As String
    Declare Operator *= (ByRef rhs As Rational)
End Type

Operator Rational.Cast () As Double
    Return numerator / denominator
End Operator

Operator Rational.Cast () As String
    Return numerator & "/" & denominator
End Operator

Operator Rational.*= (ByRef rhs As Rational)
    numerator *= rhs.numerator
    denominator *= rhs.denominator
End Operator

Dim As Rational r1 = (2, 3), r2 = (3, 4)
r1 *= r2
Dim As Double d = r1
Print r1, d

Notice that the member operator Cast (Cast) is declared twice, once for the conversion to Double and once for the conversion to String. This is the only operator (or procedure) that can be declared multiple times when only the return type differs. The compiler decides which cast overload to call based on how the object is used (in the initialization of the Double d, Rational.Cast as double is called, and in the Print statement, Rational.Cast as string is used instead).

Special Cases of Operators: '.' (Member access), '@' (Address of), '->' (Pointer to member access), and '*' (Value of)
- Overloading Operator . (Member access)
The operator '.' (member access) cannot be overloaded.

- Overloading Operator @ (Address of)
The operator @ (Adress of) is used to access the address of a variable.
There is no many interest to overload this operator for an object, and moreover if we did, we could no longer access its address.

- Overloading Operator -> (Pointer to member access) and Operator * (Value of)
The operator -> (Pointer to member access) is used to access any member of an object (instance) via a pointer to this instance.
The operator * (Value of) is used to access to variable via a pointer to this variable.
Under normal circumstances, the operand of these operators must be a pointer:
Declare Operator -> ( ByRef lhs As T Ptr ) ByRef As U
Declare Operator * ( ByRef rhs As T Ptr ) ByRef As T


Overloading of these operators allows you to create a pointer wrapper class and let it behave like the pointer itself:
Declare Operator -> ( ByRef lhs As wrapperClass ) ByRef As U
Declare Operator * ( ByRef rhs As wrapperClass ) ByRef As U


The wrapper can be then used (to access a member) like:
wrapper->member
instead of:
wrapper.realPointer->member
and:
(*wrapper).member
instead of:
(*wrapper.realPointer).member

Clarifying the particular case of overloading the operator -> (pointer to member access):
The operator -> (pointer to member access) exhibits a different behavior from the other operators with respect to overloading:
- It doesn't return only the user datatype as indicated in the overloaded procedure header,
- but it returns this user datatype implicitly followed by the operator . (member access).

The operator -> (pointer to member access) is mainly used often in conjunction with the operator * (Value of) to implement "smart pointers".

- Using smart pointer
The use of smart pointers allows automatic management of dynamic references created by New (each reference is destroyed automatically when its smart pointer goes out of scope), without even making any copy of these references.

Reminder of what a smart pointer:
- A smart pointer is an object which behaves like a pointer but does more than a pointer.
- This object is flexible as a pointer and has the advantage of being an object (like constructor and destructor called automatically).
- Therefore, the destructor of the smart pointer will be automatically called when this object goes out of scope, and it will delete the user pointer.

As the smart pointer must behave like a pointer, it must support the same interface as a pointer does.
So it must support the following operations:
- Dereferencing (operator * (Value of))
- Indirection (operator -> (pointer to member access))

The operator * (Value of) and the operator -> (pointer to member access) must return references (by means of using Byref As ..... in the declaration of there return type).

Example of a smart pointer (to UDT) with an interface:
- public default-constructor
- public copy-constructor
- public destructor
- private UDT pointer and public operator cast (Cast) to access it in read only mode
- private operator let to disallow assignment not implemented here (to avoid copying the pointers values only)
- operator * (Value of) and operator -> (pointer to member access)
Type UDT
    Declare Constructor ()
    Declare Destructor ()
    Dim As String s = "object #0"
End Type

Constructor UDT ()
    Print "  UDT construction "; @This
End Constructor

Destructor UDT ()
    Print "  UDT destruction "; @This
End Destructor

Type SmartPointer
    Public:
        Declare Constructor ()                            '' to construct smart pointer (and UDT object)
        Declare Constructor (ByRef rhs As SmartPointer)   '' to copy construct smart pointer
        Declare Operator Cast () As UDT Ptr               '' to cast private UDT pointer (for read only)
        Declare Destructor ()                             '' to destroy smart pointer (and UDT object)
    Private:
        Dim As UDT Ptr p                                  '' private UDT pointer
        Declare Operator Let (ByRef rhs As SmartPointer)  '' to disallow assignment (to avoid copy of real pointers)
End Type

Constructor SmartPointer ()
    Print "SmartPointer construction "; @This
    This.p = New UDT
End Constructor

Constructor SmartPointer (ByRef rhs As SmartPointer)
    Print "SmartPointer copy-construction "; @This; " from "; @rhs
    This.p = New UDT
    *This.p = *rhs.p
End Constructor

Operator SmartPointer.Cast () As UDT Ptr
    Return This.p
End Operator

Destructor SmartPointer ()
    Print "SmartPointer destruction "; @This
    Delete This.p
End Destructor

Operator * (ByRef sp As SmartPointer) ByRef As UDT   '' overloaded operator '*'
    Print "SmartPointer operator '*'"
    Return *Cast(UDT Ptr, sp)                        ''    (returning byref)
End Operator                                         ''    to behave as pointer
 
Operator -> (ByRef sp As SmartPointer) ByRef As UDT  '' overloaded operator '->'
    Print "SmartPointer operator '->'"
    Return *Cast(UDT Ptr, sp)                        ''    (returning byref)
End Operator                                         ''    to behave as pointer
 

Scope
    Dim sp1 As SmartPointer
    Print "'" & sp1->s & "'"
    sp1->s = "object #1"
    Print "'" & sp1->s & "'"
    Print
 
    Dim sp2 As SmartPointer = sp1
    Print "'" & (*sp2).s & "'"
    (*sp2).s = "object #2"
    Print "'" & (*sp2).s & "'"
    Print
 
    Dim sp3 As SmartPointer = sp1
    Print "'" & sp3->s & "'"
    *sp3 = *sp2
    Print "'" & sp3->s & "'"
    sp3->s = "object #3"
    Print "'" & sp3->s & "'"
    Print
End Scope

Sleep
Example of output:
SmartPointer construction 1703576
  UDT construction 10693312
SmartPointer operator '->'
'object #0'
SmartPointer operator '->'
SmartPointer operator '->'
'object #1'

SmartPointer copy-construction 1703524 from 1703576
  UDT construction 10693384
SmartPointer operator '*'
'object #1'
SmartPointer operator '*'
SmartPointer operator '*'
'object #2'

SmartPointer copy-construction 1703472 from 1703576
  UDT construction 10693456
SmartPointer operator '->'
'object #1'
SmartPointer operator '*'
SmartPointer operator '*'
SmartPointer operator '->'
'object #2'
SmartPointer operator '->'
SmartPointer operator '->'
'object #3'

SmartPointer destruction 1703472
  UDT destruction 10693456
SmartPointer destruction 1703524
  UDT destruction 10693384
SmartPointer destruction 1703576
  UDT destruction 10693312

Example of an extended smart pointer type macro for any UDT (or any predefined type), with an extended interface:
- public constructor
- public reference counter in read only mode
- public destructor
- private UDT pointer and 2 public operators cast to access it in read only mode (numeric value and string value)
- private default-constructor to disallow self construction
- private copy-constructor to disallow cloning
- private operator let to disallow assignment
- operator * (Value of) and operator -> (pointer to member access)
#Macro Define_SmartPointer (_UDTname_)

    Type SmartPointer_##_UDTname_
        Public:
            Declare Constructor (ByVal rhs As _UDTname_ Ptr)              '' to construct smart pointer
            '                                                                '' from _UDTname_ pointer,
            '                                                                '' with reference counter increment
            Declare Static Function returnCount () As Integer             '' to return reference counter value
            Declare Operator Cast () As _UDTname_ Ptr                     '' to cast private _UDTname_ pointer
            '                                                                '' to _UDTname_ pointer (read only)
            Declare Operator Cast () As String                            '' to cast private _UDTname_ pointer
            '                                                                '' to string (read only)
            Declare Destructor ()                                         '' to destroy smart pointer
            '                                                                '' and _UDTname_ object
            '                                                                '' with reference counter decrement
        Private:
            Dim As _UDTname_ Ptr p                                        '' private _UDTname_ pointer
            Static As Integer Count                                       '' private reference counter
            Declare Constructor ()                                        '' to disallow default-construction
            Declare Constructor (ByRef rhs As SmartPointer_##_UDTname_)   '' to disallow copy-construction
            Declare Operator Let (ByRef rhs As SmartPointer_##_UDTname_)  '' to disallow copy-assignment
    End Type
    Dim As Integer SmartPointer_##_UDTname_.Count = 0

    Constructor SmartPointer_##_UDTname_ (ByVal rhs As _UDTname_ Ptr)
        If rhs <> 0 Then
            This.p = rhs
            SmartPointer_##_UDTname_.count += 1
        End If
    End Constructor

    Static Function SmartPointer_##_UDTname_.returnCount () As Integer
        Return SmartPointer_##_UDTname_.count
    End Function

    Operator SmartPointer_##_UDTname_.Cast () As _UDTname_ Ptr
        Return This.p
    End Operator

    Operator SmartPointer_##_UDTname_.Cast () As String
        Return Str(This.p)
    End Operator

    Destructor SmartPointer_##_UDTname_ ()
        If This.p <> 0 Then
            Delete This.p
            SmartPointer_##_UDTname_.count -= 1
            This.p = 0
        End If
    End Destructor

    Operator * (ByRef sp As SmartPointer_##_UDTname_) ByRef As _UDTname_  '' operator '*' (return byref)
        '                                                                    '' to behave as pointer
        Return ByVal sp                                                   '' 'Return *sp' would induce an infinite loop
    End Operator

    Operator -> (ByRef sp As SmartPointer_##_UDTname_) ByRef As _UDTname_  '' operator '->' (return byref)
        '                                                                     '' to behave as pointer
        Return ByVal sp
    End Operator

#Endmacro

'--------------------------------------------------------------------------------------------------------

' Example using all eight keywords of inheritance:
'   'Extends', 'Base.', 'Base()', 'Object', 'Is' operator, 'Virtual', 'Abstract', 'Override'

Type root Extends Object ' 'Extends' to activate RTTI by inheritance of predefined Object type
    Public:
        Declare Function ObjectHierarchy () As String
        Declare Function ObjectName () As String
        Declare Abstract Function ObjectRealType () As String  '' 'Abstract' declares function without local body
        '                                                         '' which must be overridden
        Declare Virtual Destructor ()                          '' 'Virtual' declares destructor
    Protected:
        Declare Constructor ()                                 '' to avoid default-construction from outside Types
        Declare Constructor (ByRef _name As String = "")       '' to avoid construction from outside Types
        Declare Constructor (ByRef rhs As root)                '' to avoid copy-construction from outside Types
        Declare Operator Let (ByRef rhs As root)               '' to avoid copy-assignment from outside Types
    Private:
        Dim Name As String
End Type                                                   '' derived type may be member data empty

Constructor root ()  '' only to avoid compile error (due to inheritance)
End Constructor

Constructor root (ByRef _name As String = "")              '' only to avoid compile error (due to inheritance)
    This.Name = _name
    Print "root constructor:", This.Name
End Constructor

Function root.ObjectHierarchy () As String
    Return "Object(forRTTI) <- root"
End Function

Function root.ObjectName () As String
    Return This.Name
End Function

Virtual Destructor root ()
    Print "root destructor:", This.Name
End Destructor

Operator root.Let (ByRef rhs As root)                      '' only to avoid compile error (due to onheritance)
End Operator


Type animal Extends root                                           '' 'Extends' to inherit of root
    Declare Constructor (ByRef _name As String = "")
    Declare Function ObjectHierarchy () As String
    Declare Virtual Function ObjectRealType () As String Override  '' 'Virtual' declares function with local
    '                                                              ''    body which can be overridden
    '                                                              '' 'Override' to check if the function is
    '                                                              ''    well an override
    Declare virtual Destructor () Override                         '' 'Virtual' declares destructor with local body
    '                                                              '' 'Override' to check if the destructor is well an override
End Type

Constructor animal (ByRef _name As String = "")
    Base(_name)                                                    '' 'Base()' allows to call parent constructor
    Print "  animal constructor:", This.ObjectName()
End Constructor

Function animal.ObjectHierarchy () As String
    Return Base.ObjectHierarchy & " <- animal"                     '' 'Base.' allows to access to parent member function
End Function

Virtual Function animal.ObjectRealType () As String
    Return "animal"
End Function

Virtual Destructor animal ()
    Print "  animal destructor:", This.ObjectName()
End Destructor


Type dog Extends animal                                    '' 'Extends' to inherit of animal
    Declare Constructor (ByRef _name As String = "")
    Declare Function ObjectHierarchy () As String
    Declare Function ObjectRealType () As String Override  '' 'Override' to check if the function is well an
    '                                                      ''    override
    Declare Destructor () Override                         '' 'Override' to check if the destructor is well an override
End Type                                                   '' derived type may be member data empty

Constructor dog (ByRef _name As String = "")
    Base(_name)                                            '' 'Base()' allows to call parent constructor
    Print "    dog constructor:", This.ObjectName()
End Constructor

Function dog.ObjectHierarchy () As String
    Return Base.ObjectHierarchy & " <- dog"                '' 'Base.' allows to access to parent member function
End Function

Function dog.ObjectRealType () As String
    Return "dog"
End Function

Destructor dog ()
    Print "    dog destructor:", This.ObjectName()
End Destructor


Type cat Extends animal                                  '' 'Extends' to inherit of animal
    Declare Constructor (ByRef _name As String = "")
    Declare Function ObjectHierarchy () As String
    Declare Function ObjectRealType () As String Override  '' 'Override' to check if the function is well an
    '                                                      ''    override
    Declare Destructor () Override                         '' 'Override' to check if the destructor is well an override
End Type                                                   '' derived type may be member data empty

Constructor cat (ByRef _name As String = "")
    Base(_name)                                            '' 'Base()' allows to call parent constructor
    Print "    cat constructor:", This.ObjectName()
End Constructor

Function cat.ObjectHierarchy () As String
    Return Base.ObjectHierarchy & " <- cat"                '' 'Base.' allows to access to parent member function
End Function

Function cat.ObjectRealType () As String
    Return "cat"
End Function

Destructor cat ()
    Print "    cat destructor:", This.ObjectName()
End Destructor


Sub PrintInfo (ByVal p As root Ptr)                                       '' parameter is a 'root Ptr' or compatible (smart pointer)
    Print "  " & p->ObjectName, "  " & p->ObjectRealType, "           ";
    If *p Is dog Then                                                     '' 'Is' allows to check compatibility with type symbol
        Print  Cast(dog Ptr, p)->ObjectHierarchy
    ElseIf *p Is cat Then                                                 '' 'Is' allows to check compatibility with type symbol
        Print Cast(cat Ptr, p)->ObjectHierarchy
    ElseIf *p Is animal Then                                              '' 'Is' allows to check compatibility with type symbol
        Print Cast(animal Ptr, p)->ObjectHierarchy
    End If
End Sub


Define_SmartPointer(root)  '' smart pointer definition

Scope
    Print "reference counter value:"; SmartPointer_root.returnCount()
    Print
    Dim As SmartPointer_root sp(2) = {New animal("Mouse"), New dog("Buddy"), New cat("Tiger")}
    Print
    Print "reference counter value:"; SmartPointer_root.returnCount()
    For I As Integer = 0 To 2
        Print "  " & sp(I), sp(I)->ObjectName()
    Next I
    Print
    Print "Name:", "Object (real):         Hierarchy:"
    For I As Integer = 0 To 2
        PrintInfo(sp(I))
    Next I
    Print
End Scope
Print
Print "reference counter value:"; SmartPointer_root.returnCount()
Print

Sleep
Example of output:
reference counter value: 0

root constructor:           Mouse
  animal constructor:       Mouse
root constructor:           Buddy
  animal constructor:       Buddy
	dog constructor:        Buddy
root constructor:           Tiger
  animal constructor:       Tiger
	cat constructor:        Tiger

reference counter value: 3
  11145960    Mouse
  11151496    Buddy
  11151616    Tiger

Name:         Object (real):         Hierarchy:
  Mouse         animal                 Object(forRTTI) <- root <- animal
  Buddy         dog                    Object(forRTTI) <- root <- animal <- dog
  Tiger         cat                    Object(forRTTI) <- root <- animal <- cat

	cat destructor:         Tiger
  animal destructor:        Tiger
root destructor:            Tiger
	dog destructor:         Buddy
  animal destructor:        Buddy
root destructor:            Buddy
  animal destructor:        Mouse
root destructor:            Mouse

reference counter value: 0


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