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


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