Operator Let (Assign)
Indicates the assignment operator when overloading Operator = (Assignment)
{ Type | Class | Union | Enum } typename
End { Type | Class | Union }
Operator typename.Let ( [ ByRef | ByVal ] rhs As datatype )
lhs = rhs
or
lhs => rhs (from fbc version 0.90)
Syntax
{ Type | Class | Union | Enum } typename
End { Type | Class | Union }
Operator typename.Let ( [ ByRef | ByVal ] rhs As datatype )
Usage
lhs = rhs
or
Parameters
typename
lhs
The variable to assign to.
rhsThe value to assign.
Description
Let is used to overload the Operator =[>] (Assignment) operator and to distinguish it from the comparison operator Operator = (Equal).
lhs =[>] rhs will assign the rhs to lhs by invoking the Let operator procedure defined in typename.
This includes the case of an object returned from a function by value, by using Function =[>] rhs (or function_identifier =[>] rhs) assignment.
Assigning one array is not supported presently.
An operator Let (assign) must be defined if the shallow implicit copy is not sufficient. This happens in cases when the object manages dynamically allocated memory or other resources which need to be specially copied (for example if a member pointer points to dynamically allocated memory, the implicit assignment operator will simply copy the pointer value instead of allocate memory and then perform the copy of data).
Note: It is safe to do a check for self-assignment at the top of the Let body (by comparing the address of implicit 'this' instance with the address of 'rhs' parameter) to avoid object destruction if previously allocated memory is first deallocated (see example below).
Example
Type UDT
Public:
Declare Constructor (ByVal zp As Const ZString Ptr) ''constructor with string initializer
Declare Operator Let (ByRef rhs As UDT) ''operator Let (assignment)
Declare Function getString () As String ''function to get string
Declare Destructor () ''destructor
Private:
Dim zp As ZString Ptr ''private pointer to avoid direct access
End Type
Constructor UDT (ByVal zp As Const ZString Ptr)
This.zp = CAllocate(Len(*zp) + 1)
*This.zp = *zp
End Constructor
Operator UDT.Let (ByRef rhs As UDT)
If @This <> @rhs Then '' check for self-assignment to avoid object destruction
Deallocate(This.zp)
This.zp = CAllocate(Len(*rhs.zp) + 1)
*This.zp = *rhs.zp
End If
End Operator
Function UDT.getString () As String
Return *This.zp
End Function
Destructor UDT ()
Deallocate(This.zp)
End Destructor
Dim u As UDT = UDT("")
u = Type<UDT>("Thanks to the overloading operator Let (assign)")
Print u.getString
Sleep
Public:
Declare Constructor (ByVal zp As Const ZString Ptr) ''constructor with string initializer
Declare Operator Let (ByRef rhs As UDT) ''operator Let (assignment)
Declare Function getString () As String ''function to get string
Declare Destructor () ''destructor
Private:
Dim zp As ZString Ptr ''private pointer to avoid direct access
End Type
Constructor UDT (ByVal zp As Const ZString Ptr)
This.zp = CAllocate(Len(*zp) + 1)
*This.zp = *zp
End Constructor
Operator UDT.Let (ByRef rhs As UDT)
If @This <> @rhs Then '' check for self-assignment to avoid object destruction
Deallocate(This.zp)
This.zp = CAllocate(Len(*rhs.zp) + 1)
*This.zp = *rhs.zp
End If
End Operator
Function UDT.getString () As String
Return *This.zp
End Function
Destructor UDT ()
Deallocate(This.zp)
End Destructor
Dim u As UDT = UDT("")
u = Type<UDT>("Thanks to the overloading operator Let (assign)")
Print u.getString
Sleep
Output:
Thanks to the overloading operator Let (assign)
Dialect Differences
- In the -lang qb and -lang fblite dialects, this operator cannot be overloaded.
- In the -lang qb and -lang fblite dialects, an assignment expression can be preceded by the Let keyword.
Differences from QB
- None
See also