Constructor
Called automatically when a class or user defined type is created
Type typename
Constructor typename ( [ parameters ] )
Constructor methods are called when a user defined Type or Class variable is created.
typename is the name of the type for which the Constructor method is declared and defined. Name resolution for typename follows the same rules as procedures when used in a Namespace.
More than one constructor may exist for a type or class. The exact constructor method called depends on the parameter signature matched when the variable is initialized. More than one parameter may exist in a constructor method declaration.
A constructor method is passed a hidden This parameter having the same type as typename. This is optionally used to access the fields of the Type or Class which is to be initialized in the Constructor method.
Chaining of constructors in nested types is supported. Any fields that have their own default constructor are called first.
Constructors are called when declaring global or local static instances of typename and when allocating typename dynamically using the New operator.
Simple constructor example for beginners.
More advanced construction example, showing constructor overloading among other things.
Syntax
Type typename
Declare Constructor ( )
Declare Constructor ( [ ByRef | ByVal ] parameter As datatype [ = default ] [, ... ] )
End TypeDeclare Constructor ( [ ByRef | ByVal ] parameter As datatype [ = default ] [, ... ] )
Constructor typename ( [ parameters ] )
statements
End ConstructorParameters
Description
Constructor methods are called when a user defined Type or Class variable is created.
typename is the name of the type for which the Constructor method is declared and defined. Name resolution for typename follows the same rules as procedures when used in a Namespace.
More than one constructor may exist for a type or class. The exact constructor method called depends on the parameter signature matched when the variable is initialized. More than one parameter may exist in a constructor method declaration.
A constructor method is passed a hidden This parameter having the same type as typename. This is optionally used to access the fields of the Type or Class which is to be initialized in the Constructor method.
Chaining of constructors in nested types is supported. Any fields that have their own default constructor are called first.
Constructors are called when declaring global or local static instances of typename and when allocating typename dynamically using the New operator.
Example
Simple constructor example for beginners.
Type MyObj
Foo As Integer Ptr
'' Constructor to create our integer, and set its value.
Declare Constructor( ByVal DefVal As Integer = 0 )
'' Destroy our integer on object deletion.
Declare Destructor()
End Type
Constructor MyObj( ByVal DefVal As Integer = 0 )
Print "Creating a new integer in MyObj!"
Print "The Integer will have the value of: " & DefVal
Print ""
'' Create a pointer, and set its value to the one passed to the
'' Constructor.
This.Foo = New Integer
*This.Foo = DefVal
End Constructor
Destructor MyObj()
Print "Deleting our Integer in MyObj!"
Print ""
'' Delete the pointer we created in MyObj.
Delete This.Foo
This.Foo = 0
End Destructor
Scope
'' Create a MyObj type object
'' Send the value of '10' to the constructor
Dim As MyObj Bar = 10
'' See if the integer's been created. Print its value.
Print "The Value of our integer is: " & *Bar.Foo
Print ""
Sleep
End Scope
'' We've just gone out of a scope. The Destructor should be called now
'' Because our objects are being deleted.
Sleep
Foo As Integer Ptr
'' Constructor to create our integer, and set its value.
Declare Constructor( ByVal DefVal As Integer = 0 )
'' Destroy our integer on object deletion.
Declare Destructor()
End Type
Constructor MyObj( ByVal DefVal As Integer = 0 )
Print "Creating a new integer in MyObj!"
Print "The Integer will have the value of: " & DefVal
Print ""
'' Create a pointer, and set its value to the one passed to the
'' Constructor.
This.Foo = New Integer
*This.Foo = DefVal
End Constructor
Destructor MyObj()
Print "Deleting our Integer in MyObj!"
Print ""
'' Delete the pointer we created in MyObj.
Delete This.Foo
This.Foo = 0
End Destructor
Scope
'' Create a MyObj type object
'' Send the value of '10' to the constructor
Dim As MyObj Bar = 10
'' See if the integer's been created. Print its value.
Print "The Value of our integer is: " & *Bar.Foo
Print ""
Sleep
End Scope
'' We've just gone out of a scope. The Destructor should be called now
'' Because our objects are being deleted.
Sleep
More advanced construction example, showing constructor overloading among other things.
Type sample
_text As String
Declare Constructor ()
Declare Constructor ( a As Integer )
Declare Constructor ( a As Single )
Declare Constructor ( a As String, b As Byte )
Declare Operator Cast () As String
End Type
Constructor sample ()
Print "constructor sample ()"
Print
this._text = "Empty"
End Constructor
Constructor sample ( a As Integer )
Print "constructor sample ( a as integer )"
Print " a = "; a
Print
this._text = Str(a)
End Constructor
Constructor sample ( a As Single )
Print "constructor sample ( a as single )"
Print " a = "; a
Print
this._text = Str(a)
End Constructor
Constructor sample ( a As String, b As Byte )
Print "constructor sample ( a as string, b as byte )"
Print " a = "; a
Print " b = "; b
Print
this._text = Str(a) + "," + Str(b)
End Constructor
Operator sample.cast () As String
Return this._text
End Operator
Print "Creating x1"
Dim x1 As sample
Print "Creating x2"
Dim x2 As sample = 1
Print "Creating x3"
Dim x3 As sample = 99.9
Print "Creating x4"
Dim x4 As sample = sample( "aaa", 1 )
Print "Values:"
Print " x1 = "; x1
Print " x2 = "; x2
Print " x3 = "; x3
Print " x4 = "; x4
_text As String
Declare Constructor ()
Declare Constructor ( a As Integer )
Declare Constructor ( a As Single )
Declare Constructor ( a As String, b As Byte )
Declare Operator Cast () As String
End Type
Constructor sample ()
Print "constructor sample ()"
this._text = "Empty"
End Constructor
Constructor sample ( a As Integer )
Print "constructor sample ( a as integer )"
Print " a = "; a
this._text = Str(a)
End Constructor
Constructor sample ( a As Single )
Print "constructor sample ( a as single )"
Print " a = "; a
this._text = Str(a)
End Constructor
Constructor sample ( a As String, b As Byte )
Print "constructor sample ( a as string, b as byte )"
Print " a = "; a
Print " b = "; b
this._text = Str(a) + "," + Str(b)
End Constructor
Operator sample.cast () As String
Return this._text
End Operator
Print "Creating x1"
Dim x1 As sample
Print "Creating x2"
Dim x2 As sample = 1
Print "Creating x3"
Dim x3 As sample = 99.9
Print "Creating x4"
Dim x4 As sample = sample( "aaa", 1 )
Print "Values:"
Print " x1 = "; x1
Print " x2 = "; x2
Print " x3 = "; x3
Print " x4 = "; x4
Dialect Differences
- Object-related features are supported only in the -lang fb option
Differences from QB
- New to FreeBASIC
See also
