Constructor
Called automatically when a class or user defined type is created
Syntax
`
Type typename
Declare Constructor ( )
Declare Constructor ( [ ByRef | ByVal ] parameter As datatype [ = default ] [, ... ] )
End Type
Constructor typename ( [ parameters ] ) [ Export ]
statements
End Constructor
`
Parameters
`typename`
name of the `Type` or `Class`
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`, but only one will be called when a user defined `Type` or `Class` variable is created, and 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.
Constructors are called when declaring global or local static instances of `typename` and when allocating `typename` dynamically using the `New Expression` operator. See examples below for different constructor syntaxes.
A copy `Constructor` is a special constructor that initializes a new object from an existing object. There are three general cases where the copy `Constructor` is called: when instantiating one object and initializing it with another object (in one instruction), when passing an object by value, when an object is returned from a function by value (by using `Return x` statement).
Note: When an object is returned from a function by value, but by using `Function = x` (or `function_identifier = x`) assignment, the `Constructor` is called once at first, and then the `Let (Assign)` operator at each assignment.
A copy `Constructor` must be defined if the shallow implicit copy constructor is not sufficient. This happens in cases when the object manages dynamically allocated memory or other resources which need to be specially constructed or copied (for example if a member pointer points to dynamically allocated memory, the implicit copy constructor will simply do an implicit pointer construction and a copy of value instead of allocate memory and then perform the copy of data).
Note: Even if is defined an explicit default `Constructor`, it is never called by the implicit copy constructor.
A copy `Constructor` can not declare its parameter (the object to clone) by value.
Chaining of constructors in nested types is supported. Any fields that have their own default constructor are called first.
Chaining together constructors of a same type is allowed by using the keyword `Constructor(parameters)` at the top of constructor. It prevents the compiler from emitting field initialization code (instead, it relies on the chained constructor to initialize everything).
`Constructor` can be also called directly from the `typename` instance like the other member methods (`Sub`) and with the same syntax, i.e. using a member access operator, e.g. `obj.Constructor(parameters)`. In particular, doing `this.Constructor(parameters)` is not treated as chaining together constructors of a same type, and it is allowed anywhere (not only at the top of constructors). In general it's not safe to manually call the constructor on an object, because no `Destructor` is called, and the old object state - if any - is overwritten without any of its old members being destroyed, which could cause memory/resource leaks.
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
`
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
`
Example of copy constructor:
`
Type UDT
Dim As String Ptr p ''pointer to string
Declare Constructor () ''default constructor
Declare Constructor (ByRef rhs As UDT) ''copy constructor
Declare Destructor () ''destructor
End Type
Constructor UDT ()
This.p = CAllocate(1, SizeOf(String))
End Constructor
Constructor UDT (ByRef rhs As UDT)
This.p = CAllocate(1, SizeOf(String))
*This.p = *rhs.p
End Constructor
Destructor UDT ()
*This.p = ""
Deallocate This.p
End Destructor
Dim As UDT u0
*u0.p = "copy constructor exists"
Dim As UDT u = u0
*u0.p = "" ''to check the independance of the result copy with the object copied
Print *u.p
Sleep
`
Dialect Differences

- Object-related features are supported only in the -lang fb option

Differences from QB

- New to FreeBASIC

See also

- `Class`
- `Constructor (Module)`
- `New Expression`
- `Destructor`
- `Type`
- Coercion and Conversion