Constructor

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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

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.

Remarks

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

Examples

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

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Category: User Defined Types
Kind: type

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Reproduced from the FreeBASIC Wiki Manual under the GNU Free Documentation License 1.3 — with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. Copyright © the FreeBASIC project and wiki contributors. See License.