FUNCTION Pointer
Data type that stores a pointer to a FUNCTION procedure returning a value
Syntax:
dim variable as Function [CDecl|Pascal|StdCall] [( [parameter_list] )] [ ByRef ] [As return_type] [= initializer]
Parameters:
parameter_list: parameter[, parameter[, ...]]
parameter: [ByRef|ByVal] identifier [As type] [= default_value]
intializer: address of a function to set as the initial value
parameter: [ByRef|ByVal] identifier [As type] [= default_value]
identifier: the name of the variable referenced in the function
type: the type of variable
default_value: the value of the argument if none is specified in the call
return_value: the value returned from the functiontype: the type of variable
default_value: the value of the argument if none is specified in the call
intializer: address of a function to set as the initial value
Description:
A Function pointer is a procedure pointer that stores the memory location of compiled code that returns a value. If no intializer is given the default initial value is zero (0).
The memory address for the Function procedure can be assigned to the variable by taking the address of a function with ProcPtr or Operator @ (Address of).
The procedure must match the same Function declaration as the declared Function pointer.
To call the function assigned, use the variable name as if it were a normal declared Function, always with parentheses around the parameter list even empty (without parentheses, only the pointer value, ie the address of the function, would be accessed).
One of the primary uses for Function pointers is to create callback procedures:
The memory address for the Function procedure can be assigned to the variable by taking the address of a function with ProcPtr or Operator @ (Address of).
The procedure must match the same Function declaration as the declared Function pointer.
To call the function assigned, use the variable name as if it were a normal declared Function, always with parentheses around the parameter list even empty (without parentheses, only the pointer value, ie the address of the function, would be accessed).
One of the primary uses for Function pointers is to create callback procedures:
- A callback Function is a Function that is passed through an argument (a Function pointer) to another procedure which is expected to call back (execute) the "argument" at a convenient time.
- If the callback Function is completely executed before the invocation returns to the caller code, then the callback process is said to be "synchronous".
- If the invocation immediately returns to the caller code, and the callback Function and the caller's next code are running in parallel, then the callback process is said to be "asynchronous".
- If the callback Function is completely executed before the invocation returns to the caller code, then the callback process is said to be "synchronous".
- If the invocation immediately returns to the caller code, and the callback Function and the caller's next code are running in parallel, then the callback process is said to be "asynchronous".
Examples:
Function ConcatSelf( x As String ) As String
Return x & x
End Function
Dim x As Function( x As String ) As String = ProcPtr( ConcatSelf )
Print x( "Hello" )
Return x & x
End Function
Dim x As Function( x As String ) As String = ProcPtr( ConcatSelf )
Print x( "Hello" )
Function x2 (ByVal i As Integer) As Integer
Return i * 2
End Function
Function x3 (ByVal i As Integer) As Integer
Return i * 3
End Function
Function operation (ByVal i As Integer, ByVal op As Function (ByVal As Integer) As Integer) As Integer
Return op(i)
End Function
Print operation(4, @x2)
Print operation(4, @x3)
Return i * 2
End Function
Function x3 (ByVal i As Integer) As Integer
Return i * 3
End Function
Function operation (ByVal i As Integer, ByVal op As Function (ByVal As Integer) As Integer) As Integer
Return op(i)
End Function
Print operation(4, @x2)
Print operation(4, @x3)
' Example of basic callback Function mechanism (asynchronous) to implement a key pressed event:
' (the user callback Function address cannot be modified while the event thread is running)
' - An asynchronous thread tests the keyboard in a loop, and calls a user callback Function each time a key is pressed.
' - The callback Function address is passed to the thread.
' - The callback Function prints the character of the key pressed,
' but if the key pressed is <escape> it orders the thread to finish by using the function return value.
' - As the user callback address is passed to the thread as argument, it cannot be modified while the thread is running.
'' thread Sub definition
Sub threadInkey (ByVal p As Any Ptr)
If p > 0 Then '' test condition callback Function defined
Dim As Function (ByRef As String) As Integer callback = p '' convert the any ptr to a callback Function pointer
Do
Dim As String s = Inkey
If s <> "" Then '' test condition key pressed
If callback(s) Then '' test condition to finish thread
Exit Do
End If
End If
Sleep 50, 1
Loop
End If
End Sub
'' user callback Function definition
Function printInkey (ByRef s As String) As Integer
If Asc(s) = 27 Then '' test condition key pressed = <escape>
Print
Return -1 '' order thread to finish
Else
Print s;
Return 0 '' order thread to continue
End If
End Function
'' user main code
Dim As Any Ptr p = ThreadCreate(@threadInkey, @printInkey) '' launch the thread, passing the callback Function address
ThreadWait(p) '' wait for the thread finish
' (the user callback Function address cannot be modified while the event thread is running)
' - An asynchronous thread tests the keyboard in a loop, and calls a user callback Function each time a key is pressed.
' - The callback Function address is passed to the thread.
' - The callback Function prints the character of the key pressed,
' but if the key pressed is <escape> it orders the thread to finish by using the function return value.
' - As the user callback address is passed to the thread as argument, it cannot be modified while the thread is running.
'' thread Sub definition
Sub threadInkey (ByVal p As Any Ptr)
If p > 0 Then '' test condition callback Function defined
Dim As Function (ByRef As String) As Integer callback = p '' convert the any ptr to a callback Function pointer
Do
Dim As String s = Inkey
If s <> "" Then '' test condition key pressed
If callback(s) Then '' test condition to finish thread
Exit Do
End If
End If
Sleep 50, 1
Loop
End If
End Sub
'' user callback Function definition
Function printInkey (ByRef s As String) As Integer
If Asc(s) = 27 Then '' test condition key pressed = <escape>
Return -1 '' order thread to finish
Else
Print s;
Return 0 '' order thread to continue
End If
End Function
'' user main code
Dim As Any Ptr p = ThreadCreate(@threadInkey, @printInkey) '' launch the thread, passing the callback Function address
ThreadWait(p) '' wait for the thread finish
Differences from QB:
- New to FreeBASIC
See also:
Back to Standard Data Types
