Deallocatesub

Frees previously allocated memory

Syntax

Declare Sub Deallocate cdecl ( ByVal pointer As Any Ptr )

Parameters

Name		Description
`pointer`		the address of the previously allocated buffer.

Description

This procedure frees memory that was previously allocated with Allocate. pointer must be a valid pointer. After the procedure returns, pointer will be rendered invalid (pointing to an invalid memory address), and its use (dereferencing or calling Deallocate again) will result in undefined behavior.

When memory was allocated to hold a string descriptor, the string must always be destroyed (setting to "") before deallocate the string descriptor (allowing to deallocate the memory taken up by the string data), otherwise, it is not possible to deallocate it later, and it may induce memory leak in the program continuation.

Calling Deallocate on a null pointer induces no action.

Remarks

Usage



Deallocate( pointer )

Dialect Differences

Not available in the -lang qb dialect unless referenced with the alias __Deallocate.

Differences from QB

New to FreeBASIC

Example

The following example shows how to free previously allocated memory. Note that the pointer is set to null following the deallocation:

Sub DeallocateExample1()

   Dim As Integer Ptr integerPtr = Allocate( Len( Integer ) )  '' initialize pointer to

                                                               '' new memory address



   *integerPtr = 420                                     '' use pointer

   Print *integerPtr



   Deallocate( integerPtr )                              '' free memory back to system

   integerPtr = 0                                        '' and zero the pointer

End Sub



   DeallocateExample1()

   End 0

Although in this case it is unnecessary since the function immediately exits afterwards, setting the pointer to null is a good habit to get into. If the function deallocated memory from a pointer that was passed in by reference, for instance, the pointer that was used in the function call will be rendered invalid, and it is up to the caller to either reassign the pointer or set it to null. Example 3 shows how to correctly handle this kind of situation, and the next example shows the effects of deallocating memory with multiple references.

In the following example, a different pointer is used to free previously allocated memory:

'' WARNING: "evil" example showing how things should NOT be done



Sub DeallocateExample2()

   Dim As Integer Ptr integerPtr = Allocate( Len( Integer ) )  

   '' initialize ^^^ pointer to new memory



   Dim As Integer Ptr anotherIntegerPtr = integerPtr

   '' initialize ^^^ another pointer to the same memory



   *anotherIntegerPtr = 69                     '' use other pointer

   Print *anotherIntegerPtr



   Deallocate( anotherIntegerPtr )             '' free memory back to system

   anotherIntegerPtr = 0                       '' and zero other pointer



'' *integerPtr = 420                           '' undefined behavior; original

                                               '' pointer is invalid

End Sub



   DeallocateExample2()

   End 0

Note that after the deallocation, both pointers are rendered invalid. This illustrates another one of the ways that bugs can arise when working with pointers. As a general rule, only deallocate memory previously allocated when you know that there is only one (1) pointer currently pointing at it.

Function createInteger() As Integer Ptr

   Return Allocate( Len( Integer ) )                     '' return pointer to newly

End Function                                             '' allocated memory



Sub destroyInteger( ByRef someIntegerPtr As Integer Ptr )

   Deallocate( someIntegerPtr )                          '' free memory back to system

   someIntegerPtr = 0                                    '' null original pointer

End Sub



Sub DeallocateExample3()

   Dim As Integer Ptr integerPtr = createInteger()       '' initialize pointer to

                                                         '' new memory address



   *integerPtr = 420                                     '' use pointer

   Print *integerPtr



   destroyInteger( integerPtr )                          '' pass pointer by reference

   Assert( integerPtr = 0 )                              '' pointer should now be null

End Sub



   DeallocateExample3()

   End 0

In the program above, a reference pointer in a function is set to null after deallocating the memory it points to. An Assert macro is used to test if the original pointer is in fact null after the function call. This example implies the correct way to pass pointers to functions that deallocate the memory they point to is by reference.

Reference

Documented in KeyPgDeallocate.html