Frees previously allocated memory

declare sub Deallocate cdecl ( byval pointer as any ptr )

Deallocate( pointer )

the address of the previously allocated buffer.

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.

Calling Deallocate on a null pointer induces no action.

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

   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

   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

   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.

Dialect Differences:
Differences from QB:
See also:
Back to Memory Functions
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