Operator Placement New


Operator to construct an object at a specified memory address.

Syntax:
result = New(address) datatype
or
result = New(address) datatype ( initializers, ... )
or
result = New(address) datatype[ count ]

Parameters:
address
the location in memory to construct. the parenthesis are not optional.
initializers
Initial value(s) for the variable.
datatype
name of the data type to construct.
count
Number of elements to construct.

Return Value:
A pointer of type datatype to the newly constructed data.

Description:
The Placement New operator constructs a specified data type at the specified memory location.

For simple types, like integers, an initial value can be given. For types without constructors, initial values can be specified for each field (either with default initializer at data-field declaration, or with initializer list as in New datatype (initializers, ..) if all type data-fields are numeric primitives only and without any default initializers). Types that have constructors can have their constructors called by Placement New as well. If no initializers are given, the default values for those types will be set.

Memory is not allocated when using the Placement New operator. Instead, the memory at the specified address is used (the provided memory size must be large enough to contain all the placement).
It is incorrect to call Delete on the address. The proper way is to only call the destructor if one exists (implicitly or explicitly), with syntax as for a member method by using member access operator.
See examples below for proper placement new usage.

Specifying an initial value of Any, as in New(address)datatype (Any) or New(address)datatype[count] {Any} will not initialize the data. This is only valid on data types that do not have constructors (otherwise for data types with constructors, syntax of simple pointer conversion, like Cptr(datatype Ptr, address), can be substituted to the invalid use of New...Any).

Examples:
'' "placement new" example

Type Rational
    As Integer    numerator, denominator
    Declare Constructor ( ByVal n As Integer, ByVal d As Integer )
    As String ratio = "/"
End Type

Constructor Rational ( ByVal n As Integer, ByVal d As Integer )
    This.numerator = n
    This.denominator = d
End Constructor

Scope
   
    '' allocate some memory to construct as a Rational
    Dim As Any Ptr ap = CAllocate(Len(Rational))
   
    '' make the placement new call
    Dim As Rational Ptr r = New (ap) Rational( 3, 4 )
   
    '' you can see, the addresses are the same, just having different types in the compiler
    Print ap, r
   
    '' confirm all is okay
    Print r->numerator & r->ratio & r->denominator
   
    '' delete must not be used with placement new
    '' destroying must be done explicitly if a destructor exists (implicitly or explicitly)
    ''   (in this example, the var-string member induces an implicit destructor)
    r->Destructor( )
   
    '' we explicitly allocated, so we explicitly deallocate
    Deallocate( ap )
   
End Scope

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