Operator Placement New

Home › Operators › Operator Placement New

Operator Placement New

Operator to construct an object at a specified memory address.

Syntax

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

Parameters

• address
• initializers
• datatype
• count

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). For types with at least one constructor, the initialize list (if any) must match an existing constructor. 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 Statement 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 operator usage.

Placement New[] operator is the (one-dimensional) array-version of the Placement New operator and constructs the specified number of objects from the specified memory location. The default constructor for the type will be used to set the initial values for each item.

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).

Because it does not provide any dynamic memory allocation process, the Placement New operator (unlike the New Expression operator) does not allow any overloading by a member operator for user-defined types.

Note: Using pointer = New(address)datatype[count] may be unsafe if pointer was declared with a type different from datatype (for sub-type polymorphism purpose for example), because the pointer arithmetic fails to access the elements if the pointer type size is different from the size of datatype (when using Operator [] (Pointer Index) or adding an offset (element number) to the pointer).

Return Value

A pointer of type datatype to the newly constructed data.

Remarks

Dialect Differences

• Only available in the -lang fb dialect.

Differences from QB

• New to FreeBASIC

See also

• Destructor
• New Expression

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

---

Category: Operators
Kind: operator

---

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.