Operator Pointer Index

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Operator [] (Pointer Index)

Returns a reference to memory offset from an address

Syntax

Declare Operator [] ( ByRef lhs As T Pointer, ByRef rhs As Integer ) ByRef As T

Parameters

• lhs
• rhs
• lhs
• T

Description

This operator returns a reference to a value some distance in memory from a base address. It is essentially shorthand for "*(lhs + rhs)" because the reference can be thought of as a pointer having as value the memory location "(lhs + rhs)", and which is implicitly dereferenced; both do exactly the same thing.
Like pointer arithmetic, any type of Pointer can be indexed except for an Any Pointer. Also, like pointer arithmetic, it is up to the user to make sure meaningful data is being accessed.

When indexing a '2-dimensional' pointer (i.e. a T Ptr Ptr), the first (leftmost) index is applied before the second: For example, Pt[I1][I2] = *(Pt[I1] + I2) = *(*(Pt + I1) + I2)
In general, when using an 'n-dimensional' pointer: Pt[I1][I2].....[In], the index order (from left to right) corresponds to the dereferencing order.

This operator must not be used in case of null pointer because reference is undefined (inducing runtime error).
Otherwise, the user must ensure that the offset value (rhs) is in a range that allows an access to valid memory. Outside this range, results are undefined.

This operator can be overloaded for user-defined types as a member Operator using the appropriate syntax.

Remarks

Usage

result = lhs [ rhs ]

or

lhs [ rhs ] = value

Differences from QB

• New to FreeBASIC

See also

• Operator * (Value Of)
• Operator [] (String Index)
• Operator () (Array Index)
• Operator + (Add)
• Operator - (Subtract)
• Pointer Operators

Examples

'' initialize a 5-element array

Dim array(4) As Integer = { 0, 1, 2, 3, 4 }

'' point to the first element

Dim p As Integer Ptr = @array(0)

'' use pointer indexing to output array elements

For index As Integer = 0 To 4

    Print p[index];

Next

Print

Will give the output,

 0 1 2 3 4

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Category: Operators
Kind: operator

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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.