5.6.13 Index Expressions

Note

This section describes the implementation of MS-VBAL §5.6.13 Index Expressions.

Syntax

Expression	AST node
Index	IndexExpressionNode

See RD-VBAL §3.0.2 Node Types.

Runtime Semantics

RuntimeExpressionEvaluator.EvaluateIndex evaluates an index expression.

Procedure Callee

For an IndexExpressionNode, whether its Callee is a bare name resolving to a Sub, Function or Property Get is checked before the usual recursive Evaluate(Callee). Such an index expression invokes the procedure with the index expression's own arguments. See RD-VBAL §5.3.1.11 Procedure Invocation Argument Processing. A member of the class module whose code it is, named bare, is invoked on the current instance (RD-VBAL §5.6.10 Simple Name Expressions).

The check precedes Evaluate(Callee) because, otherwise, a bare-name Callee would already have been auto-invoked with zero arguments by SimpleName's own dispatch, before the index expression could supply its arguments. See RD-VBAL §5.6.10 Simple Name Expressions.

An index expression with a procedure Callee is the only expression form that recurses (e.g. Foo(n - 1), even from within Foo's own body). A bare Foo, from within Foo's own body, reads the function result variable instead (RD-VBAL §5.6.10 Simple Name Expressions).

Variant Holding an Array

RuntimeExpressionEvaluator.EvaluateIndex unwraps a VBVariantValue before matching a wrapped VBArrayValue, so v(0) indexes a Variant holding an array the same as a declared array. See RD-VBAL §5.5.1.2.12 Let-coercion to Variant.

Subscripts

An index expression whose Callee is an array selects an element: one subscript per dimension, each Let-coerced to Long, as a bound is. MS-VBAL §5.6.13 only says that each argument must be "within its respective array dimension"; MS-VBA coerces it as any value is coerced to a Long (RD-VBAL §5.5.1.2):

Subscript	Selects
2.5, 1.5	2, rounded to even.
"2"	2.
True, Empty	-1, 0.
A Byte, a Currency, a Date, a Variant holding a number	Its value as a Long.

Condition	Run-time error
A subscript past the range of a Long.	6 — Overflow
A subscript of text that is no number.	13 — Type mismatch
A Null subscript.	94 — Invalid use of Null
A subscript outside its dimension, or a number of subscripts other than the array's rank.	9 — Subscript out of range

Object Callee

An index expression whose Callee is an object - or a Variant holding one - calls the object's default member, the one its class marks with VB_UserMemId = 0: c(1) is c.Item(1), and c(1)(2) indexes what that returns. A class of the workspace and a class of the library are found alike (RD-VBAL §6.1.3.1 Collection Object). An object with no such member is runtime error 438, and Nothing is runtime error 91. The target of an assignment is the same call: c(1) = 5 assigns c.Item(1) (RD-VBAL §5.4.3.8 Let Statement).

5.6.13.1 Argument Lists

This section corresponds to MS-VBAL §5.6.13.1 Argument Lists.

The ByVal keyword (argument-expression = ["byval"] expression) is written before one argument. It is a token of the source and stays one in the tree: a ByValArgumentExpressionNode wraps the argument it is written before, in an argument list of a call, of a call statement and of a named argument alike. Such an argument is a value bound to nothing, so it is never aliased to a ByRef parameter (RD-VBAL §5.3.1.11).

Where ByVal is written

Important

MS-VBAL §5.6.13.1 says it is invalid for an argument list to have a ByVal argument unless it is that of an invocation of an external procedure. MS-VBA compiles it elsewhere too, and RD-VBA follows MS-VBA: a ByVal argument is valid where MS-VBA compiles it, and is the compile error MS-VBA reports where it does not (recorded in Excel, 100 cases).

What decides is the parameter the argument is for, not the procedure. MS-VBA takes ByVal only where the argument is passed by value already, and where it means something:

The argument is for	ByVal is	Why
A ByVal parameter of any procedure - of the workspace, of a class, of a library, of a Declare; an index of a Property Let	Valid, and changes nothing. RDC00310 says so, a Hint.	The parameter takes its argument by value with the keyword or without: the value is let-coerced to the parameter's type as it is without it (Twice(ByVal 2.7) is 6, as Twice(2.7) is), and an object is passed as itself, not as the value of its default member - which (c) would be, so ByVal c is not the parentheses of RD-VBAL §5.6.6.
A ByRef or As Any parameter of a Declare, or an As Any parameter of the library (VarPtr)	Valid.	The argument is passed by value where a reference goes: the value itself where an address is expected (RD-VBAL §6.1.2.13).
A ByRef parameter of a procedure of the workspace or of a class, an Optional ByRef one too	VBC09310 Type mismatch.	A procedure of VBA code is given a reference to a variable, which a value is not.
A ByRef array parameter (a() As Long)	VBC09371 Array argument must be ByRef.	An array is always passed by reference.
A ParamArray, the Array keyword's elements	VBC09310 Type mismatch.	A ParamArray collects references to its arguments.
A parameter of the library the type library declares by reference: every Variant one (VARIANT*) but the String of a $ function, and every one of Replace's	VBC09310 Type mismatch.	As for the workspace's ByRef: Left(ByVal "abc", 1) is refused, Left("abc", ByVal 1) and Left$(ByVal "abc", 1) are not.
A member called late-bound (on an Object or a Variant)	VBC09310 Type mismatch.	Its parameters are not known, and are taken to be ByRef.
A function MS-VBA compiles inline: the reserved names (Abs, Fix, Int, Sgn, Len, LenB, the conversion functions but CDec, String), InStr, InStrB, StrComp, LBound, UBound	VBC00001 Syntax error.	They are parsed with their arguments, and the parser takes no ByVal there.
An index of an array (a(ByVal 0))	VBC00001 Syntax error.	An index is no argument.

The argument list of a RaiseEvent takes no ByVal at all: there it is a syntax error (RD-VBAL §5.4.2.20). A parameter declared ByVal and an array (ByVal a() As Long) is VBC09371 too, wherever it is declared (RD-VBAL §5.3.1.5).

ByValArgumentStaticSemantics checks each ByVal argument against its parameter, and records the redundant ones as a fact of the argument (ValueExpressionSemanticFlags.RedundantByVal). Which library parameters the type library declares by reference is the parameter's SymbolProperties.TypeLibraryByRef, read off the declarations (a Variant parameter, or StdLibTypeLibraryByRefAttribute); it says nothing of how the argument is passed. VBC09327, MS-VBAL's own error, is no longer raised.

See RD-VBAL §5.3.1.11 Procedure Invocation Argument Processing.

5.6.13.2 Argument List Queues

This section corresponds to MS-VBAL §5.6.13.2 Argument List Queues.

Note

Reserved. This section has no content yet.

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⏮️ RD-VBAL §5.6.12 Member Access Expressions | ⏭️ RD-VBAL §5.6.14 Dictionary Access Expressions