target

integration into IS ISO/IEC 9899:202y

document history

1 Motivation

It seems that the current terminology that is applied for variable length arrays (VLA) and variably modified types (VM types) is often misunderstood because the term “variable length array type” is introduced quite confusingly spread over the text and by applying double negation. We think that the dual term, “complete type with known constant size”, is much easier to introduce consistently.

The goal of this papers to strictly define the term “complete type with known constant size” recursively. VLA then are just “complete array types for which the size is not known during translation”.

This is not intended to change semantics of the text.

Whether or not we then keep up the term VLA (to which people are accustomed) and whether or not we use size/length/extend/count… is of secondary interest and could be done in a second phase.

2 Suggested additions and changes to the wording.

New text is underlined green, removed text is stroke-out red.

2.1 In 6.2.4, Storage durations of objects

6 For such an object that does not have a variable length array type have a known constant size, see 6.2.5, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way. (Entering an enclosed block or calling a function suspends, but does not end, execution of the current block.) If the block is entered recursively, a new instance of the object is created each time. The initial representation of the object is indeterminate. If an initialization is specified for the object and it is not specified with constexpr, it is performed each time the declaration or compound literal is reached in the execution of the block; if it is specified with constexpr the initializer is evaluated once at translation time and the new instance of the object is initialized to that fixed value each time the specification is reached; otherwise, the representation of the object becomes indeterminate each time the declaration is reached.

7 For such an object that does have a variable length array type not have a known constant size, its lifetime extends from the declaration of the object until execution of the program leaves the scope of the declaration.²⁶⁾ If the scope is entered recursively, a new instance of the object is created each time. The initial representation of the object is indeterminate.

2.2 In 6.2.5 (Types)

28 A complete type shall have a size that is less than or equal to SIZE_MAX. A type has known constant size if it is complete and is not a variable length array type.^FNT)

^FNT) That is, any complete type that is not an array type has a known constant size. For types that are not derived, for pointer types and for atomic types this follows directly from their definitions. For structure and union types this follows from the fact that they are composed of members that all have a known constant size, see 6.7.3.2. For array types specific rules apply to determine if they have known constant size, see 6.7.7.3.

2.3 In 6.5.4.5, The sizeof and alignof operators

2 The sizeof operator yields the size (in bytes) of its operand, which can be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type does not have a known constant size, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant expression.

2.4 In 6.7.3.2 Structure and union specifiers

3 A structure or union shall not contain a member with incomplete or function type that does not have a known constant size (hence …

Note to the editors: It might be good to profit from the change to transform the weird subsentences in parenthesis that are obviously not meant to be part of the normative text into footnotes.

…

11 A member of a structure or union can have any complete object type type of known constant size other than a variably modified type.¹³²⁾ …

2.5 In 6.7.7.3, Array declarators

4 If the size is not present, the array type is an incomplete type. If the size is * instead of being an expression, the array type is a variable length array type of unspecified size, which can only be used as part of the nested sequence of declarators or abstract declarators for a parameter declaration, not including anything inside an array size expression in one of those declarators;¹⁶¹⁾ such arrays are nonetheless complete types. If the size assignment expression is an integer constant expression and the element type has a known constant size, the array type is not a variable length array typethe array type has a known constant size; otherwise, the array type is a variable length array type.^FNT)

^FNT)(Variable length arrays with automatic storage duration are a conditional feature that implementations may not support; see 6.10.10.4.)

3 Interaction with other proposals

Note that n3414 also proposes changes to 6.7.7.3, but that these are independent of each other.