Defect Report #059

Submission Date: 15 Jun 93
Submittor: Project Editor (P.J. Plauger)
Source: Martin Ruckert
Question 1
The ISO Standard for the programming language C explains the notion of incomplete type in subclause 6.1.2.5 and subclause 6.5.2.3 (for structures). Both sections do not explicitly require that an incomplete type eventually must be completed, nor do they explicitly allow incomplete types to remain incomplete for the whole compilation unit.
Since this feature is of importance for the declaration of true opaque data types, it deserves clarification. I propose to add to the fourth paragraph on page 24 (subclause 6.1.2.5) the sentence: ``It is admissable that an incomplete type remain incomplete in the whole compilation unit.''
The type void is already an incomplete type which is never completed.
The examples given in the standard document explain that incomplete types are exclusively needed to define mutual referential structures. Opaque data types, however, constitute a second use for this feature. Considering mutual referential structures defined and implemented in different compilation units makes the idea of an opaque data type a natural extension of an incomplete data type.
Response
The C Standard intentionally contains no prohibition against leaving a type incomplete. (As you so aptly observe, void is an incomplete type that is never completed.) There is no need to make a positive statement about the absence of a prohibition.
Moreover, the examples were not intended to represent that mutual referencing was the only reason for permitting incomplete structure types. Opaque data types were considered, and endorsed, by the Committee when drafting the C Standard.
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