Defect Report #059
Submission Date: 15 Jun 93
Submittor: Project Editor (P.J. Plauger)
Source: Martin Ruckert
The ISO Standard for the programming language C explains the notion
of incomplete type in subclause 18.104.22.168 and subclause 22.214.171.124
(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
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 126.96.36.199) the sentence: ``It is
admissable that an incomplete type remain incomplete in the whole
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.
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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