Editing C syntax (section)

===Incomplete types===
An incomplete type is a [[#Structures_and_unions|structure or union]] type whose members have not yet been specified, an [[#Arrays|array type]] whose dimension has not yet been specified, or the {{code|void}} type (the {{code|void}} type cannot be completed). Such a type may not be instantiated (its size is not known), nor may its members be accessed (they, too, are unknown); however, the derived pointer type may be used (but not dereferenced).

They are often used with pointers, either as forward or external declarations. For instance, code could declare an incomplete type like this:

<syntaxhighlight lang=C>
struct thing *pt;
</syntaxhighlight>

This declares {{code|pt}} as a pointer to {{code|struct thing}} ''and'' the incomplete type {{code|struct thing}}. Pointers to data always have the same byte-width regardless of what they point to, so this statement is valid by itself (as long as {{code|pt}} is not dereferenced). The incomplete type can be completed later in the same scope by redeclaring it:

<syntaxhighlight lang=C>
struct thing {
    int num;
}; /* thing struct type is now completed */
</syntaxhighlight>

Incomplete types are used to implement [[Recursion (computer science)|recursive]] structures; the body of the type declaration may be deferred to later in the translation unit:

<syntaxhighlight lang=C>
typedef struct Bert Bert;
typedef struct Wilma Wilma;

struct Bert {
    Wilma *wilma;
};

struct Wilma {
    Bert *bert;
};
</syntaxhighlight>

Incomplete types are also used for [[data hiding]]; the incomplete type is defined in a [[header file]], and the body only within the relevant source file.