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ライブラリFreeImagePlusの には、同じ名前のとを作成しているように見えるFreeImage.h面白い があります。#definetypedefenum

#define FI_ENUM(x)      typedef int x; enum x

これは、プリプロセッサによって次のようなコードに展開されます。

typedef int FREE_IMAGE_FILTER;
enum FREE_IMAGE_FILTER {
 FILTER_BOX = 0,
 FILTER_BICUBIC = 1,
[...]

これは何をしますか？同じ名前のatypedefと anを持つことは合法ですか? とにかく互換性enumはありませんか？FreeImage がこれを行うのはなぜですか?enumint

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Quoting C99 N1256 draft 6.2.1 "Scopes of identifiers":

An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a typedef name; a label name; a macro name; or a macro parameter.

which means that in:

typedef int id;

id is an identifier.

And from 6.2.3 "Name spaces of identifiers":

BEGIN QUOTE

If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows:

  • label names (disambiguated by the syntax of the label declaration and use);
  • the tags of structures, unions, and enumerations (disambiguated by following any of the keywords struct, union, or enum);
  • the members of structures or unions; each structure or union has a separate name space for its members (disambiguated by the type of the expression used to access the member via the . or -> operator);
  • all other identifiers, called ordinary identifiers (declared in ordinary declarators or as enumeration constants).

END QUOTE

So in:

typedef int id;
enum id {ID0};
  • the first id is an ordinary identifier
  • the second is a tag identifier

an both can coexist pacifically.

On the other hand, we could not do something like:

typedef int id;
int id;

because both would be ordinary identifiers.

于 2015-05-28T20:13:03.820 に答える