Defect Report #207

Submitter: Clive Feather (UK)
Submission Date: 1999-06-27
Source:
Reference Document: ISO/IEC WG14 N892
Version: 1.2
Date: 2001-09-18 02:48:44
Subject: Handling of imaginary types

Summary
The handling of imaginary types in the Standard is somewhat inconsistent. For example, they are not mentioned at all in 6.2.5 (other than a footnote), but are treated as first-class types in 6.7.2. Annex G makes certain assumptions about such types but these assumptions are not supported by the Standard.

Details
There are two reasonable approaches that could be followed. The first is to remove all mention of imaginary types from the main text of the Standard and put them all in Annex G. The second is to make the basic properties of imaginary types part of the main language (while still making them optional), leaving Annex G to handle the details of ISO 60559 imaginary types.

After some thought, the author of this DR feels that imaginary types are experimental enough that the first approach is better and has worded the Suggested Technical Corrigendum on that basis.

The keyword _Imaginary is mentioned in 6.4.1, 6.7.2, and 7.3.1. These references - and any related text - are all to be removed and replacement wording added to Annex G.

A new subclause G.4.4 is added. This specifies the practical implications of giving imaginary types the same representation and alignment as real floating types.

Suggested Technical Corrigendum
Delete "_Imaginary" from the list of keywords in 6.4.1. If this is felt to be too radical, instead add the following text to paragraph 2:

The keyword _Imaginary is not used in the C language, but is reserved for specifying imaginary types such as described in Annex G.

Delete "_Imaginary" from 6.7.2p1 and the three imaginary cases from 6.7.2p2.

Change 6.7.2p3 to read:

The type specifier _Complex shall not be used if the implementation does not provide complex types.

Delete 7.3.1p3.

Delete "imaginary" from 7.3.1p5.

Replace 7.3.1p4 with:

The macro I expands to _Complex_I.

Add a new paragraph before G.2p1:

There is a new keyword _Imaginary used to specify imaginary types. It is used as a type-specifier within declaration-specifiers in the same way as _Complex is (thus "_Imaginary float" is a valid type name).

Add a new subclause G.4.4

G.4.4 Interchangeable values

Though imaginary types are not compatible with the corresponding real type, values of one may be used where the other is expected in the following cases. In each case the value is converted to the value of the other type that has the same representation (that is, by multiplying by the imaginary unit when converting to an imaginary type, and by dividing by the imaginary unit when converting to a real type).

• one type is the type of the parameter, and the other type the type of the argument, when a function is called without a prototype in scope; [*]
• one type is the type of an argument corresponding to a trailing ellipsis in a function call and the other is specified as the type argument of an invocation of the va_arg macro;
• one type is the type of an argument to a function such as fprintf or the type pointed to by an argument to a function such as fscanf, and the other is the type implied by the corresponding conversion specifier.

[*] If a prototype is in scope, conversion is as if by assignment and the value will be converted to zero.

Replace G.6p1 with:

The macros

imaginary

and

_Imaginary_I

are defined, respectively, as _Imaginary and a constant expression of type const float _Imaginary with the value of the imaginary unit. The macro I is defined to be _Imaginary_I (not _Complex_I as stated in 7.3). Notwithstanding the provisions of 7.1.3, a program may undefine and then perhaps redefine the macro imaginary.

Afternote
If WG14 wishes to take the alternative approach of moving _Imaginary types more firmly into the body of the Standard, then the following areas would be affected.

• Do not make any of the above changes.
• Add text to 4p6 explaining that imaginary types are never required.
• Merge the text from G.2 into 6.2.5.
• Merge the existing text from G.4 into 6.3.1.
• Make the cases described in the new G.4.4 above further cases of the relevant subclauses (6.5.2.2, 7.15.1.1, 7.19.6.1, 7.19.6.2, 7.24.2.1, and 7.24.2.2).
• Move G.5.1p1 and G.5.2p1 into 6.5.5 and 6.5.6.
• Delete G.6p1.

Technical Corrigendum:
In 6.4.1 append to paragraph 2:

The keyword _Imaginary is reserved for specifying imaginary types.^footnote

^footnoteOne possible specification for imaginary types is Annex G.

• float _Imaginary
• double _Imaginary
• long double _Imaginary

[#3] The type specifier _Complex shall not be used if the implementation does not provide complex types.¹⁰¹

¹⁰¹Freestanding implementations are not required to provide complex types.

In 7.3.1 replace paragraphs 3 to 5 with:

[#3] The macro

I

expands to _Complex_I.¹⁶²

[#4] Notwithstanding the provisions of subclause 7.1.3, a program may undefine and perhaps then redefine the macros complex and I.

Add a new paragraph to the start of G.2:

[#0] There is a new keyword _Imaginary, which is used to specify imaginary types. It is used as a type-specifier within declaration-specifiers in the same way as _Complex is (thus "_Imaginary float" is a valid type name).

Replace G.6 paragraph 1 with:

[#1] The macro

imaginary

and

_Imaginary_I

are defined, respectively, as _Imaginary and a constant expression of type const float _Imaginary with the value of the imaginary unit. The macro

I

is defined to be _Imaginary_I (not _Complex_I as stated in 7.3). Notwithstanding the provisions of 7.1.3, a program may undefine and then perhaps redefine the macro imaginary.

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