**Submitter:** Fred Tydeman (USA)

**Submission Date:** 2004-08-26

**Source:** WG 14

**Reference Document:**

**Version:** 1.1

**Date:** 2006-03-05

**Subject:**Translation-time expresssion evaluation

**Summary**

The standard does not require translation-time expression
evaluation to produce "obvious" values. For example, there is
no requirement that ** static const double d = 1.23/4.56 -
1.23/4.56;** be zero.

**Details**

5.2.4.2.2p7 discusses ** FLT_EVAL_FORMAT**, range
and precision of floating-point constants and operations.

6.4.4.2p5 discusses translation time conversion of floating-point constants to internal format.

6.6p5 discusses translation time evaluation of floating-point constant expressions.

F.7.2p2 and footnote 306 discusses conversion of floating-point constants to internal representation.

F.7.5p3 discusses initialization and internal representation of floating-point constants.

Currently, if ** FLT_EVAL_METHOD** is negative,
there is no requirement that the first 1.23 and the second 1.23
convert to the same internal representation; one could have
more precision than the other. While the same range and
precision is a requirement when

Currently, if ** FLT_EVAL_METHOD** is negative,
there is no requirement that the first '/' produce a result
with the same precision as the second '/' (for the same
operands). This is a requirement when

Without these two requirements, the best one can do to try
to get zero is: ** static const double d =
(double)((double)1.23/(double)4.56) -
(double)((double)1.23/(double)4.56);** and even that
could fail if the first 1.23 and the second 1.23 convert to
different values.

For translation-time expression evaluation to produce the "obvious" value, two requirments must be met: the same decimal constants must result in the same value and internal representation, and operators must use the same precision.

**Suggested Technical Corrigendum**

6.4.4.2 Floating constants: Add to paragraph 5: All floating-constants of the same source form in the same translation unit shall convert to the same internal format with the same value. Footnote: 1.23, 1.230, 123e-2, 123e-02, 1.23L are all different source forms, so need not convert to the same internal format and value.

Add to 6.4.4.2p7, All floating-constants with the same mathematical value and type in the same translation unit should convert to the same internal format and value. Footnote: 1.23, 1.230, 123e-2 and 123e-02 have the same mathematical value, so should convert to the same internal format and value.

6.6: Constant expression: Add a 12th paragraph: All operators with the same type(s) of floating-point operand(s) in the same translation unit shall use the same precision.

**Committee Discussion**

**Committee Response**

This is not really a requirement, but an area to be investigated that could be addressed in a future revision of the C Standard.