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Section: 17.3.5.3 [numeric.special] Status: CD1 Submitter: Gabriel Dos Reis Opened: 1999-07-21 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
bools are defined by the standard to be of integer types, as per 6.8.2 [basic.fundamental] paragraph 7. However "integer types" seems to have a special meaning for the author of 18.2. The net effect is an unclear and confusing specification for numeric_limits<bool> as evidenced below.
18.2.1.2/7 says numeric_limits<>::digits is, for built-in integer types, the number of non-sign bits in the representation.
4.5/4 states that a bool promotes to int ; whereas 4.12/1 says any non zero arithmetical value converts to true.
I don't think it makes sense at all to require numeric_limits<bool>::digits and numeric_limits<bool>::digits10 to be meaningful.
The standard defines what constitutes a signed (resp. unsigned) integer types. It doesn't categorize bool as being signed or unsigned. And the set of values of bool type has only two elements.
I don't think it makes sense to require numeric_limits<bool>::is_signed to be meaningful.
18.2.1.2/18 for numeric_limits<integer_type>::radix says:
For integer types, specifies the base of the representation.186)
This disposition is at best misleading and confusing for the standard requires a "pure binary numeration system" for integer types as per 3.9.1/7
The footnote 186) says: "Distinguishes types with base other than 2 (e.g BCD)." This also erroneous as the standard never defines any integer types with base representation other than 2.
Furthermore, numeric_limits<bool>::is_modulo and numeric_limits<bool>::is_signed have similar problems.
Proposed resolution:
Append to the end of 17.3.5.3 [numeric.special]:
The specialization for bool shall be provided as follows:
namespace std { template<> class numeric_limits<bool> { public: static const bool is_specialized = true; static bool min() throw() { return false; } static bool max() throw() { return true; } static const int digits = 1; static const int digits10 = 0; static const bool is_signed = false; static const bool is_integer = true; static const bool is_exact = true; static const int radix = 2; static bool epsilon() throw() { return 0; } static bool round_error() throw() { return 0; } static const int min_exponent = 0; static const int min_exponent10 = 0; static const int max_exponent = 0; static const int max_exponent10 = 0; static const bool has_infinity = false; static const bool has_quiet_NaN = false; static const bool has_signaling_NaN = false; static const float_denorm_style has_denorm = denorm_absent; static const bool has_denorm_loss = false; static bool infinity() throw() { return 0; } static bool quiet_NaN() throw() { return 0; } static bool signaling_NaN() throw() { return 0; } static bool denorm_min() throw() { return 0; } static const bool is_iec559 = false; static const bool is_bounded = true; static const bool is_modulo = false; static const bool traps = false; static const bool tinyness_before = false; static const float_round_style round_style = round_toward_zero; }; }
[Tokyo: The LWG desires wording that specifies exact values rather than more general wording in the original proposed resolution.]
[Post-Tokyo: At the request of the LWG in Tokyo, Nico Josuttis provided the above wording.]