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### 873. signed integral type and unsigned integral type are not clearly defined

Section: 6.8.2 [basic.fundamental] Status: NAD Editorial Submitter: Travis Vitek Opened: 2008-06-30 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Neither the term "signed integral type" nor the term "unsigned integral type" is defined in the core language section of the standard, therefore the library section should avoid its use. The terms signed integer type and unsigned integer type are indeed defined (in 6.8.2 [basic.fundamental]), thus the usages should be replaced accordingly.

Note that the key issue here is that "signed" + "integral type" != "signed integral type". The types `bool`, `char`, `char16_t`, `char32_t` and `wchar_t` are all listed as integral types, but are neither of signed integer type or unsigned integer type. According to 6.8 [basic.types] p7, a synonym for integral type is integer type. Given this, one may choose to assume that an integral type that can represent values less than zero is a signed integral type. Unfortunately this can cause ambiguities. As an example, if `T` is `unsigned char`, the expression `make_signed<T>::type`, is supposed to name a signed integral type. There are potentially two types that satisfy this requirement, namely `signed char` and `char` (assuming `CHAR_MIN < 0`).

[ San Francisco: ]

> Plum, Sebor to review.

[ Post Summit Daniel adds: ]

> The proposed resolution needs to be "conceptualized". Currently we have in [concept.support] only concept `IntegralType` for all "integral types", thus indeed the current `Container` concept and Iterator concepts are sufficiently satisfied with "integral types". If the changes are applied, we might ask core for concept `BilateralIntegerType` and add proper restrictions to the library concepts.

Proposed resolution:

I propose to use the terms "signed integer type" and "unsigned integer type" in place of "signed integral type" and "unsigned integral type" to eliminate such ambiguities.

The proposed change makes it absolutely clear that the difference between two pointers cannot be `char` or `wchar_t`, but could be any of the signed integer types. 7.6.6 [expr.add] paragraph 6...

> 1. When two pointers to elements of the same array object are subtracted, the result is the difference of the subscripts of the two array elements. The type of the result is an implementation-defined signed integral type signed integer type; this type shall be the same type that is defined as `std::ptrdiff_t` in the `<cstdint>` header (18.1)...

The proposed change makes it clear that `X::size_type` and `X::difference_type` cannot be `char` or `wchar_t`, but could be one of the signed or unsigned integer types as appropriate. 16.4.4.6 [allocator.requirements] table 40...

> Table 40: Allocator requirements

> expression | return type | assertion/note/pre/post-condition
> --- | --- | ---
> `X::size_type` | unsigned integral type unsigned integer type | a type that can represent the size of the largest object in the allocation model.
> `X::difference_type` | signed integral type signed integer type | a type that can represent the difference between any two pointers in the allocation model.

The proposed change makes it clear that `make_signed<T>::type` must be one of the signed integer types as defined in 3.9.1. Ditto for `make_unsigned<T>type` and unsigned integer types. 21.3.8.4 [meta.trans.sign] table 48...

> Table 48: Sign modifications

> Template | Comments
> --- | ---
> `template <class T> struct make_signed;` | If `T` names a (possibly cv-qualified) signed integral type signed integer type (3.9.1) then the member typedef `type` shall name the type `T`; otherwise, if `T` names a (possibly cv-qualified) unsigned integral type unsigned integer type then `type` shall name the corresponding signed integral type signed integer type, with the same cv-qualifiers as `T`; otherwise, `type` shall name the signed integral type signed integer type with the smallest rank (4.13) for which `sizeof(T) == sizeof(type)`, with the same cv-qualifiers as `T`. Requires: `T` shall be a (possibly cv-qualified) integral type or enumeration but not a `bool` type.
> `template <class T> struct make_unsigned;` | If `T` names a (possibly cv-qualified) unsigned integral type unsigned integer type (3.9.1) then the member typedef `type` shall name the type `T`; otherwise, if `T` names a (possibly cv-qualified) signed integral type signed integer type then `type` shall name the corresponding unsigned integral type unsigned integer type, with the same cv-qualifiers as `T`; otherwise, `type` shall name the unsigned integral type unsigned integer type with the smallest rank (4.13) for which `sizeof(T) == sizeof(type)`, with the same cv-qualifiers as `T`. Requires: `T` shall be a (possibly cv-qualified) integral type or enumeration but not a `bool` type.

Note: I believe that the basefield values should probably be prefixed with `ios_base::` as they are in 30.4.3.3.3 [facet.num.put.virtuals] The listed virtuals are all overloaded on signed and unsigned integer types, the new wording just maintains consistency. 30.4.3.2.3 [facet.num.get.virtuals] table 78...

> Table 78: Integer Conversions

> State | `stdio` equivalent
> --- | ---
> `basefield == oct` | `%o`
> `basefield == hex` | `%X`
> `basefield == 0` | `%i`
> signed integral type signed integer type | `%d`
> unsigned integral type unsigned integer type | `%u`

Rationale is same as above. 30.4.3.3.3 [facet.num.put.virtuals] table 80...

> Table 80: Integer Conversions

> State | `stdio` equivalent
> --- | ---
> `basefield == ios_base::oct` | `%o`
> `(basefield == ios_base::hex) && !uppercase` | `%x`
> `(basefield == ios_base::hex)` | `%X`
> `basefield == 0` | `%i`
> for a signed integral type signed integer type | `%d`
> for a unsigned integral type unsigned integer type | `%u`

24.2 [container.requirements] table 80...

> Table 89: Container requirements

> expression | return type | operational semantics | assertion/note/pre/post-condition | complexity
> --- | --- | --- | --- | ---
> `X::difference_type` | signed integral type signed integer type |  | is identical to the difference type of `X::iterator` and `X::const_iterator` | compile time
> `X::size_type` | unsigned integral type unsigned integer type |  | `size_type` can represent any non-negative value of `difference_type` | compile time

25.3.4 [iterator.concepts] paragraph 1...

> Iterators are a generalization of pointers that allow a C++ program to work with different data structures (containers) in a uniform manner. To be able to construct template algorithms that work correctly and efficiently on different types of data structures, the library formalizes not just the interfaces but also the semantics and complexity assumptions of iterators. All input iterators `i` support the expression `*i`, resulting in a value of some class, enumeration, or built-in type `T`, called the value type of the iterator. All output iterators support the expression `*i = o` where `o` is a value of some type that is in the set of types that are writable to the particular iterator type of `i`. All iterators `i` for which the expression `(*i).m` is well-defined, support the expression `i->m` with the same semantics as `(*i).m`. For every iterator type `X` for which equality is defined, there is a corresponding signed integral type signed integer type called the difference type of the iterator.

I'm a little unsure of this change. Previously this paragraph would allow instantiations of `linear_congruential_engine` on `char`, `wchar_t`, `bool`, and other types. The new wording prohibits this. 28.5.4.2 [rand.eng.lcong] paragraph 2...

> The template parameter `UIntType` shall denote an unsigned integral type unsigned integer type large enough to store values as large as `m - 1`. If the template parameter `m` is 0, the modulus `m` used throughout this section 26.4.3.1 is `numeric_limits<result_type>::max()` plus 1. [Note: The result need not be representable as a value of type `result_type`. --end note] Otherwise, the following relations shall hold: `a < m` and `c < m`.

Same rationale as the previous change. 99 [rand.adapt.xor] paragraph 6...

> Both `Engine1::result_type` and `Engine2::result_type` shall denote (possibly different) unsigned integral types unsigned integer types. The member result_type shall denote either the type Engine1::result_type or the type Engine2::result_type, whichever provides the most storage according to clause 3.9.1.

28.5.8.1 [rand.util.seedseq] paragraph 7...

> Requires:`RandomAccessIterator` shall meet the requirements of a random access iterator (24.1.5) such that `iterator_traits<RandomAccessIterator>::value_type` shall denote an unsigned integral type unsigned integer type capable of accomodating 32-bit quantities.

By making this change, integral types that happen to have a signed representation, but are not signed integer types, would no longer be required to use a two's complement representation. This may go against the original intent, and should be reviewed. 33.5.8.2 [atomics.types.operations] paragraph 24...

> Remark: For signed integral types signed integer types, arithmetic is defined using two's complement representation. There are no undefined results. For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.