[iterator.concept.winc] (original) (raw)

24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.4 Iterator concepts [iterator.concepts]

24.3.4.4 Concept weakly_incrementable [iterator.concept.winc]

The weakly_incrementable concept specifies the requirements on types that can be incremented with the pre- and post-increment operators.

The increment operations are not required to be equality-preserving, nor is the type required to be equality_comparable.

template<class T> constexpr bool is-integer-like = see below; template<class T> constexpr bool is-signed-integer-like = see below; template<class I> concept weakly_incrementable = movable<I> && requires(I i) { typename iter_difference_t<I>;requires is-signed-integer-like<iter_difference_t<I>>;{ ++i } -> same_as<I&>; i++; };

A type I is an integer-class typeif it is in a set of implementation-defined types that behave as integer types do, as defined below.

[Note 1:

An integer-class type is not necessarily a class type.

— _end note_]

The range of representable values of an integer-class type is the continuous set of values over which it is defined.

For any integer-class type, its range of representable values is either to (inclusive) for some integer N, in which case it is a signed-integer-class type, or0 to (inclusive) for some integer N, in which case it is an unsigned-integer-class type.

In both cases, N is called the width of the integer-class type.

The width of an integer-class type is greater than that of every integral type of the same signedness.

A type I other than cv bool is integer-likeif it models integral<I> or if it is an integer-class type.

An integer-like type I is signed-integer-likeif it models signed_integral<I> or if it is a signed-integer-class type.

An integer-like type I is unsigned-integer-likeif it models unsigned_integral<I> or if it is an unsigned-integer-class type.

For every integer-class type I, let B(I) be a unique hypothetical extended integer type of the same signedness with the same width ([basic.fundamental]) as I.

[Note 2:

The corresponding hypothetical specialization numeric_limits<B(I)>meets the requirements on numeric_limits specializations for integral types ([numeric.limits]).

— _end note_]

For every integral type J, let B(J) be the same type as J.

Expressions of integer-class type are explicitly convertible to any integer-like type, and implicitly convertible to any integer-class type of equal or greater width and the same signedness.

Expressions of integral type are both implicitly and explicitly convertible to any integer-class type.

Conversions between integral and integer-class types and between two integer-class types do not exit via an exception.

The result of such a conversion is the unique value of the destination type that is congruent to the source modulo , where N is the width of the destination type.

Let a be an object of integer-class type I, let b be an object of integer-like type I2such that the expression b is implicitly convertible to I, let x and y be, respectively, objects of type B(I) and B(I2) as described above that represent the same values as a and b, and let c be an lvalue of any integral type.

• The expressions a++ and a-- shall be prvalues of type Iwhose values are equal to that of a prior to the evaluation of the expressions.
  The expression a++ shall modify the value of aby adding 1 to it.
  The expression a-- shall modify the value of aby subtracting 1 from it.
• The expressions ++a, --a, and &a shall be expression-equivalent toa += 1, a -= 1, and addressof(a), respectively.
• For every unary-operator @ other than & for which the expression @x is well-formed, @a shall also be well-formed and have the same value, effects, and value category as @x.
  If @x has type bool, so too does @a; if @x has type B(I), then @a has type I.
• For every assignment operator @= for which c @= x is well-formed,c @= a shall also be well-formed and shall have the same value and effects as c @= x.
  The expression c @= a shall be an lvalue referring to c.
• For every assignment operator @=for which x @= y is well-formed,a @= b shall also be well-formed and shall have the same effects as x @= y, except that the value that would be stored into xis stored into a.
  The expression a @= b shall be an lvalue referring to a.
• For every non-assignment binary operator @ for which x @ y and y @ x are well-formed,a @ b and b @ a shall also be well-formed and shall have the same value, effects, and value category asx @ y and y @ x, respectively.
  If x @ y or y @ x has type B(I), then a @ b or b @ a, respectively, has type I; if x @ y or y @ x has type B(I2), then a @ b or b @ a, respectively, has type I2; if x @ y or y @ x has any other type, then a @ b or b @ a, respectively, has that type.

An expression E of integer-class type I is contextually convertible to boolas if by bool(E != I(0)).

A value-initialized object of integer-class type has value 0.

For every (possibly cv-qualified) integer-class type I,numeric_limits<I> is specialized such that each static data member mhas the same value as numeric_limits<B(I)>​::​m, and each static member function freturns I(numeric_limits<B(I)>​::​f()).

For any two integer-like types I1 and I2, at least one of which is an integer-class type,common_type_t<I1, I2> denotes an integer-class type whose width is not less than that of I1 or I2.

If both I1 and I2 are signed-integer-like types, then common_type_t<I1, I2> is also a signed-integer-like type.

is-integer-like<I> is trueif and only if I is an integer-like type.

is-signed-integer-like<I> is trueif and only if I is a signed-integer-like type.

Let i be an object of type I.

When i is in the domain of both pre- and post-increment, i is said to be incrementable.

I models weakly_incrementable<I> only if:

• The expressions ++i and i++ have the same domain.
• If i is incrementable, then both ++i and i++ advance i to the next element.
• If i is incrementable, thenaddressof(++i) is equal toaddressof(i).

Recommended practice: The implementation of an algorithm on a weakly incrementable type should never attempt to pass through the same incrementable value twice; such an algorithm should be a single-pass algorithm.

[Note 3:

For weakly_incrementable types, a equals b does not imply that ++aequals ++b.

(Equality does not guarantee the substitution property or referential transparency.)

Such algorithms can be used with istreams as the source of the input data through the istream_iterator class template.

— _end note_]