The STL brought the notion of a range to C++, expressed as a
pair of iterators. C++11 added the range-based for loop, which iterates
over a single object for which begin(x)
and end(x)
return that pair of iterators. The Boost.Range library extends this
to a full library of algorithms based on ranges as single objects. We'd like
to be able to experiment with such a library in a series of Technical
Specifications between now and C++17, but the LWG preference is that TSes
shouldn't change the definitions of any existing types, so we need to add a
minimal amount of range-object support to the C++14 standard library so that
range TSes can interoperate. This paper attempts to add that support.
I drew inspiration from two places in adding this support. First, the range-based for loop ([stmt.ranged]) defines the minimal interface for a range object:
begin(range)
and end(range)
, with
std::begin
and std::end
in the candidate set,
return types that can initialize variables in the same
auto
-typed declaration. (Note that [stmt.ranged] specifies
individual cases for arrays, objects with .begin()
and
.end()
members, and objects for which begin()
and end()
can be found via ADL, but std::begin()
and std::end()
include code for arrays and objects with
.begin()
and .end()
members, so this
library-oriented proposal simply relies on them.)begin(range)
and
end(range)
supports operator*
,
operator++
, and operator!=
in the pattern
defined by Input Iterators. This proposal slightly strengthens that into
a requirement that begin(range)
and end(range)
actually return an Input Iterator type, although the
enable_if
logic for implementations is only required to check
for the presence of operator*
.Second, many container methods have an overload taking an
initializer_list<value_type>
argument. This proposal takes
that as a good indication of the methods that can usefully take a range
argument and adds such an overload parallel to each one of those. This is
the same as the set of methods taking a templated Iterator pair except for
one priority_queue
constructor.
Range
template argumentThere are many sorts of range types, so container methods taking ranges either have to be templated, or we'd need to define a single range type with a templated converting constructor. I proposed such a type in N3350, but the exact set of methods that the type needs is somewhat contentious, so this paper proposes templating the methods instead.
A templated method could either take a const Range&
or a
Range&&
(where Range
is a template
argument). Both of these can capture arguments that should implicitly
convert to the argument types of another overload of the same method, so we
need some enable_if
logic for both. const
Range&
would naturally leave Container&
arguments for the const Container&
overload, but it would
incorrectly capture DerivedFromContainer
arguments, just like
Range&&
would. Range&&
lets us
allow library authors to move elements from rvalue arguments. Because the
necessary enable_if
logic seems similar in both cases, I chose
to take Range&&
.
The enable_if
logic is
specified as:
Several functions in the standard library take a template argument named "
Range
". These functions shall not participate in overload resolution ifRange
is not deduced to a range type. Further, if the function is a container or string’s constructor or operator=, anddecay<Range>::type
is the type of the container or a type derived from the container type, the function shall not participate in overload resolution.Additionally, implementations may exclude these functions from overload resolution if Range's iterator type is not an Input Iterator type ([input.iterators]) or its value type is not convertible to the container’s value type, but the extent to which they do so is unspecified.
Even with this text, range types that define an implicit conversion to the container type with a non-default allocator, comparator, or hash instance are going to have strange behavior when a conversion is requested. With current language rules, it appears that copy-initialization will call the conversion operator, but direct-initialization will call the templated range constructor, losing any custom allocator, comparator, or hash instance the conversion operator attempts to set. It's possible to work around this by explicitly passing them to the range constructor, but it's unlikely users will know to do so. I believe such types are rare enough that this surprise is acceptable.
The proposed text also says that ranges passed as rvalues are "left in an unspecified state after the call." When a range is just a reference to objects owned elsewhere, this text doesn't allow moving those objects, since that leaves more than just the range in an unspecified state. However, if the implementation can detect that the range owns the objects it iterates over, this wording allows those objects to be moved. I leave the technique for detecting this as a QoI issue.
Boost has a fairly extensive collection of range-based algorithms, but they can't quite interoperate perfectly with standard containers because the containers are missing appropriate constructors. This paper allows the following code (adapted from the Boost.Range docs) to work:
#include <boost/range/adaptor/replaced.hpp>
#include <boost/range/adaptor/reversed.hpp>
#include <boost/range/algorithm/copy.hpp>
#include <deque>
#include <iostream>
#include <vector>
int main() {
using namespace boost::adaptors;
std::deque<int> input{1,2,3,2,5,2,7,2,9};
std::vector<int> output{
input | replaced(2, 10) | reversed};
boost::copy(output, std::ostream_iterator<int>(std::cout, ","));
}
This prints "9,10,7,10,5,10,3,10,1,".
You'll note that this paper doesn't propose any new algorithm overloads
taking ranges, so the above example still needs to call
boost::copy
instead of std::copy
. That's because a
TS can add new functions in its own namespace, so we can go through several
revisions getting them exactly right, rather than needing to debate a whole
algorithms library for C++14.
The primary discomfort the LWG had with the split()
proposal
was that its implicit conversion operator to any container type was just a
hack around the lack of range support (Portland
discussion). This paper delivers enough range support to remove
split()
's conversion operator.
vector<string> v{std::split("a,b,c", ",")};
deque<string> d{std::split("a,b,c", ",")};
set<string> s{std::split("a,b,c", ",")};
list<string> l{std::split("a,b,c", ",")};
vector<string_ref> v2{std::split("a,b,c", ",")}; // No data copied.
assert(v.size() == 3); // "a", "b", "c"
Conversion to either string
or string_ref
is
accomplished by having split()
's result's iterator return proxy
objects that are implicitly convertible to either type. The enable_if
logic specifically allows
implicit conversions to the container's value_type
so that this
works.
This paper updates N3456 by moving the range requirements out of [containers] and into the library's introduction ([utility.requirements]).
The most recent version of this paper is maintained on GitHub.
Wording changes are being maintained at https://github.com/google/cxx-std-draft/compare/range-args and a snapshot of the changes, relative to N3485, is copied below. An early implementation is at https://github.com/google/libcxx/compare/range-args. Patches and pull requests are welcome against both.
As an editorial matter, several of these new functions could be specified more concisely using [structure.specifications]'s "equivalent to" wording. They're specified more verbosely in order to match the existing functions around them.
[utility.arg.requirements] describes requirements on types and expressions used to instantiate templates defined in the C++ standard library. [swappable.requirements] describes the requirements on swappable types and swappable expressions. [nullablepointer.requirements] describes the requirements on pointer-like types that support null values. [hash.requirements] describes the requirements on hash function objects. [allocator.requirements] describes the requirements on storage allocators. [range.requirements] describes the requirements on range types.
Objects of Range
type (Range
objects) refer to a
sequence of other objects using a pair of iterators accessed by
begin()
and end()
functions. Range
objects may or may not contain and own these other objects.
This subclause provides definitions for Range
types and
expressions. In this subclause, r
is an instance of a
Range
type R
. i
is an instance of
I
, known as R
's iterator type. V
is
R
's and I
's value type.
[Note: These requirements are intended to match the requirements on
_RangeT
in the range-based for loop ([stmt.ranged]). —endnote]
R is a Range
type if it satisfies the requirements in Table 29
when the expressions are evaluated in the context described below.
Expression | Return type |
---|---|
begin(r) | I |
end(r) | I |
*i | implicitly convertible to V |
The context in which begin(r)
and end(r)
are
evaluated shall ensure that a unary non-member function named "begin" or "end"
respectively is selected via overload resolution ([over.match]) on a candidate
set that includes:
[Note: If R is an array of fundamental type and the declarations from the header <iterator> are in scope, the overall lookup set described above is equivalent to that of the qualified name lookup applied to the expression std::begin(r) or std::end(r) as appropriate. — end note ]
[ Note: It is unspecified whether a library component that has a
Range
requirement includes the header <iterator> to ensure an
appropriate evaluation context. — end note ]
Several functions in the standard library take a template argument named
"Range
". These functions shall not participate in overload
resolution if Range
is not deduced to a Range
type. Further, if the function is a container or string’s constructor or
operator=
, and decay<Range>::type
is the type of
the container or a type derived from the container type, the function shall not
participate in overload resolution.
Additionally, implementations may exclude these functions from overload
resolution if Range
's iterator type is not an Input Iterator type
([input.iterators]) or its value type is not convertible to the container's
value type, but the extent to which they do so is unspecified.
If the Range
is passed as an rvalue, it
is left in an unspecified state after the call. [Footnote: This allows
implementations to detect arguments that are containers and move, instead of
copying, their contents. -- end footnote]
In calls to functions taking template arguments named Range
, if
Range
's iterator type does not satisfy at least the Input Iterator
requirements ([input.iterators]), the program is ill-formed, no diagnostic
required. If a Range object r
is passed such that
[begin(r),end(r))
is not a valid range
([iterator.requirements.general]), the behavior is undefined.
template<class Range>
explicit basic_string(Range&&, const Allocator& = Allocator());
template<class Range>
basic_string& operator=(Range&&);
template<class Range>
basic_string& operator+=(Range&&);
template<class Range>
basic_string& append(Range&&);
template<class Range>
basic_string& assign(Range&&);
template<class Range>
iterator insert(const_iterator p, Range&&);
template<class Range>
basic_string& replace(const_iterator, const_iterator, Range&&);
template<typename Range>
explicit basic_string(Range&& range, const Allocator& a = Allocator());
Effects: Same as basic_string(begin(range), end(range), a).
template<typename Range>
basic_string& operator=(Range&& range);
Effects: *this = basic_string(std::forward<Range>(range)).
Returns: *this.
template<class Range>
basic_string& operator+=(Range&& range);
Effects: Calls append(std::forward<Range>(range)).
Returns: *this.
template<class Range>
basic_string& append(Range&& range);
Effects: Calls append(begin(range), end(range)).
Returns: *this.
template<class Range>
basic_string& assign(Range&& range);
Effects: Calls assign(begin(range), end(range)).
Returns: *this.
template<class Range>
iterator insert(const_iterator p, Range&& range);
Effects: insert(p, begin(range), end(range)).
Returns: An iterator which refers to the copy of the first inserted
character, or p if [begin(range),end(range))
is an empty
range.
template<class Range>
basic_string& replace(const_iterator i1, const_iterator i2,
Range&& range);
Requires: [begin(),i1), [i1,i2), and [begin(range),end(range)) are valid ranges.
Effects: Calls replace(i1 - begin(), i2 - i1, begin(range), end(range)).
Returns: *this.
In Tables 101 and 102, X denotes a sequence container class, a denotes a
value of X containing elements of type T, A denotes X::allocator_type if it
exists and std::allocator<T> if it doesn’t, i and j denote iterators
satisfying input iterator requirements and refer to elements implicitly
convertible to value_type, [i, j) denotes a valid range, r denotes a
Range
object ([range.requirements.implementations]) whose value
type is implicitly convertible to value_type and for which [begin(r),end(r)) is
a valid range ([iterator.requirements.general]), il designates an object
of type
initializer_list<value_type>, n denotes a value of X::size_type, p denotes a
valid const iterator to a, q denotes a valid dereferenceable const iterator to
a, [q1, q2) denotes a valid range of const iterators in a, t denotes an lvalue
or a const rvalue of X::value_- type, and rv denotes a non-const rvalue of
X::value_type. Args denotes a template parameter pack; args denotes a function
parameter pack with the pattern Args&&.
Expression | Return type | Assertion/note pre-/post-condition |
---|---|---|
X(r); | Equivalent to X(begin(r), end(r)) | |
a = r; | X& | Requires: T is CopyInsertable into X and CopyAssignable. Assigns the range [begin(r),end(r)) into a. All existing elements of a are either assigned to or destroyed. Returns: *this. |
a.insert(p, r); | iterator | a.insert(p, begin(r), end(r)). |
a.assign(r) | void | a.assign(begin(r), end(r)). |
In Table 103, X denotes an associative container class, a denotes a value of
X, a_uniq denotes a value of X when X supports unique keys, a_eq denotes a value
of X when X supports multiple keys, u denotes an identifier, i and j satisfy
input iterator requirements and refer to elements implicitly convertible to
value_type, [i,j) denotes a valid range, p denotes a valid const iterator to a,
q denotes a valid dereferenceable const iterator to a, [q1, q2) denotes a valid
range of const iterators in a, r denotes a Range
object
([range.requirements.implementations]) whose value type is implicitly
convertible to value_type and for which [begin(r),end(r)) is a valid range
([iterator.requirements.general]), il designates an object of type
initializer_list<value_type>, t denotes a value of X::value_type, k denotes a
value of X::key_type and c denotes a value of type X::key_compare. A denotes the
storage allocator used by X, if any, or std::allocator<X::value_type>
otherwise, and m denotes an allocator of a type convertible to A.
Expression | Return type | Assertion/note pre-/post-condition | Complexity |
---|---|---|---|
X(r); | Same as X(begin(r), end(r)). | same as X(begin(r), end(r)). | |
a = r | X& | Requires: value_type is CopyInsertable into X and CopyAssignable. Effects: Assigns the range [begin(r),end(r)) into a. All existing elements of a are either assigned to or destroyed. | NlogN in general (where N has the value distance(begin(r), end(r)) + a.size()); linear if [begin(r),end(r)) is sorted with value_comp(). |
a.insert(r) | void | Equivalent to a.insert(begin(r), end(r)). |
In table 104: X is an unordered associative container class, a is an object
of type X, b is a possibly const object of type X, a_uniq is an object of type X
when X supports unique keys, a_eq is an object of type X when X supports
equivalent keys, i and j are input iterators that refer to value_type, [i, j) is
a valid range, p and q2 are valid const iterators to a, q and q1 are valid
dereferenceable const iterators to a, [q1, q2) is a valid range in a, r
denotes a Range
object ([range.requirements.implementations]) whose
value type is implicitly convertible to value_type and for which
[begin(r),end(r)) is a valid range ([iterator.requirements.general]), il
designates an object of type
initializer_list<value_type>, t is a value of type X::value_type, k is a
value of type key_type, hf is a possibly const value of type hasher, eq is a
possibly const value of type key_equal, n is a value of type size_type, and z is
a value of type float.
Expression | Return type | Assertion/note pre-/post-condition | Complexity |
---|---|---|---|
X(r) | X | Same as X(begin(r), end(r)). | Same as X(begin(r), end(r)). |
a = r | X& | Requires: value_type is CopyInsertable into X and CopyAssignable. Effects: Assigns the range [begin(r),end(r)) into a. All existing elements of a are either assigned to or destroyed. | Same as a = X(r). |
a.insert(r) | void | Same as a.insert(begin(r), end(r)). | Same as a.insert( begin(r), end(r)). |
template <class Range>
explicit deque(Range&&, const Allocator& = Allocator());
template <class Range>
deque& operator=(Range&&);
template <class Range>
void assign(Range&&);
template <class Range>
iterator insert(const_iterator position, Range&&);
template <class Range>
iterator insert(const_iterator position, Range&&);
template <class Range>
explicit forward_list(Range&&, const Allocator& = Allocator());
template <class Range>
forward_list& operator=(Range&&);
template <class Range>
void assign(Range&&);
template <class Range>
iterator insert_after(const_iterator position, Range&& range);
template <class Range>
iterator insert_after(const_iterator position, Range&& range);
Effects: insert_after(p, begin(range), end(range)).
Returns: An iterator pointing to the last inserted element or
position
if [begin(range),end(range))
is an empty
range.
template <class Range>
explicit list(Range&&, const Allocator& = Allocator());
template <class Range>
list& operator=(Range&&);
template <class Range>
void assign(Range&&);
template <class Range>
iterator insert(const_iterator position, Range&& range);
template <class Range>
iterator insert(const_iterator position, Range&&);
template <class Range>
explicit vector(Range&&, const Allocator& = Allocator());
template <class Range>
vector& operator=(Range&&);
template <class Range>
void assign(Range&&);
template <class Range>
iterator insert(const_iterator position, Range&& range);
template <class Range>
iterator insert(const_iterator position, Range&&);
template <class Range>
vector(Range&&, const Allocator& = Allocator()));
template <class Range>
vector operator=(Range&&);
template <class Range>
void assign(Range&&);
template <class Range>
iterator insert(const_iterator position, Range&& range);
template <class Range>
explicit map(Range&&,
const Compare& = Compare(),
const Allocator& = Allocator());
template <class Range>
map& operator=(Range&&);
template <class Range>
void insert(Range&&);
template <class Range>
explicit multimap(Range&&,
const Compare& = Compare(),
const Allocator& = Allocator());
template <class Range>
multimap& operator=(Range&&);
template <class Range> void insert(Range&&);
template <class Range>
explicit set(Range&&,
const Compare& = Compare(),
const Allocator& = Allocator());
template <class Range>
set& operator=(Range&&);
template <class Range>
void insert(Range&&);
template <class Range>
explicit multiset(Range&&,
const Compare& = Compare(),
const Allocator& = Allocator());
template <class Range>
multiset& operator=(Range&&);
template <class Range>
void insert(Range&&);
template <class Range>
explicit unordered_map(Range&&,
size_type = see below,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
template <class Range>
unordered_map& operator=(Range&&);
template <class Range> void insert(Range&&);
template <class Range>
explicit unordered_multimap(Range&&,
size_type = see below,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
template <class Range>
unordered_multimap& operator=(Range&&);
template <class Range> void insert(Range&&);
template <class Range>
explicit unordered_set(Range&&,
size_type = see below,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
template <class Range>
unordered_set& operator=(Range&&);
template <class Range> void insert(Range&&);
template <class Range>
explicit unordered_multiset(Range&&,
size_type = see below,
const hasher& hf = hasher(),
const key_equal& eql = key_equal(),
const allocator_type& a = allocator_type());
template <class Range>
unordered_multiset& operator=(Range&&);
template <class Range> void insert(Range&&);
I'd like to thank Daniel Krügler for help with the wording in this paper, although any remaining mistakes are mine.