std::experimental::ranges::equal

class Pred = ranges::equal_to <>,
class Proj1 = ranges::identity, class Proj2 = ranges::identity >
requires IndirectlyComparable<I1, I2, Pred, Proj1, Proj2>
bool equal( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = Pred{},

Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{} );

(1) (ranges TS)

template< InputRange R1, InputRange R2, class Pred = ranges::equal_to <>,

class Proj1 = ranges::identity, class Proj2 = ranges::identity >
requires IndirectlyComparable<ranges::iterator_t <R1>, ranges::iterator_t <R2>,
Pred, Proj1, Proj2>
bool equal( R1&& r1, R2&& r2, Pred pred = Pred{},

Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{} );

(2) (ranges TS)

template< InputIterator I1, Sentinel<I1> S1, class I2,

class Pred = ranges::equal_to <>,
class Proj1 = ranges::identity, class Proj2 = ranges::identity >
requires InputIterator<std::decay_t <I2>> && !Range<I2> &&
IndirectlyComparable<I1, std::decay_t <I2>, Pred, Proj1, Proj2>
bool equal( I1 first1, S1 last1, I2&& first2_, Pred pred = Pred{},

Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{} );

(3) (ranges TS)
(deprecated)

template< InputRange R1, class I2, class Pred = ranges::equal_to <>,

class Proj1 = ranges::identity, class Proj2 = ranges::identity >
requires InputIterator<std::decay_t <I2>> && !Range<I2> &&
IndirectlyComparable<ranges::iterator_t <R1>, std::decay_t <I2>, Pred, Proj1, Proj2>
bool equal( R1&& r1, I2&& first2_, Pred pred = Pred{},

Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{} );

(4) (ranges TS)
(deprecated)

1) Returns true if the range [first1, last1) is equal to the range [first2, last2), and false otherwise.

2) Same as (1), but uses r1 as the first source range and r2 as the second source range, as if using ranges::begin (r1) as first1, ranges::end (r1) as last1, ranges::begin (r2) as first2, and ranges::end (r2) as last2.

3) Same as (1), except that the second range is considered to end when either the first range is exhausted or the first mismatch is detected. Equivalent to return last1 == ranges::mismatch (first1, last1, std::forward <I2>(first2_), comp, proj1, proj2).in1();

4) Same as (3), but uses r1 as the first source range, as if using ranges::begin (r1) as first1 and ranges::end (r1) as last1.

Two ranges are considered equal if they have the same number of elements and, for every iterator i in the range [first1, last1), ranges::invoke (pred, ranges::invoke (proj1, *i), ranges::invoke (proj2, *(first2 + (i - first1)))) is true.

Notwithstanding the declarations depicted above, the actual number and order of template parameters for algorithm declarations is unspecified. Thus, if explicit template arguments are used when calling an algorithm, the program is probably non-portable.

ranges::equal should not be used to compare the ranges formed by the iterators from std::unordered_set , std::unordered_multiset , std::unordered_map , or std::unordered_multimap because the order in which the elements are stored in those containers may be different even if the two containers store the same elements.

When comparing entire containers for equality, operator== for the corresponding container are usually preferred.

[edit] Complexity

1,2) If SizedSentinel<S1, I1> && SizedSentinel<S2, I2> is satisfied and last1 - first1 != last2 - first2, no applications of the predicate and projections. Otherwise, at most min(last1 - first1, last2 - first2) applications of the predicate and each projection.

3,4) At most last1 - first1 applications of the predicate and each projection.

[edit] Possible implementation

namespace detail 
{
 template<InputIterator I1, SizedSentinel<I1> S1,
 InputIterator I2, SizedSentinel<I1> S2>
 bool check_size(I1& first1, S1& last1, I2& first2, S2& last2)
 {
 return last1 - first1 != last2 - first2;
 }
 
 template<InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I1> S2>
 bool check_size(I1& first1, S1& last1, I2& first2, S2& last2)
 {
 return false;
 }
}
 
template<InputIterator I1, Sentinel<I1> S1, InputIterator I2, Sentinel<I2> S2,
 class Pred = ranges::equal_to <>, 
 class Proj1 = ranges::identity, class Proj2 = ranges::identity >
 requires IndirectlyComparable<I1, I2, Pred, Proj1, Proj2>
bool equal(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = Pred{},
 Proj1 proj1 = Proj1{}, Proj2 proj2 = Proj2{}) 
{
 if (detail::check_size(first1, last1, first2, last2))
 return false;
 for (; first1 != last1 && first2 != last2; (void) ++first1, (void)++first2)
 if (!ranges::invoke (pred, ranges::invoke (proj1, *first1), 
 ranges::invoke (proj2, *first2)))
 return false;
 return first1 == last1 && first2 == last2;
}