std::ranges::prev

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Iterator library

Iterator concepts

indirectly_readable (C++20)
indirectly_writable (C++20)
weakly_incrementable (C++20)
incrementable (C++20)
is-integer-like is-signed-integer-like (C++20)(C++20)

sentinel_for (C++20)
sized_sentinel_for (C++20)
input_iterator (C++20)
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forward_iterator (C++20)
bidirectional_iterator (C++20)
random_access_iterator (C++20)
contiguous_iterator (C++20)

Iterator primitives

input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag

(C++20)

iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t

(C++20)(C++20)(C++20)(C++23)(C++20)(C++20)

iterator (deprecated in C++17)
iterator_traits
incrementable_traits (C++20)
indirectly_readable_traits (C++20)

Algorithm concepts and utilities

Indirect callable concepts

indirectly_unary_invocableindirectly_regular_unary_invocable

(C++20)(C++20)

indirect_unary_predicate (C++20)
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indirect_equivalence_relation (C++20)
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Common algorithm requirements

indirectly_movable (C++20)
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permutable (C++20)
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(C++20)

(C++20)

(C++26)

reverse_iterator
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move_iterator (C++11)
make_move_iterator (C++11)
default_sentinel_tdefault_sentinel (C++20)(C++20)
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front_insert_iterator
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common_iterator (C++20)
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ranges::iter_move (C++20)
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Iterator operations

advance
distance
prev (C++11)
next (C++11)

ranges::advance (C++20)
ranges::distance (C++20)
ranges::prev (C++20)
ranges::next (C++20)

Range access

begincbegin (C++11)(C++14)
rbegincrbegin (C++14)(C++14)

endcend (C++11)(C++14)
rendcrend (C++14)(C++14)

sizessize (C++17)(C++20)
empty (C++17)
data (C++17)

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Defined in header`<iterator>`
Call signature
template<std::bidirectional_iterator I> constexpr I prev( I i);	(1)	(since C++20)
template<std::bidirectional_iterator I> constexpr I prev( I i,std::iter_difference_t<I> n);	(2)	(since C++20)
template<std::bidirectional_iterator I> constexpr I prev( I i,std::iter_difference_t<I> n, I bound);	(3)	(since C++20)

Return then^th predecessor of iteratori.

The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible toargument-dependent lookup.
When any of them are found bynormal unqualified lookup as the name to the left of the function-call operator,argument-dependent lookup is inhibited.

[edit]Parameters

i	-	an iterator
n	-	number of elementsi should be descended
bound	-	iterator denoting the beginning of the rangei points to

[edit]Return value

1) The predecessor ofi.

2) Then^th predecessor of iteratori.

3) Then^th predecessor of iteratori, or the first iterator that compares equal tobound, whichever is first.

[edit]Complexity

1) Constant.

2,3) Constant ifI modelsstd::random_access_iterator<I>; otherwise linear.

[edit]Possible implementation

struct prev_fn{template<std::bidirectional_iterator I>constexpr I operator()(I i)const{--i;return i;} template<std::bidirectional_iterator I>constexpr I operator()(I i,std::iter_difference_t<I> n)const{ranges::advance(i,-n);return i;} template<std::bidirectional_iterator I>constexpr I operator()(I i,std::iter_difference_t<I> n, I bound)const{ranges::advance(i,-n, bound);return i;}}; inlineconstexprauto prev= prev_fn();

[edit]Notes

Although the expression--r.end() often compiles for containers, it is not guaranteed to do so:r.end() is an rvalue expression, and there is no iterator requirement that specifies that decrement of an rvalue is guaranteed to work. In particular, when iterators are implemented as pointers or itsoperator-- is lvalue-ref-qualified,--r.end() does not compile, whileranges::prev(r.end()) does.

This is further exacerbated by ranges that do not modelranges::common_range. For example, for some underlying ranges,ranges::transform_view::end doesn't have the same return type asranges::transform_view::begin, and so--r.end() won't compile. This isn't something thatranges::prev can aid with, but there are workarounds.

[edit]Example

Run this code

#include <iostream>#include <iterator>#include <vector> int main(){std::vector<int> v{3,1,4};auto pv= std::ranges::prev(v.end(),2);std::cout<<*pv<<'\n';     pv= std::ranges::prev(pv,42, v.begin());std::cout<<*pv<<'\n';}

Output:

[edit]See also

ranges::next (C++20)	increment an iterator by a given distance or to a bound (algorithm function object)[edit]
ranges::advance (C++20)	advances an iterator by given distance or to a given bound (algorithm function object)[edit]
prev (C++11)	decrement an iterator (function template)[edit]