NAME¶

std::search - std::search

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt1, class ForwardIt2 >

ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, (1) (constexpr since C++20)

ForwardIt2 s_first, ForwardIt2 s_last
);
template< class ExecutionPolicy, class ForwardIt1, class
ForwardIt2 >

ForwardIt1 search( ExecutionPolicy&& policy, (2) (since C++17)
ForwardIt1 first, ForwardIt1 last,

ForwardIt2 s_first, ForwardIt2 s_last
);
template< class ForwardIt1, class ForwardIt2, class
BinaryPred >

ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, (3) (constexpr since C++20)
ForwardIt2 s_first, ForwardIt2
s_last,

BinaryPred p );
template< class ExecutionPolicy,

class ForwardIt1, class ForwardIt2, class
BinaryPred >
ForwardIt1 search( ExecutionPolicy&& policy, (4) (since C++17)
ForwardIt1 first, ForwardIt1 last,
ForwardIt2 s_first, ForwardIt2
s_last,

BinaryPred p );
template< class ForwardIt, class Searcher >
(since C++17)
ForwardIt search( ForwardIt first, ForwardIt last, (5) (constexpr since C++20)

const Searcher& searcher );

1-4) Searches for the first occurrence of the sequence of elements [s_first, s_last)
in the range [first, last).
1) Elements are compared using operator==.
3) Elements are compared using the given binary predicate p.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
C++20)

5) Searches the range [first, last) for the pattern specified in the constructor of
searcher.

The standard library provides the following searchers:

default_searcher standard C++ library search algorithm
(C++17) implementation
(class template)
boyer_moore_searcher Boyer-Moore search algorithm (since C++17)
(C++17) implementation
(class template)
boyer_moore_horspool_searcher Boyer-Moore-Horspool search algorithm
(C++17) implementation
(class template)

Parameters¶

first, last - the range of elements to examine
s_first, s_last - the range of elements to search for
policy - the execution policy to use. See execution policy for details.
searcher - the searcher encapsulating the search algorithm and the pattern to
look for
binary predicate which returns true if the elements should be
treated as equal.

The signature of the predicate function should be equivalent to
the following:

bool pred(const Type1 &a, const Type2 &b);

p - While the signature does not need to have const &, the function
must not modify the objects passed to it and must be able to
accept all values of type (possibly const) Type1 and Type2
regardless of value category (thus, Type1 & is not allowed
, nor is Type1 unless for Type1 a move is equivalent to a copy
(since C++11)).
The types Type1 and Type2 must be such that objects of types
ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly
converted to Type1 and Type2 respectively.

Type requirements¶

-
ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.
-
BinaryPred must meet the requirements of BinaryPredicate.

Return value¶

1-4) Iterator to the beginning of first occurrence of the sequence [s_first, s_last)
in the range [first, last). If no such occurrence is found, last is returned.
If [s_first, s_last) is empty, first is returned.
5) searcher(first, last).first.

Complexity¶

1-4) Given \(\scriptsize N\)N as std::distance(first, last) and \(\scriptsize S\)S
as std::distance(s_first, s_last):
1,2) At most \(\scriptsize N\cdot S\)N·S comparisons using operator==.
3,4) At most \(\scriptsize N\cdot S\)N·S applications of the predicate p.
5) Depends on searcher.

Exceptions¶

The overloads with a template parameter named ExecutionPolicy report errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation¶

search (1)
template<class ForwardIt1, class ForwardIt2>
constexpr //< since C++20
ForwardIt1 search(ForwardIt1 first, ForwardIt1 last,
ForwardIt2 s_first, ForwardIt2 s_last)
{
while (true)
{
ForwardIt1 it = first;
for (ForwardIt2 s_it = s_first; ; ++it, ++s_it)
{
if (s_it == s_last)
return first;
if (it == last)
return last;
if (!(*it == *s_it))
break;
}
++first;
}
}
search (3)
template<class ForwardIt1, class ForwardIt2, class BinaryPred>
constexpr //< since C++20
ForwardIt1 search(ForwardIt1 first, ForwardIt1 last,
ForwardIt2 s_first, ForwardIt2 s_last, BinaryPred p)
{
while (true)
{
ForwardIt1 it = first;
for (ForwardIt2 s_it = s_first; ; ++it, ++s_it)
{
if (s_it == s_last)
return first;
if (it == last)
return last;
if (!p(*it, *s_it))
break;
}
++first;
}
}

Example¶

// Run this code

#include <algorithm>
#include <cassert>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string_view>
#include <vector>

using namespace std::literals;

bool contains(const auto& cont, std::string_view s)
{
// str.find() (or str.contains(), since C++23) can be used as well
return std::search(cont.begin(), cont.end(), s.begin(), s.end()) != cont.end();
}

int main()
{
const auto str{"why waste time learning, when ignorance is instantaneous?"sv};
assert(contains(str, "learning"));
assert(not contains(str, "lemming"));

const std::vector vec(str.begin(), str.end());
assert(contains(vec, "learning"));
assert(not contains(vec, "leaning"));

// The C++17 overload with searchers demo:
constexpr auto quote
{
"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed "
"do eiusmod tempor incididunt ut labore et dolore magna aliqua"sv
};

for (const auto word : {"pisci"sv, "Pisci"sv})
{
std::cout << "The string " << std::quoted(word) << ' ';
const std::boyer_moore_searcher searcher(word.begin(), word.end());
const auto it = std::search(quote.begin(), quote.end(), searcher);
if (it == quote.end())
std::cout << "not found\n";
else
std::cout << "found at offset " << std::distance(quote.begin(), it) << '\n';
}
}

Output:¶

The string "pisci" found at offset 43
The string "Pisci" not found

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 1205 C++98 the return value was unclear if returns first in this
[s_first, s_last) is empty case
the resolution of LWG issue 1205 was
LWG 1338 C++98 incorrectly applied, returns last in this
making first to be returned if no case
occurence is found
LWG 2150 C++98 the condition of “sequence occurence” was corrected
incorrect

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