# std::search

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt1, class ForwardIt2 > ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt1 search( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last ); | (2) | (since C++17)
template< class ForwardIt1, class ForwardIt2, class BinaryPred > ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred > ForwardIt1 search( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPred p ); | (4) | (since C++17)
template< class ForwardIt, class Searcher > ForwardIt search( ForwardIt first, ForwardIt last, const Searcher& searcher ); | (5) | (since C++17) (constexpr since C++20)

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 all following conditions are satisfied:

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

(since C++17)

### 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
searcher | - | the searcher encapsulating the search algorithm and the pattern to look for

p

-

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);

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 `[`s_first`,` s_last`)` is empty | returns first in this case
LWG 1338 | C++98 | the resolution of LWG issue 1205 was incorrectly applied, making first to be returned if no occurence is found | returns last in this case
LWG 2150 | C++98 | the condition of “sequence occurence” was incorrect | corrected

### See also

find_end | finds the last sequence of elements in a certain range (function template)
includes | returns true if one sequence is a subsequence of another (function template)
equal | determines if two sets of elements are the same (function template)
findfind_iffind_if_not (C++11) | finds the first element satisfying specific criteria (function template)
lexicographical_compare | returns true if one range is lexicographically less than another (function template)
mismatch | finds the first position where two ranges differ (function template)
search_n | searches for the first occurrence of a number consecutive copies of an element in a range (function template)
default_searcher (C++17) | standard C++ library search algorithm implementation (class template)
boyer_moore_searcher (C++17) | Boyer-Moore search algorithm implementation (class template)
boyer_moore_horspool_searcher (C++17) | Boyer-Moore-Horspool search algorithm implementation (class template)
ranges::search (C++20) | searches for the first occurrence of a range of elements (algorithm function object)

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