# std::search

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt1, class ForwardIt2 > ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last ); |  | (until C++20)
template< class ForwardIt1, class ForwardIt2 > constexpr ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last ); |  | (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)
| (3) |
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p ); |  | (until C++20)
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate > constexpr ForwardIt1 search( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt1 search( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p ); | (4) | (since C++17)
| (5) |
template< class ForwardIt, class Searcher > ForwardIt search( ForwardIt first, ForwardIt last, const Searcher& searcher ); |  | (since C++17) (until C++20)
template< class ForwardIt, class Searcher > constexpr ForwardIt search( ForwardIt first, ForwardIt last, const Searcher& searcher ); |  | (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 do not participate in overload resolution unless

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. `Searcher` need not be CopyConstructible.

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. See execution policy for details.
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.

### 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 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 BinaryPredicate>
    constexpr ForwardIt1 search(ForwardIt1 first, ForwardIt1 last,
                                ForwardIt2 s_first, ForwardIt2 s_last,
                                BinaryPredicate 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

### 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)
find find_if find_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 a range for a number of consecutive copies of an element (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 a range of elements (niebloid)

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