std::search(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💖 std::search — C++ Standard Library 💖

okay so bestie, welcome to std::search(3), the C++ Standard Library page for the algorithm that literally goes "where's my pattern at" and finds it for you. no cap, this one is a whole vibe. let's get into it ✨

🌸 NAME 🌸

std::search — std::search. that's it. that's the name. she's self-titled and honestly? iconic 💅

🦋 SYNOPSIS 🦋

first things first, she lives in a header:

Defined in header <algorithm>

then we got five overloads, and yes you need to know all five, i don't make the rules:

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 >
ForwardIt search( ForwardIt first, ForwardIt last,             (5) (since C++17) (constexpr since C++20)
                  const Searcher& searcher );

✨ what they actually do ✨

1–4) these search for the first occurrence of the sequence of elements [s_first, s_last) inside the range [first, last). like, you hand her a haystack and a needle and she's like "got u" 🫶

  • 1) elements get compared using operator==. classic.
  • 3) elements get compared using the given binary predicate p. fancy, we love a custom comparator.
  • 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. she doesn't even need you to tell her the pattern, the searcher already knows. main character energy tbh 👑

🎀 the searchers the standard library hands you 🎀

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

three searchers, all class templates, all valid, all slay 💅

🌈 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);
okay so this is the part where the standard gets picky and honestly she's right. 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. phew. drink some water bestie 🍓

🧚 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. and if [s_first, s_last) is empty, first is returned. (write that down, there's a whole defect report about it later 👀)

5) searcher(first, last).first.

🔥 COMPLEXITY 🔥

1–4) given N as std::distance(first, last) and S as std::distance(s_first, s_last):

  • 1,2) at most N·S comparisons using operator==.
  • 3,4) at most N·S applications of the predicate p.

5) depends on searcher. like, it literally depends on which searcher you picked, she's not gonna do your homework for you.

💀 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.
fr fr, read that first bullet again. std::terminate. she does not do warnings, she goes straight to the end. 😭

👑 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, it's cute, it's got asserts, it's got a boyer_moore_searcher cameo:

#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

(case sensitivity said "not today, Pisci" 🙈)

💜 DEFECT REPORTS 💜

the following behavior-changing defect reports were applied retroactively to previously published C++ standards. yes, the past got patched. we love a retroactive glow-up ✨

  • LWG 1205 — applied to C++98. behavior as published: the return value was unclear if [s_first, s_last) is empty. correct behavior: returns first in this case.
  • LWG 1338 — applied to C++98. behavior as published: the resolution of LWG issue 1205 was incorrectly applied, making first to be returned if no occurence is found. correct behavior: returns last in this case.
  • LWG 2150 — applied to C++98. behavior as published: the condition of "sequence occurence" was incorrect. correct behavior: corrected. ✅

move over to the next part, bestie, there's more and i'm not done hyping her up 💅

✨ SEE ALSO ✨

okay bestie, here’s the friend group of other algorithms that are lowkey adjacent to std::search — every name, every little type label, and every C++ version tag is staying exactly where it belongs:

  • 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 — finds the first element satisfying specific criteria (function template) 🔥
  • find_if — finds the first element satisfying specific criteria (function template) 🔥
  • find_if_not — finds the first element satisfying specific criteria (C++11) (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 — standard C++ library search algorithm implementation (C++17) (class template) 🎀
  • boyer_moore_searcher — Boyer-Moore search algorithm implementation (C++17) (class template) 🧚
  • boyer_moore_horspool_searcher — Boyer-Moore-Horspool search algorithm implementation (C++17) (class template) 🧚
  • ranges::search — searches for a range of elements (C++20) (niebloid) 💜

💖 source/version: cppreference.com, 2024.06.10 — this iconic little page is std::search(3). periodt. 🎀

📦 where this page comes from