std::find() algorithm

od C++20

// (1)
template< class InputIt, class T >
constexpr InputIt find( InputIt first, InputIt last, const T& value );

// (2)
template< class ExecutionPolicy, class ForwardIt, class T >
ForwardIt find( ExecutionPolicy&& policy,
                ForwardIt first, ForwardIt last, const T& value );

od C++17

// (1)
template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );

// (2)
template< class ExecutionPolicy, class ForwardIt, class T >
ForwardIt find( ExecutionPolicy&& policy,
                ForwardIt first, ForwardIt last, const T& value );

do C++17

// (1)
template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );

Returns an iterator to the first element in the range satisfiying specific criteria (or last iterator if there is no such iterator):

• (1) Searches for an element equal to value (using operator==).

• (2) Same as (1), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (od C++20) is true.

Parameters

first last	The range of elements to apply the function to.
value	Value to compare the elements to.
policy	The execution policy to use. See execution policy for details.

Type requirements

InputIt	LegacyInputIterator
ForwardIt	LegacyForwardIterator

Return value

The first iterator it in the range [first, last) for which *it == value is true.

Complexity

Given N as std::distance(first, last):

At most N comparisons with value using operator==.

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

find (1)

template<class InputIt, class T>
constexpr InputIt find(InputIt first, InputIt last, const T& value)
{
    for (; first != last; ++first)
        if (*first == value)
            return first;

    return last;
}

Examples

Main.cpp

#include <algorithm>
#include <array>
#include <iostream>

int main()
{
    const auto v = {1, 2, 3, 4};

    for (int n : {3, 5})
        (std::find(v.begin(), v.end(), n) == std::end(v))
            ? std::cout << "v does not contain " << n << '\n'
            : std::cout << "v contains " << n << '\n';

    auto is_even = [](int i) { return i % 2 == 0; };

    for (auto const& w : {std::array{3, 1, 4}, {1, 3, 5}})
        if (auto it = std::find_if(begin(w), end(w), is_even); it != std::end(w))
            std::cout << "w contains an even number " << *it << '\n';
        else
            std::cout << "w does not contain even numbers\n";
}

Output

v contains 3
v does not contain 5
w contains an even number 4
w does not contain even numbers