std:: find, std:: find_if, std:: find_if_not

Returns an iterator to the first element in the range [ first , last ) that satisfies specific criteria (or last if there is no such iterator).

Parameters

Return value

The first iterator it in the range [ first , last ) satisfying the following condition or last if there is no such iterator:

Complexity

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

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

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

template<class InputIt, class UnaryPred>
constexpr InputIt find_if(InputIt first, InputIt last, UnaryPred p)
{
    for (; first != last; ++first)
        if (p(*first))
            return first;
    return last;
}

template<class InputIt, class UnaryPred>
constexpr InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
{
    for (; first != last; ++first)
        if (!q(*first))
            return first;
    return last;
}

Notes

If C++11 is not available, an equivalent to std::find_if_not is to use std::find_if with the negated predicate.

template<class InputIt, class UnaryPred>
InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
{
    return std::find_if(first, last, std::not1(q));
}

Example

#include <algorithm>
#include <array>
#include <cassert>
#include <complex>
#include <initializer_list>
#include <iostream>
#include <vector>
bool is_even(int i)
{
    return i % 2 == 0;
}
void example_contains()
{
    const auto haystack = {1, 2, 3, 4};
    for (const int needle : {3, 5})
        if (std::find(haystack.begin(), haystack.end(), needle) == haystack.end())
            std::cout << "haystack does not contain " << needle << '\n';
        else
            std::cout << "haystack contains " << needle << '\n';
}
void example_predicate()
{
    for (const auto& haystack : {std::array{3, 1, 4}, {1, 3, 5}})
    {
        const auto it = std::find_if(haystack.begin(), haystack.end(), is_even);
        if (it != haystack.end())
            std::cout << "haystack contains an even number " << *it << '\n';
        else
            std::cout << "haystack does not contain even numbers\n";
    }
}
void example_list_init()
{
    std::vector<std::complex<double>> haystack{{4.0, 2.0}};
#ifdef __cpp_lib_algorithm_default_value_type
    // T gets deduced making list-initialization possible
    const auto it = std::find(haystack.begin(), haystack.end(), {4.0, 2.0});
#else
    const auto it = std::find(haystack.begin(), haystack.end(), std::complex{4.0, 2.0});
#endif
    assert(it == haystack.begin());  
}
int main()
{
    example_contains();
    example_predicate();
    example_list_init();
}

haystack contains 3
haystack does not contain 5
haystack contains an even number 4
haystack does not contain even numbers

Defect reports

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

See also