std::find_if() algorithm

since C++20

// (1)
template< class InputIt, class UnaryPredicate >
constexpr InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt find_if( ExecutionPolicy&& policy,
                   ForwardIt first, ForwardIt last, UnaryPredicate p );

since C++17

// (1)
template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt find_if( ExecutionPolicy&& policy,
                   ForwardIt first, ForwardIt last, UnaryPredicate p );

until C++17

// (1)
template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

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 for which predicate p returns true.

• (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>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Parameters

first second	The range of elements to apply the function to.
policy	The execution policy to use. See execution policy for details.
p	Unary predicate which returns true for the required element. The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT& is not allowed , nor is VT unless for VT a move is equivalent to a copy. (since C++11).

Type requirements

InputIt	LegacyInputIterator
ForwardIt	LegacyForwardIterator

Return value

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

Complexity

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

At most N applications of the predicate p.

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_if (1)

template<class InputIt, class UnaryPredicate>
constexpr InputIt find_if(InputIt first, InputIt last, UnaryPredicate p)
{
    for (; first != last; ++first)
        if (p(*first))
            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_if(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_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