std::for_each() algorithm

od C++20

// (1)
template< class InputIt, class UnaryFunction >
constexpr UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryFunction2 >
void for_each( ExecutionPolicy&& policy,
               ForwardIt first, ForwardIt last, UnaryFunction2 f );

od C++17

// (1)
template< class InputIt, class UnaryFunction >
UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );

// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryFunction2 >
void for_each( ExecutionPolicy&& policy,
               ForwardIt first, ForwardIt last, UnaryFunction2 f );

do C++17

// (1)
template< class InputIt, class UnaryFunction >
UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );

Applies the given function to all elements in range.

• (1) Applies the given function object f to the result of dereferencing every iterator in the range [first, last), in order.

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

  warning

  There is no guarantee the function will be applied to elements in order.

  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.

For both overloads, if the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range. If f returns a result, the result is ignored.

warning

Unlike the rest of the parallel algorithms, for_each is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

Parameters

first last	The range of elements to apply the function to.
policy	The execution policy to use. See execution policy for details.
f	Function object, to be applied to every element of the range. The signature of the function should be equivalent to the following: void fun(const Type& a); The signature does not need to have const&. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.

Type requirements

InputIt	LegacyInputIterator
ForwardIt	LegacyForwardIterator
UnaryFunction	MoveConstructible
UnaryFunction2	CopyConstructible

Return value

• (1) - f (do C++11) std::move(f) (od C++11).
• (2) - (none)

Complexity

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

Exactly N applications of f.

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

for_each (1)

template<class InputIt, class UnaryFunction>
constexpr UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
{
    for (; first != last; ++first)
        f(*first);

    return f; // implicit move since C++11
}

Examples

Main.cpp

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v {3, -4, 2, -8, 15, 267};

    auto print = [](const int& n) { std::cout << n << ' '; };

    std::cout << "before:\t";
    std::for_each(v.cbegin(), v.cend(), print);
    std::cout << '\n';

    // increment elements in-place
    std::for_each(v.begin(), v.end(), [](int &n) { n++; });

    std::cout << "after:\t";
    std::for_each(v.cbegin(), v.cend(), print);
    std::cout << '\n';

    struct Sum
    {
        void operator()(int n) { sum += n; }
        int sum {0};
    };

    // invoke Sum::operator() for each element
    Sum s = std::for_each(v.cbegin(), v.cend(), Sum());
    std::cout << "sum:\t" << s.sum << '\n';
}

Output

before:	3 -4 2 -8 15 267 
after:	4 -3 3 -7 16 268 
sum:	281