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# std::for_each

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class UnaryFunction > UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f ); | (1) |
template< class ExecutionPolicy, class ForwardIt, class UnaryFunction2 > void for_each( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryFunction2 f ); | (2) | (since C++17)

1) Applies the given function object `f` to the result of dereferencing every iterator in the range `[first, last)`, in order.
2) Applies the given function object `f` to the result of dereferencing every iterator in the range `[first, last)` (not necessarily in order). The algorithm is executed according to `policy`. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

For both overloads, if the iterator type is mutable, `f` may modify the elements of the range through the dereferenced iterator. If `f` returns a result, the result is ignored.

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

### [edit] Parameters

first, last | - | the range to apply the function to
policy | - | the execution policy to use. See execution policy for details.
f | - | function object, to be applied to the result of dereferencing every iterator in the range `[first, last)` 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` must meet the requirements of `InputIterator`.
- `ForwardIt` must meet the requirements of `ForwardIterator`.
- `UnaryFunction` must meet the requirements of `MoveConstructible`. Does not have to be `CopyConstructible`
- `UnaryFunction2` must meet the requirements of `CopyConstructible`.

### [edit] Return value

1) `f` (until C++11) std::move(f) (since C++11)
2) (nothing)

### [edit] Complexity

Exactly `last` - `first` applications of `f`

### [edit] Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the three 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.

### [edit] Possible implementation

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

### [edit] Example

The following example uses a lambda function to increment all of the elements of a vector and then uses an overloaded `operator()` in a functor to compute their sum:

Run this code

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

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

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

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

        std::cout << "before:";
        std::for_each(nums.begin(), nums.end(), print);
        std::cout << '\n';

        std::for_each(nums.begin(), nums.end(), [](int &n){ n++; });

        // calls Sum::operator() for each number
        Sum s = std::for_each(nums.begin(), nums.end(), Sum());

        std::cout << "after: ";
        std::for_each(nums.begin(), nums.end(), print);
        std::cout << '\n';
        std::cout << "sum: " << s.sum << '\n';
    }

Output:

    before: 3 4 2 8 15 267
    after:  4 5 3 9 16 268
    sum: 305

### [edit] See also

transform | applies a function to a range of elements (function template) [edit]
range-for loop | executes loop over range (since C++11) [edit]
for_each_n (C++17) | applies a function object to the first n elements of a sequence (function template) [edit]

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