Std::fill_n (3) Linux Manual Page

std::fill_n – std::fill_n

Synopsis

Defined in header<algorithm>

    template <class OutputIt, class Size, class T>
    (until C++ 11)

        void fill_n(OutputIt first, Size count, const T &value);

template <class OutputIt, class Size, class T>
(since C++ 11)

    OutputIt fill_n(OutputIt first, Size count, const T &value);
(1)(until C++ 20)

    template <class OutputIt, class Size, class T>
    (since C++ 20)

        constexpr OutputIt fill_n(OutputIt first, Size count, const T &value);

template <class ExecutionPolicy, class ForwardIt, class Size, class T>
(2)(since C++ 17)

    ForwardIt fill_n(ExecutionPolicy &&policy, ForwardIt first, Size count, const T &value);

1) Assigns the given value to the first count elements in the range beginning at first if count > 0. Does nothing otherwise.
2) Same as (1), but executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first – the beginning of the range of elements to modify
count – number of elements to modify
value – the value to be assigned
policy – the execution policy to use. See execution_policy for details.

Type requirements

–
OutputIt must meet the requirements of LegacyOutputIterator.
–
ForwardIt must meet the requirements of LegacyForwardIterator.

Return value

(none) (until C++11)
Iterator one past the last element assigned if count > 0, first otherwise. (since C++11)

Complexity

Exactly count assignments, for count > 0.

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 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 OutputIt, class Size, class T>
  OutputIt fill_n(OutputIt first, Size count, const T& value)
  {
      for (Size i = 0; i < count; i++) {
          *first++ = value;
      }
      return first;
  }

Example

The following code uses fill_n() to assign -1 to the first half of a vector of integers:
// Run this code

#include <algorithm>

#include <vector>

#include <iostream>

#include <iterator>
int main()

{
    std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::fill_n(v1.begin(), 5, -1);
    std::copy(begin(v1), end(v1), std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\n";
}

Output:

-1 -1 -1 -1 -1 5 6 7 8 9

std::fill_n (3) Linux Manual Page