std::fill_n (3) - Linux Manuals

std::fill_n: std::fill_n

NAME

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