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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../algorithm/fill_n.html below:

std::fill_n - cppreference.com

(1)

template< class OutputIt, class Size, class T > OutputIt fill_n( OutputIt first, Size count, const T& value );

(constexpr since C++20)
(until C++26) template< class OutputIt, class Size, class T = typename std::iterator_traits <OutputIt>::value_type > constexpr OutputIt fill_n( OutputIt first, Size count, const T& value ); (since C++26) (2) template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value ); (since C++17)
(until C++26) template< class ExecutionPolicy, class ForwardIt, class Size, class T = typename std::iterator_traits <OutputIt>::value_type > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value ); (since C++26)

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 participates in overload resolution only if all following conditions are satisfied:

If any of the following conditions is satisfied, the program is ill-formed:

value is not writable to first.
Size is not convertible to an integral type.

[edit] 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 Type requirements -OutputIt must meet the requirements of LegacyOutputIterator. -ForwardIt must meet the requirements of LegacyForwardIterator. [edit] Return value

Iterator one past the last element assigned if count > 0, first otherwise.

[edit] Complexity

Exactly std::max(0, count) assignments.

[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 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 fill_n

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

[edit] Notes [edit] Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
 
    // replace values of the first 5 elements with -1
    std::fill_n(v1.begin(), 5, -1);
 
    std::copy_n(v1.cbegin(), v1.size(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
 
    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
    #ifdef __cpp_lib_algorithm_default_value_type
        std::fill_n(nums.begin(), 2, {4, 2});
    #else
        std::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
    #endif
    std::copy_n(nums.cbegin(), nums.size(),
                std::ostream_iterator<std::complex<double>>(std::cout, " "));
    std::cout << '\n';
}

Output:

-1 -1 -1 -1 -1 5 6 7 8 9
(4,2) (4,2) (4,8)

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 283 C++98 T was required to be CopyAssignable, but
T is not always writable to OutputIt required to be writable instead LWG 426 C++98 the complexity requirement was âexactly count
assignmentsâ, which is broken if count is negative no assignment if
count is non-positive LWG 865 C++98 the location of the first element following
the filling range was not returned returned [edit] See also copy-assigns the given value to every element in a range
(function template) [edit] assigns a value to a number of elements
(algorithm function object)[edit]

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