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    std::fill_n

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Execution policies (C++17)
    Non-modifying sequence operations
    Batch operations
    (C++17)
    Search operations
    (C++11)                (C++11)(C++11)

    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations
    C library
    Numeric operations
    Operations on uninitialized memory
     
    Defined in header <algorithm>
    (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:
    See More

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

    		(since C++20)

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

    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.

    Return value

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

    Complexity

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

    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

    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;
}

    Notes

    Feature-test macro Value Std Feature
    __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (1,2)

    Example

    Run this code
    #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)

    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 “exactly count
    assignments”, 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

    See also

    		 copy-assigns the given value to every element in a range
    (function template) [edit]
    (C++20)
    		 assigns a value to a number of elements
    (algorithm function object)[edit]