std:: fill_n

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 .

Parameters

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

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

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';
}

-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.

See also