std::ranges::fill_n
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< class T, std::output_iterator<const T&> O >
constexpr O fill_n( O first, std::iter_difference_t<O> count, const T& value );
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(1) | (since C++20) (until C++26) |
template< class O, class T = std::iter_value_t<O> >
requires std::output_iterator<O, const T&>
constexpr O fill_n( O first, std::iter_difference_t<O> count, const T& value );
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(since C++26) | |
template< /*execution-policy*/ Ep,
std::random_access_iterator O, class T = std::iter_value_t<O> >
requires std::indirectly_writable<O, const T&>
O fill_n( Ep&& policy, O first, iter_difference_t<O> count, const T& value );
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(2) | (since C++26) |
For the definition of /*execution-policy*/, see this page.
1) If
count is positive, assigns value to all elements in the target range [first, ranges::next(first, count)). Otherwise does nothing.2) Same as (1), but executed according to
policy.The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
Parameters
| first | - | the beginning of the target range |
| count | - | number of elements to modify |
| value | - | the value to be assigned |
| policy | - | the execution policy to use |
Return value
The past-the-end iterator of the target range, or first if count is non-positive.
Complexity
Exactly \(\scriptsize \max(\mathtt{count},0)\)max(count,0) assignments.
Exceptions
2) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 |
(C++26) | List-initialization for algorithms |
Possible implementation
struct fill_n_fn
{
template<class O, class T = std::iter_value_t<O>>
requires std::output_iterator<O, const T&>
constexpr O operator()(O first, std::iter_difference_t<O> count,
const T& value) const
{
for (std::iter_difference_t<O> i {}; i != count; ++first, ++i)
*first = value;
return first;
}
};
inline constexpr fill_n_fn fill_n{};
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Example
Run this code
#include <algorithm>
#include <complex>
#include <iostream>
#include <string>
#include <vector>
void println(const auto& v)
{
for (const auto& elem : v)
std::cout << ' ' << elem;
std::cout << '\n';
}
int main()
{
constexpr auto n{8};
std::vector<std::string> v(n, "");
println(v);
std::ranges::fill_n(v.begin(), n, "");
println(v);
std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
println(nums);
#ifdef __cpp_lib_algorithm_default_value_type
std::ranges::fill_n(nums.begin(), 2, {4, 2});
#else
std::ranges::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
#endif
println(nums);
}
Output:
(1,3) (2,2) (4,8)
(4,2) (4,2) (4,8)
See also
| copy-assigns the given value to N elements in a range (function template) | |
(C++20) |
assigns a range of elements a certain value (algorithm function object) |
(C++20) |
copies a number of elements to a new location (algorithm function object) |
(C++20) |
saves the result of a function in a range (algorithm function object) |
(C++20) |
applies a function to a range of elements (algorithm function object) |
(C++26) |
fills a range with random numbers from a uniform random bit generator (algorithm function object) |