NAME¶

std::fill - std::fill

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class T > (constexpr since
void fill( ForwardIt first, ForwardIt last, C++20)
const T& value ); (until C++26)
template< class ForwardIt, class T = typename
std::iterator_traits

<ForwardIt>::value_type > (since C++26)
constexpr void fill( ForwardIt first,
ForwardIt last,

const T& value );
template< class ExecutionPolicy, class
ForwardIt, class T >
(1) (since C++17)
void fill( ExecutionPolicy&& policy, (until C++26)

ForwardIt first, ForwardIt last,
const T& value );
template< class ExecutionPolicy,
(2)
class ForwardIt, class T = typename
std::iterator_traits
(since C++26)
<ForwardIt>::value_type >
void fill( ExecutionPolicy&& policy,

ForwardIt first, ForwardIt last,
const T& value );

1) Assigns the given value to all elements in the range [first, last).
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if

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 value is not writable to first, the program is ill-formed.

Parameters¶

first, last - the range of elements to modify
value - the value to be assigned
policy - the execution policy to use. See execution policy for details.

Type requirements¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.

Complexity¶

Exactly std::distance(first, last) 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
template<class ForwardIt,
class T = typename std::iterator_traits<ForwardIt>::value_type>
void fill(ForwardIt first, ForwardIt last, const T& value)
{
for (; first != last; ++first)
*first = value;
}

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 <vector>

void println(const auto& seq)
{
for (const auto& e : seq)
std::cout << e << ' ';
std::cout << '\n';
}

int main()
{
std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8};
println(v);

// set all of the elements to 8
std::fill(v.begin(), v.end(), 8);
println(v);

std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
println(nums);
#ifdef __cpp_lib_algorithm_default_value_type
std::fill(nums.begin(), nums.end(), {4, 2});
#else
std::fill(nums.begin(), nums.end(), std::complex<double>{4, 2});
#endif
println(nums);
}

Output:¶

0 1 2 3 4 5 6 7 8
8 8 8 8 8 8 8 8 8
(1,3) (2,2) (4,8)
(4,2) (4,2) (4,2)

Defect reports

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

DR Applied to Behavior as published Correct behavior
T was required to be CopyAssignable, required to be writable
LWG 283 C++98 but instead
T is not always writable to ForwardIt

See also¶

fill_n copy-assigns the given value to N elements in a range
(function template)
copy copies a range of elements to a new location
copy_if (function template)
(C++11)
assigns the results of successive function calls to every element in a
generate range
(function template)
applies a function to a range of elements, storing results in a
transform destination range
(function template)
ranges::fill assigns a range of elements a certain value
(C++20) (niebloid)