< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Concepts and utilities: std::Sortable, std::projected, ...
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Numeric operations
C library
Defined in header <algorithm>
template< class ForwardIt, class T >
void fill( ForwardIt first, ForwardIt last, const T& value );
(until C++20)
template< class ForwardIt, class T >
constexpr void fill( ForwardIt first, ForwardIt last, const T& value );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class T >
void fill( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );
(2) (since C++17)
1) Assigns the given value to the elements in the range [first, last).
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true


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.

Return value



Exactly last - first assignments.


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 ForwardIt, class T >
void fill(ForwardIt first, ForwardIt last, const T& value)
    for (; first != last; ++first) {
        *first = value;


The following code uses fill() to set all of the elements of a vector of integers to -1:

#include <algorithm>
#include <vector>
#include <iostream>
int main()
    std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::fill(v.begin(), v.end(), -1);
    for (auto elem : v) {
        std::cout << elem << " ";
    std::cout << "\n";


-1 -1 -1 -1 -1 -1 -1 -1 -1 -1

See also

copy-assigns the given value to N elements in a range
(function template)
copies a range of elements to a new location
(function template)
assigns the results of successive function calls to every element in a range
(function template)
applies a function to a range of elements
(function template)