std::fill

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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
fill
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
(C++11)
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
(C++11)


C library
Numeric operations
Operations on uninitialized memory
Defined in header <algorithm>
(1)
template < class ForwardIt, class T >
void fill( ForwardIt first, ForwardIt last, const T& value ) ;
(constexpr since C++20)
(until C++26)
template < class ForwardIt, class T = typename std::iterator_traits

<ForwardIt> :: value_type >
constexpr void fill( ForwardIt first, ForwardIt last,

const T& value ) ;
(since C++26)
(2)
template < class ExecutionPolicy, class ForwardIt, class T >

void fill( ExecutionPolicy&& policy,

           ForwardIt first, ForwardIt last, const T& value ) ;
(since C++17)
(until C++26)
template < class ExecutionPolicy,

class ForwardIt, class T = typename std::iterator_traits
<ForwardIt> :: value_type >
void fill( ExecutionPolicy&& policy,

           ForwardIt first, ForwardIt last, const T& value ) ;
(since C++26)
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 all following conditions are satisfied:

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

Parameters

first, last - the range of elements to modify
value - the value to be assigned
policy - the execution policy to use
Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator

Complexity

Exactly std::distance(first, last)

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
  • 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
LWG 283 C++98 T was required to be CopyAssignable, but
T is not always writable to ForwardIt 		required to be writable instead

See also

 copy-assigns the given value to N elements in a range
(function template)
(C++11)
 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, storing results in a destination range
(function template)
(C++20)
 assigns a range of elements a certain value
(algorithm function object)
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