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    std::fill

    From cppreference.com
     
    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
    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
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations
    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, the program is ill-formed.

    Parameters

    first, last - the pair of iterators defining 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) 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 (1) 
    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) [edit]
    (C++11)
    		 copies a range of elements to a new location
    (function template) [edit]
    		assigns the results of successive function calls to every element in a range
    (function template) [edit]
    		applies a function to a range of elements, storing results in a destination range
    (function template) [edit]
    (C++20)
    		 assigns a range of elements a certain value
    (algorithm function object)[edit]