# std::fill

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
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. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### 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.
- value must be writable to first.

### Return value

(none)

### 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

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

### Example

The following code uses `fill()` to set all of the elements of a std::vector<int> to -1:

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    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';
    }

Output:

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

### 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

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

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