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# std::uninitialized_fill_n

From cppreference.com
< cpp‎ | memory

Defined in header `<memory>` |  |
| (1) |
template< class ForwardIt, class Size, class T > void uninitialized_fill_n( ForwardIt first, Size count, const T& value ); |  | (until C++11)
template< class ForwardIt, class Size, class T > ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value ); |  | (since C++11)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value ); | (2) | (since C++17)

1) Copies the given value `value` to the first `count` elements in an uninitialized memory area beginning at `first` as if by

    for (; n--; ++first)
      ::new (static_cast<void*>(std::addressof(*first)))
         typename std::iterator_traits<ForwardIt>::value_type(x);

If an exception is thrown during the initialization, the function has no effects.
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

### [edit] Parameters

first | - | the beginning of the range of the elements to initialize
count | - | number of elements to construct
value | - | the value to construct the elements with
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.
- No increment, assignment, comparison, or indirection through valid instances of `ForwardIt` may throw exceptions.

### [edit] Return value

(none) | (until C++11)
Iterator to the element past the last element copied. | (since C++11)

### [edit] Complexity

Linear in `count`.

### [edit] 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 three 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.

### [edit] Possible implementation

    template< class ForwardIt, class Size, class T >
    ForwardIt uninitialized_fill_n(ForwardIt first, Size count, const T& value)
    {
        typedef typename std::iterator_traits<ForwardIt>::value_type Value;
        ForwardIt current = first;
        try {
            for (; count > 0; ++current, (void) --count) {
                ::new (static_cast<void*>(std::addressof(*current))) Value(value);
            }
            return current;
        } catch (...) {
            for (; first != current; ++first) {
                first->~Value();
            }
            throw;
        }
    }

### [edit] Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <memory>
    #include <string>
    #include <tuple>

    int main()
    {
        std::string* p;
        std::size_t sz;
        std::tie(p, sz) = std::get_temporary_buffer<std::string>(4);
        std::uninitialized_fill_n(p, sz, "Example");

        for (std::string* i = p; i != p+sz; ++i) {
            std::cout << *i << '\n';
            i->~basic_string<char>();
        }
        std::return_temporary_buffer(p);
    }

Output:

    Example
    Example
    Example
    Example

### [edit] See also

uninitialized_fill | copies an object to an uninitialized area of memory, defined by a range (function template) [edit]

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