std::uninitialized_fill_n

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< cpplrm; | memory
Dynamic memory management
Uninitialized storage
(C++17)
Garbage collection support
Miscellaneous
(C++20)
(C++11)
(C++11)
C Library
Low level memory management
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

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.

Return value

(none) (until C++11)

Iterator to the element past the last element copied.

(since C++11)

Complexity

Linear in count.

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.

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;
    }
}

Example

#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

See also

copies an object to an uninitialized area of memory, defined by a range
(function template)