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std::uninitialized_fill_n

From cppreference.com
< cpp‎ | 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 objects already constructed are destroyed in an unspecified order.
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

Contents

[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

#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

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