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std:: uninitialized_fill_n

From cppreference.net
C++
Memory management library
( exposition only* )
Allocators
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Memory resources
Uninitialized storage (until C++20)
( until C++20* )
( until C++20* )
( until C++20* )

Garbage collector support (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
Defined in header <memory>
template < class NoThrowForwardIt, class Size, class T >

NoThrowForwardIt uninitialized_fill_n ( NoThrowForwardIt first,

Size count, const T & value ) ;
(1) (constexpr since C++26)
template < class ExecutionPolicy,

class NoThrowForwardIt, class Size, class T >
NoThrowForwardIt uninitialized_fill_n ( ExecutionPolicy && policy,
NoThrowForwardIt first,

Size count, const T & value ) ;
(2) (since C++17)
1) Copies value to an uninitialized memory area first + [ ​ 0 ​ , count ) as if by

for ( ; count -- ; ++ first )
:: new ( voidify ( * first ) )
typename std:: iterator_traits < NoThrowForwardIt > :: value_type ( value ) ;
return first ;

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 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)

Contents

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
-
NoThrowForwardIt must meet the requirements of LegacyForwardIterator .
-
No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions. Applying & * to a NoThrowForwardIt value must yield a pointer to its value type. (until C++11)

Return value

As described above.

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 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.

Notes

Feature-test macro Value Std Feature
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms , ( 1 )

Possible implementation 

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

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

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 866 C++98 given T as the value type of NoThrowForwardIt , if
T :: operator new exists, the program might be ill-formed 		uses global placement new instead
LWG 1339 C++98 the location of the first element following
the filling range was not returned 		returned
LWG 2433 C++11 this algorithm might be hijacked by overloaded operator & uses std::addressof
LWG 3870 C++20 this algorithm might create objects on a const storage kept disallowed

See also

copies an object to an uninitialized area of memory, defined by a range
(function template)
(C++20)
copies an object to an uninitialized area of memory, defined by a start and a count
(algorithm function object)