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    std::uninitialized_fill

    From cppreference.com
     
    C++
     
    Algorithm library
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    Specialized <memory> algorithms
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    (C++17)
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    (C++17)
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    Defined in header <memory> 
    template< class NoThrowForwardIt, class T >
void uninitialized_fill( NoThrowForwardIt first,
                         NoThrowForwardIt last, const T& value );
    		(1) (constexpr since C++26) 
    template< class ExecutionPolicy, class NoThrowForwardIt, class T >
void uninitialized_fill( ExecutionPolicy&& policy,
                         NoThrowForwardIt first,
                         NoThrowForwardIt last, const T& value );
    		(2) (since C++17)
    1) Constructs elements in the destination range [first, last) with the given value value as if by

    for (; first != last; ++first)
        ::new (voidify(*first))
            typename std::iterator_traits<NoThrowForwardIt>::value_type(value);

    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 the value of the following expression is true:

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>

    		(since C++20)

    Parameters

    first, last - the pair of iterators defining the range of elements to initialize
    value - the value to construct the elements with
    policy - the execution policy to use
    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)

    Exceptions

    2) During the execution process:
    • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
    • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

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

    Example

    Run this code
    #include <algorithm>
#include <iostream>
#include <memory>
#include <string>

int main()
{
    const std::size_t sz = 4;
    std::allocator<std::string> alloc;
    std::string* p = alloc.allocate(sz);
    
    std::uninitialized_fill(p, p + sz, "Example");
    
    for (std::string* i = p; i != p + sz; ++i)
    {
        std::cout << *i << '\n';
        i->~basic_string<char>();
    }
    
    alloc.deallocate(p, sz);
}

    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 2433 C++11 this algorithm might be hijacked by overloaded operator& uses std::addressof

    See also

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