std::uninitialized_fill_n(3)

🌸 std::uninitialized_fill_n 🌸

okay bestie, welcome to the C++ Standard Libary manual entry for std::uninitialized_fill_n — yes, we're doing raw uninitialized memory today, and it's lowkey iconic.

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✨ NAME ✨

std::uninitialized_fill_n — that's literally the name, no cap. It's giving uninitialized-memory-filling main character energy.

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🦋 Synopsis 🦋

Defined in header <memory> — write that down, bestie.

(1)

template< class ForwardIt, class Size, class T >
ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value );

(2) (since C++17)

template< class ExecutionPolicy, class ForwardIt, class Size, class T >
ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy,
                                ForwardIt first, Size count, const T& value );

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💖 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.

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🧚 Type requirements 🧚

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

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👑 Description 👑

Okay so here's the tea, bestie:

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

for (; n--; ++first)
    ::new (/* VOIDIFY */(*first))
        typename std::iterator_traits<ForwardIt>::value_type(value);

where /* VOIDIFY */(e) is:

static_cast<void*>(&e)                (until C++11)
static_cast<void*>(std::addressof(e)) (since C++11)

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order. That's the drama of it.

2) Same as (1), but executed according to policy. This overload participates in overload resolution only if:

• 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)

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⭐ Return value ⭐

Iterator to the element past the last element copied. Slay.

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🌈 Complexity 🌈

Linear in count. That's the whole vibe.

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💀 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.

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🎀 Possible implementation 🎀

Here's the possible implementation, looking all cute and try/catch-y:

template<class ForwardIt, class Size, class T>
ForwardIt uninitialized_fill_n(ForwardIt first, Size count, const T& value)
{
    using V = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt 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;
    }
}

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🍓 Example 🍓

// Run this code — go off, bestie:

#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

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💀 Defect reports 💀

Real talk: the following behavior-changing defect reports were applied retroactively to previously published C++ standards. So this is the lore.

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

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👀 See also 👀

• uninitialized_fill — copies an object to an uninitialized area of memory, defined by a range (function template).
• ranges::uninitialized_fill_n (C++20) — copies an object to an uninitialized area of memory, defined by a start and a count (niebloid).

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Alright bestie, that's the whole page. Source/version: http://cppreference.com, date: 2024.06.10. Mwah 💋✨