1) Constructs objects of type std::iter_value_t<I> in the uninitialized memory area [first, last) by default-initialization, as if by

for (; first != last; ++first)
    ::new (voidify(*first))
        std::remove_reference_t<std::iter_reference_t<I>>;
return first;

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

2) Equivalent to ranges::uninitialized_default_construct(ranges::begin(r), ranges::end(r)).

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

Return value

As described above.

Complexity

Linear in the distance between first and last.

Exceptions

Any exception thrown on construction of the elements in the destination range.

Notes

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible.

Possible implementation

struct uninitialized_default_construct_fn
{
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
        requires std::default_initializable<std::iter_value_t<I>>
    constexpr I operator()(I first, S last) const
    {
        using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::next(first, last); // skip initialization
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (static_cast<void*>(std::addressof(*first))) ValueType;
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
    
    template<no-throw-forward-range R>
        requires std::default_initializable<ranges::range_value_t<R>>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r) const
    {
        return (*this)(ranges::begin(r), ranges::end(r));
    }
};

inline constexpr uninitialized_default_construct_fn uninitialized_default_construct{};

Example

#include <cstring>
#include <iostream>
#include <memory>
#include <string>

int main()
{
    struct S { std::string m{"▄▀▄▀▄▀▄▀"}; };
    
    constexpr int n{4};
    alignas(alignof(S)) char out[n * sizeof(S)];
    
    try
    {
        auto first{reinterpret_cast<S*>(out)};
        auto last{first + n};
        
        std::ranges::uninitialized_default_construct(first, last);
        
        auto count{1};
        for (auto it{first}; it != last; ++it)
            std::cout << count++ << ' ' << it->m << '\n';
        
        std::ranges::destroy(first, last);
    }
    catch (...) { std::cout << "Exception!\n"; }
    
    // Notice that for "trivial types" the uninitialized_default_construct
    // generally does not zero-fill the given uninitialized memory area.
    constexpr char sample[]{'A', 'B', 'C', 'D', '\n'};
    char v[]{'A', 'B', 'C', 'D', '\n'};
    std::ranges::uninitialized_default_construct(std::begin(v), std::end(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
    {
        std::cout << "  ";
        // Maybe undefined behavior, pending CWG 1997 to be resolved:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : sample)
            std::cout << c << ' ';
    }
    else
        std::cout << "Unspecified\n";
}

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
  A B C D

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also