std::ranges:: uninitialized_default_construct_n

Constructs objects of type std:: iter_value_t < I > in the uninitialized memory area first + [ ​ 0 ​ , count ) by default-initialization , as if by return ranges:: uninitialized_default_construct ( std:: counted_iterator ( first, count ) ,
std:: default_sentinel ) . base ( ) ;

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

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 count .

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 .

Notes

Possible implementation

struct uninitialized_default_construct_n_fn
{
    template<no-throw-forward-iterator I>
        requires std::default_initializable<std::iter_value_t<I>>
    constexpr I operator()(I first, std::iter_difference_t<I> count) const
    {
        auto iter = std::counted_iterator(first, count);
        return ranges::uninitialized_default_construct(iter, std::default_sentinel).base();
    }
};
inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

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 = std::ranges::uninitialized_default_construct_n(first, n);
        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";
    }
    // For scalar types, uninitialized_default_construct_n
    // generally does not zero-fill the given uninitialized memory area.
    constexpr int sample[]{1, 2, 3, 4, 5, 6};
    int v[]{1, 2, 3, 4, 5, 6};
    std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
    if (std::memcmp(v, sample, sizeof(v)) == 0)
    {
        // Maybe undefined behavior, pending CWG 1997:
        // for (const int i : v) { std::cout << i << ' '; }
        for (const int i : sample)
            std::cout << i << ' ';
    }
    else
        std::cout << "Unspecified!";
    std::cout << '\n';
}

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Defect reports

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

See also