◐ Shell
clean mode source ↗

std::ranges::uninitialized_default_construct - cppreference.com

From cppreference.com

Defined in header <memory>

Call signature

 
template< /*nothrow-forward-iterator*/ I, /*nothrow-sentinel-for*/<I> S >
    requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct( I first, S last );
(1) (since C++20)
(constexpr since C++26)		 
template< /*nothrow-forward-range*/ R >
    requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>
    uninitialized_default_construct( R&& r );
(2) (since C++20)
(constexpr since C++26)		 
template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I,
          /*nothrow-sized-sentinel-for*/<I> S >
    requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct( Ep&& policy, I first, S last );
(3) (since C++26) 
template< /*execution-policy*/ Ep, /*nothrow-sized-random-access-range*/ R >
    requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>
    uninitialized_default_construct( Ep&& policy, R&& r );
(4) (since C++26)

For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.

1) Constructs elements in the destination range [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) Same as (1), but uses r as the destination range.

3,4) Same as (1,2), but executed according to policy.

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

Parameters

first, last - the iterator-sentinel pair defining the range of elements to initialize
r - the range of the elements to initialize
policy - the execution policy to use

Return value

As described above.

Exceptions

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

3,4) 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

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.

Feature-test macro Value Std Feature
__cpp_lib_parallel_algorithm 202506L (C++26) Parallel range algorithms
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized <memory> algorithms, (1,2)

Possible implementation

struct uninitialized_default_construct_fn
{
    template</*nothrow-forward-iterator*/ I, /*nothrow-sentinel-for*/<I> S>
        requires std::default_initializable<std::iter_value_t<I>>
    constexpr I operator()(I first, S last) const
    {
        using value_type = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<value_type>)
            return ranges::next(first, last); // skip initialization
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (static_cast<void*>(std::addressof(*first))) value_type;
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
    
    template</*nothrow-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::next(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";
}

Possible output: 

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

See also