std::uninitialized_default_construct

Defined in header <memory>

Defined in header `<memory>`
`template< class ForwardIt > void uninitialized_default_construct( ForwardIt first, ForwardIt last );`	(1)	(since C++17)
`template< class ExecutionPolicy, class ForwardIt > void uninitialized_default_construct( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );`	(2)	(since C++17)

template< class ForwardIt >
void uninitialized_default_construct( ForwardIt first, ForwardIt last );

(1)

(since C++17)

template< class ExecutionPolicy, class ForwardIt >
void uninitialized_default_construct( ExecutionPolicy&& policy,
                                      ForwardIt first, ForwardIt last );

(2)

(since C++17)

1) Constructs objects of type typename std::iterator_traits<ForwardIt>::value_type in the uninitialized storage designated by the range [first, last) by default-initialization, as if by

for (; first != last; ++first)
 ::new (static_cast<void*>(std::addressof(*first)))
 typename std::iterator_traits<ForwardIt>::value_type;

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 does not participate in overload resolution unless

`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)

Parameters

first, last	-	the range of the elements to initialize
policy	-	the execution policy to use. See execution policy for details.
Type requirements
-`ForwardIt` must meet the requirements of LegacyForwardIterator.
-No increment, assignment, comparison, or indirection through valid instances of `ForwardIt` may throw exceptions.

Return value

(none)

Complexity

Linear in the distance between first and last.

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.

Possible implementation

template<class ForwardIt>
void uninitialized_default_construct(ForwardIt first, ForwardIt last)
{
    using Value = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
    try
    {
        for (; current != last; ++current)
        {
            ::new (const_cast<void*>(static_cast<const volatile void*>(
                std::addressof(*current)))) Value;
        }
    }
    catch (...)
    {
        std::destroy(first, current);
        throw;
    }
}

Example

#include <cstring>
#include <iostream>
#include <memory>
#include <string>
 
struct S
{
    std::string m{"Default value"};
};
 
int main()
{
    constexpr int n{3};
    alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
 
    try
    {
        auto first{reinterpret_cast<S*>(mem)};
        auto last{first + n};
 
        std::uninitialized_default_construct(first, last);
 
        for (auto it{first}; it != last; ++it)
            std::cout << it->m << '\n';
 
        std::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.
    int v[]{1, 2, 3, 4};
    const int original[]{1, 2, 3, 4};
    std::uninitialized_default_construct(std::begin(v), std::end(v));
 
    // Maybe undefined behavior, pending CWG 1997.
    // for (const int i : v)
    //     std::cout << i << ' ';
 
    // The result is unspecified.
    std::cout <<
        (std::memcmp(v, original, sizeof(v)) == 0 ? "Unmodified\n" : "Modified\n");
}

Possible output:

Default value
Default value
Default value
Unmodified

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 3870	C++20	this algorithm might create objects on a const storage	kept disallowed

uninitialized_default_construct_n (C++17)	constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template)
uninitialized_value_construct (C++17)	constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template)
ranges::uninitialized_default_construct (C++20)	constructs objects by default-initialization in an uninitialized area of memory, defined by a range (niebloid)

Docs

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