std:: uninitialized_default_construct_n

Parameters

Return value

The end of the range of objects (i.e., std:: next ( first, n ) ).

Complexity

Linear in n .

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, class Size>
ForwardIt uninitialized_default_construct_n(ForwardIt first, Size n)
{
    using T = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
 
    try
    {
        for (; n > 0; (void) ++current, --n)
            ::new (const_cast<void*>(static_cast<const volatile void*>(
                std::addressof(*current)))) T;
        return current;
    }
    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 = std::uninitialized_default_construct_n(first, n);
 
        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_n
    // generally does not zero-initialize the given uninitialized memory area.
    int v[]{1, 2, 3, 4};
    const int original[]{1, 2, 3, 4};
    std::uninitialized_default_construct_n(std::begin(v), std::size(v));
 
    // An attempt to access v might be an 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 ? "un" : "") << "modified\n";
}

default value
default value
default value
unmodified

Defect reports

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

See also