std:: uninitialized_default_construct

Parameters

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");
}

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