std::uninitialized_default_construct

Since C++17

Header: <memory>

Constructs objects of type typename std::iterator_traits::value_type in the uninitialized memory area [first,last) by default-initialization, as if by for (; first != last; ++first) ::new (voidify(*first)) typename std::iterator_traits::value_type;

# Declarations

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

(since C++17) (constexpr since C++26)

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

(since C++17)

# Parameters

first, last: the range of the elements to initialize
policy: the execution policy to use

# Notes

Feature-test macro Value Std Feature __cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms, (1)

# 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";
    }
 
    // For scalar types, 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 to be resolved.
    // 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");
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 3870	C++20	this algorithm might create objects on a const storage	kept disallowed