Difference between revisions of "cpp/memory/uninitialized move n"

Revision as of 03:21, 13 February 2023

for ( ; n > 0; ++d_first, (void) ++first, --n)
   ::new (static_cast<void*>(std::addressof((*d_first)))
      typename std::iterator_traits<NoThrowForwardIt>::value_type(std::move(*first));

Parameters

Return value

A pair whose first element is an iterator to the element past the last element moved in the source range, and whose second element is an iterator to the element past the last element moved in the destination range.

Complexity

Linear in count.

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 InputIt, class Size, class NoThrowForwardIt>
std::pair<InputIt, NoThrowForwardIt>
    uninitialized_move_n(InputIt first, Size count, NoThrowForwardIt d_first)
{
    using Value = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try {
        for (; count > 0; ++first, (void) ++current, --count) {
            ::new (static_cast<void*>(std::addressof(*current))) Value(std::move(*first));
        }
    } catch (...) {
        std::destroy(d_first, current);
        throw;
    }
    return {first, current};
}

Example

#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <memory>
#include <string>
 
void print(auto rem, auto first, auto last) {
    for (std::cout << rem; first != last; ++first)
        std::cout << std::quoted(*first) << ' ';
    std::cout << '\n';
}
 
int main() {
    std::string in[] { "One", "Definition", "Rule" };
    print("initially, in: ", std::begin(in), std::end(in));
 
    if (
        constexpr auto sz = std::size(in);
        void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz)
    ) {
        try {
            auto first {static_cast<std::string*>(out)};
            auto last {first + sz};
            std::uninitialized_move_n(std::begin(in), sz, first);
 
            print("after move, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);
 
            std::destroy(first, last);
        }
        catch (...) {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

initially, in: "One" "Definition" "Rule" 
after move, in: "" "" "" 
after move, out: "One" "Definition" "Rule"

Defect reports

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

See also