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

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< cpp‎ | memory
m (→‎Complexity: + '.')
m (→‎Example: +)
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===Example===
 
===Example===
−
{{example
+
{{example|code=
 +
#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[] { "Home", "Work!" };
 +
    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(std::begin(in), std::end(in), 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);
 +
    }
 +
}
 +
| p=true <!-- move results are indeterminate -->
 +
| output=
 +
initially, in: "Home" "Work!"
 +
after move, in: "" ""
 +
after move, out: "Home" "Work!"
 
}}
 
}}
  

Revision as of 06:03, 25 December 2020

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)



 
Defined in header <memory>
template< class InputIt, class ForwardIt >
ForwardIt uninitialized_move( InputIt first, InputIt last, ForwardIt d_first );
 (1) (since C++17)
template< class ExecutionPolicy, class InputIt, class ForwardIt >
ForwardIt uninitialized_move( ExecutionPolicy&& policy, InputIt first, InputIt last, ForwardIt d_first );
 (2) (since C++17)
1) Moves elements from the range [first, last) to an uninitialized memory area beginning at d_first as if by 
for (; first != last; ++d_first, (void) ++first)
   ::new (static_cast<void*>(std::addressof(*d_first)))
      typename std::iterator_traits<ForwardIt>::value_type(std::move(*first));
If an exception is thrown during the initialization, some objects in [first, last) are left in a valid but unspecified state, and the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy. This overload participates in overload resolution only if

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)

Contents

Parameters

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

Return value

Iterator to the element past the last element moved.

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 InputIt, class ForwardIt>
ForwardIt uninitialized_move(InputIt first, InputIt last, ForwardIt d_first)
{
    typedef typename std::iterator_traits<ForwardIt>::value_type Value;
    ForwardIt current = d_first;
    try {
        for (; first != last; ++first, (void) ++current) {
            ::new (static_cast<void*>(std::addressof(*current))) Value(std::move(*first));
        }
        return current;
    } catch (...) {
        for (; d_first != current; ++d_first) {
            d_first->~Value();
        }
        throw;
    }
}

Example

Run this code
#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[] { "Home", "Work!" };
    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(std::begin(in), std::end(in), 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);
    }
}

Possible output: 

initially, in: "Home" "Work!"
after move, in: "" ""
after move, out: "Home" "Work!"

See also

 copies a range of objects to an uninitialized area of memory
(function template) [edit]
(C++17)
 moves a number of objects to an uninitialized area of memory
(function template) [edit]
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