Namespaces
Variants
Views
Actions

Difference between revisions of "cpp/memory/uninitialized value construct n"

From cppreference.com
< cpp‎ | memory
m (link to zh)
m (fix)
 
(14 intermediate revisions by 6 users not shown)
Line 2: Line 2:
 
{{cpp/memory/navbar}}
 
{{cpp/memory/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | memory}}
+
{{dcl header|memory}}
−
{{dcl | since=c++17 | num= 1 |1=   
+
{{dcl|since=c++17|num=1|1=   
 
template< class ForwardIt, class Size >
 
template< class ForwardIt, class Size >
 
ForwardIt uninitialized_value_construct_n( ForwardIt first, Size n );
 
ForwardIt uninitialized_value_construct_n( ForwardIt first, Size n );
 
}}
 
}}
−
{{dcl | since=c++17 | num= 2 |1=
+
{{dcl|since=c++17|num=2|1=
 
template< class ExecutionPolicy, class ForwardIt, class Size >
 
template< class ExecutionPolicy, class ForwardIt, class Size >
−
ForwardIt uninitialized_value_construct_n( ExecutionPolicy&& policy, ForwardIt first, Size n );}}
+
ForwardIt uninitialized_value_construct_n( ExecutionPolicy&& policy,
 +
                                          ForwardIt first, Size n );}}
 
{{dcl end}}
 
{{dcl end}}
−
@1@ Constructs {{tt|n}} objects of type {{tt|typename iterator_traits<ForwardIt>::value_type}} in the uninitialized storage starting at {{tt|first}} by [[cpp/language/value_initialization|value-initialization]], as if by
+
@1@ Constructs {{c|n}} objects of type {{c|typename iterator_traits<ForwardIt>::value_type}} in the uninitialized storage starting at {{c|first}} by [[cpp/language/value_initialization|value-initialization]], as if by
−
{{source|1=
+
{{c multi|
−
for (; n > 0; (void) ++first, --n)
+
for (; n > 0; (void) ++first, --n)|
−
  ::new (static_cast<void*>(std::addressof(*first)))
+
    ::new (static_cast<void*>(std::addressof(*first)))|
−
      typename std::iterator_traits<ForwardIt>::value_type();
+
        typename std::iterator_traits<ForwardIt>::value_type();
 
}}
 
}}
  
−
@@If an exception is thrown during the initialization, the function has no effects.  
+
@@ If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.  
−
@2@ Same as {{v|1}}, but executed according to {{tt|policy}}. This overload does not participate in overload resolution unless {{c|std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>}} is true
+
@2@ Same as {{v|1}}, but executed according to {{c|policy}}. {{cpp/algorithm/parallel overload precondition}}
 +
 
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | first | the beginning of the range of elements to initialize}}
+
{{par|first|the beginning of the range of elements to initialize}}
−
{{par | n | the number of elements to initialize}}
+
{{par|n|the number of elements to initialize}}
 
{{par exec pol}}
 
{{par exec pol}}
 
{{par hreq}}
 
{{par hreq}}
−
{{par req concept | ForwardIt | ForwardIterator}}
+
{{par req named|ForwardIt|ForwardIterator}}
−
{{par req | No increment, assignment, comparison, or indirection through valid instances of {{tt|ForwardIt}} may throw exceptions.}}
+
{{par req|No increment, assignment, comparison, or indirection through valid instances of {{tt|ForwardIt}} may throw exceptions.}}
 
{{par end}}
 
{{par end}}
  
Line 34: Line 36:
  
 
===Complexity===
 
===Complexity===
−
Linear in {{tt|n}}.
+
Linear in {{c|n}}.
  
 
===Exceptions===
 
===Exceptions===
Line 40: Line 42:
  
 
===Possible implementation===
 
===Possible implementation===
−
{{eq fun | 1=
+
{{eq fun|1=
 
template<class ForwardIt, class Size>
 
template<class ForwardIt, class Size>
−
ForwardIt uninitialized_value_construct_n( ForwardIt first, Size n )
+
ForwardIt uninitialized_value_construct_n(ForwardIt first, Size n)
 
{
 
{
−
     typedef typename std::iterator_traits<ForwardIt>::value_type Value;
+
     using T = typename std::iterator_traits<ForwardIt>::value_type;
 
     ForwardIt current = first;
 
     ForwardIt current = first;
−
     try {
+
     try
−
         for (; n > 0 ; (void) ++current, --n) {
+
    {
−
             ::new (static_cast<void*>(std::addressof(*current))) Value();
+
         for (; n > 0 ; (void) ++current, --n)
−
        }
+
             ::new (const_cast<void*>(static_cast<const volatile void*>(
 +
                std::addressof(*current)))) T();
 
         return current;
 
         return current;
−
     } catch (...) {
+
     }
−
         for (; first != current; ++first) {
+
    catch (...)
−
            first->~Value();
+
    {
−
        }
+
         std::destroy(first, current);
 
         throw;
 
         throw;
 
     }
 
     }
Line 61: Line 64:
  
 
===Example===
 
===Example===
−
{{example
+
{{example|code=
 +
#include <iostream>
 +
#include <memory>
 +
#include <string>
 +
 
 +
int main()
 +
{
 +
    struct S { std::string m{"Default value"}; };
 +
 
 +
    constexpr int n{3};
 +
    alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
 +
 
 +
    try
 +
    {
 +
        auto first{reinterpret_cast<S*>(mem)};
 +
        auto last = std::uninitialized_value_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_value_construct_n
 +
    // zero-initializes the given uninitialized memory area.
 +
    int v[]{1, 2, 3, 4};
 +
    for (const int i : v)
 +
        std::cout << i << ' ';
 +
    std::cout << '\n';
 +
    std::uninitialized_value_construct_n(std::begin(v), std::size(v));
 +
    for (const int i : v)
 +
        std::cout << i << ' ';
 +
    std::cout << '\n';
 +
}
 +
|output=
 +
Default value
 +
Default value
 +
Default value
 +
1 2 3 4
 +
0 0 0 0
 
}}
 
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=3870|std=C++20|before=this algorithm might create objects on a {{c/core|const}} storage|after=kept disallowed}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/memory/dsc uninitialized_value_construct}}
+
{{dsc inc|cpp/memory/dsc uninitialized_value_construct}}
−
{{dsc inc | cpp/memory/dsc uninitialized_default_construct_n}}
+
{{dsc inc|cpp/memory/dsc uninitialized_default_construct_n}}
 +
{{dsc inc|cpp/memory/ranges/dsc uninitialized_value_construct_n}}
 
{{dsc end}}
 
{{dsc end}}
  
−
{{langlinks|zh}}
+
{{langlinks|es|ja|ru|zh}}

Latest revision as of 18:59, 25 July 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Dynamic memory management
Uninitialized memory algorithms
(C++11)
(C++17)
(C++17)
uninitialized_value_construct_n
(C++17)
(C++17)
(C++20)

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 ForwardIt, class Size >
ForwardIt uninitialized_value_construct_n( ForwardIt first, Size n );
 (1) (since C++17)
template< class ExecutionPolicy, class ForwardIt, class Size >

ForwardIt uninitialized_value_construct_n( ExecutionPolicy&& policy,

                                           ForwardIt first, Size n );
 (2) (since C++17)
1) Constructs n objects of type typename iterator_traits<ForwardIt>::value_type in the uninitialized storage starting at first by value-initialization, as if by for (; n > 0; (void) ++first, --n)
    ::new (static_cast<void*>(std::addressof(*first)))
        typename std::iterator_traits<ForwardIt>::value_type();
If an exception is thrown during the initialization, 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

[edit] Parameters

first - the beginning of the range of elements to initialize
n - the number of elements to initialize
policy - the execution policy to use. See execution policy for details.
Type requirements
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.

[edit] Return value

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

[edit] Complexity

Linear in n.

[edit] 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.

[edit] Possible implementation

template<class ForwardIt, class Size>
ForwardIt uninitialized_value_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;
    }
}

[edit] Example

Run this code
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    struct S { std::string m{"Default value"}; };
 
    constexpr int n{3};
    alignas(alignof(S)) unsigned char mem[n * sizeof(S)];
 
    try
    {
        auto first{reinterpret_cast<S*>(mem)};
        auto last = std::uninitialized_value_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_value_construct_n
    // zero-initializes the given uninitialized memory area.
    int v[]{1, 2, 3, 4};
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
    std::uninitialized_value_construct_n(std::begin(v), std::size(v));
    for (const int i : v)
        std::cout << i << ' ';
    std::cout << '\n';
}

Output: 

Default value
Default value
Default value
1 2 3 4
0 0 0 0

[edit] Defect reports

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

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

[edit] See also

(C++17)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(function template) [edit]
(C++17)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count
(function template) [edit]
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(niebloid)[edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/uninitialized_value_construct_n&oldid=155423"