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

Latest revision as of 21:48, 24 July 2023

Defined in header `<memory>`
template< class ForwardIt, class Size, class T > ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value );	(1)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value );	(2)	(since C++17)

1) Copies the given value value to the first count elements in an uninitialized memory area beginning at first as if by for (; n--; ++first)
::new (/* VOIDIFY */(*first))
typename std::iterator_traits<ForwardIt>::value_type(value);

where /* VOIDIFY */(e) is:

static_cast<void*>(&e)	(until C++11)
static_cast<void*>(std::addressof(e))	(since C++11)

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)

first	-	the beginning of the range of the elements to initialize
count	-	number of elements to construct
value	-	the value to construct the elements with
Type requirements
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of `ForwardIt` may throw exceptions. Applying &* to a `ForwardIt` value must yield a pointer to its value type.(until C++11)

[edit] Return value

Iterator to the element past the last element copied.

[edit] Complexity

Linear in count.

[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, class T>
ForwardIt uninitialized_fill_n(ForwardIt first, Size count, const T& value)
{
    using V = typename std::iterator_traits<ForwardIt>::value_type;
    ForwardIt current = first;
    try
    {
        for (; count > 0; ++current, (void) --count)
            ::new (static_cast<void*>(std::addressof(*current))) V(value);
        return current;
    }
    catch (...)
    {
        for (; first != current; ++first)
            first->~V();
        throw;
    }
}

[edit] Example

Run this code

#include <algorithm>
#include <iostream>
#include <memory>
#include <string>
#include <tuple>
 
int main()
{
    std::string* p;
    std::size_t sz;
    std::tie(p, sz) = std::get_temporary_buffer<std::string>(4);
    std::uninitialized_fill_n(p, sz, "Example");
 
    for (std::string* i = p; i != p + sz; ++i)
    {
        std::cout << *i << '\n';
        i->~basic_string<char>();
    }
    std::return_temporary_buffer(p);
}

Output:

Example
Example
Example
Example

[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 866	C++98	given `T` as the value type of `ForwardIt`, if T::operator new exists, the program might be ill-formed	uses global replacement- new instead
LWG 1339	C++98	the location of the first element following the filling range was not returned	returned
LWG 2433	C++11	this algorithm might be hijacked by overloaded operator&	uses std::addressof
LWG 3870	C++20	this algorithm might create objects on a const storage	kept disallowed

[edit] See also

uninitialized_fill	copies an object to an uninitialized area of memory, defined by a range (function template) [edit]
ranges::uninitialized_fill_n (C++20)	copies an object to an uninitialized area of memory, defined by a start and a count (niebloid)[edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|uninitialized_fill_n}}
-{{cpp/memory/sidebar}}
+{{cpp/memory/navbar}}
-{{ddcl | header=memory | notes={{mark c++11 feature}} |
+{{dcl begin}}
-template< class ForwardIterator, class Size, class T >
+{{dcl header|memory}}
-void uninitialized_fill_n( ForwardIterator first, Size count
+{{dcl|num=1|1=
-                           const T& value )
+template< class ForwardIt, class Size, class T >
+ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value );
 }}
+{{dcl|since=c++17|num=2|1=
+template< class ExecutionPolicy, class ForwardIt, class Size, class T >
+ForwardIt uninitialized_fill_n( ExecutionPolicy&& policy,
+                                ForwardIt first, Size count, const T& value );
+}}
+{{dcl end}}
+@1@ Copies the given value {{c|value}} to the first {{c|count}} elements in an uninitialized memory area beginning at {{c|first}} as if by
+{{c multi|
+for (; n--; ++first)|
+    ::new (/* VOIDIFY */(*first))|
+        typename std::iterator_traits<ForwardIt>::value_type(value);
+}}
+@@ {{cpp/memory/uninitialized memory voidify|c++98=yes}}
+@@ If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
-Copies the given value {{tt|value}} to the first {{tt|count}} elements in an uninitialized memory area beginning at {{tt|first}}. The elements in the uninitialized area are constructed using copy constructor.
+@2@ Same as {{v|1}}, but executed according to {{c|policy}}. {{cpp/algorithm/parallel overload precondition}}
 ===Parameters===
-{{param list begin}}
+{{par begin}}
-{{param list item | first | the beginning of the range of the elements to initialize}}
+{{par|first|the beginning of the range of the elements to initialize}}
-{{param list item | count | number of elements to construct}}
+{{par|count|number of elements to construct}}
-{{param list item | value | the value to construct the elements with}}
+{{par|value|the value to construct the elements with}}
-{{param list end}}
+{{par hreq}}
+{{par req named|ForwardIt|ForwardIterator}}
+{{par req|No increment, assignment, comparison, or indirection through valid instances of {{tt|ForwardIt}} may throw exceptions. {{rev inl|until=c++11|Applying {{c|&*}} to a {{tt|ForwardIt}} value must yield a pointer to its value type.}}}}
+{{par end}}
 ===Return value===
-iterator to the element past the last element copied.
+Iterator to the element past the last element copied.
 ===Complexity===
-linear in {{tt|count}}
+Linear in {{c|count}}.
-===Equivalent function===
+===Exceptions===
-{{eq fun cpp | 1=
+{{cpp/algorithm/parallel_exceptions_reporting_behavior|singular=yes}}
-template< class ForwardIterator, class Size, class T >
-void uninitialized_fill_n(ForwardIterator first, Size count
+===Possible implementation===
-                          const T& value)
+{{eq fun|1=
+template<class ForwardIt, class Size, class T>
+ForwardIt uninitialized_fill_n(ForwardIt first, Size count, const T& value)
 {
-     typedef typename std::iterator_traits<ForwardIterator>::value_type Value;
+     using V = typename std::iterator_traits<ForwardIt>::value_type;
-     for (; count > 0; ++first, --count) {
+     ForwardIt current = first;
-        ::new (static_cast<void*>(&*first)) Value(value);
+    try
+    {
+        for (; count > 0; ++current, (void) --count)
+            ::new (static_cast<void*>(std::addressof(*current))) V(value);
+        return current;
+    }
+    catch (...)
+    {
+        for (; first != current; ++first)
+            first->~V();
+        throw;
      }
-    return first;
 }
 }}
 ===Example===
-{{example cpp
+{{example
- | code=
+|code=
- | output=
+#include <algorithm>
+#include <iostream>
+#include <memory>
+#include <string>
+#include <tuple>
+int main()
+{
+    std::string* p;
+    std::size_t sz;
+    std::tie(p, sz) = std::get_temporary_buffer<std::string>(4);
+    std::uninitialized_fill_n(p, sz, "Example");
+    for (std::string* i = p; i != p + sz; ++i)
+    {
+        std::cout << *i << '\n';
+        i->~basic_string<char>();
+    }
+    std::return_temporary_buffer(p);
+}
+|output=
+Example
+Example
+Example
+Example
 }}
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=866|std=C++98|before=given {{tt|T}} as the value type of {{tt|ForwardIt}}, if<br>{{c/core|T::operator new}} exists, the program might be ill-formed|after=uses global replacement-<br>new instead}}
+{{dr list item|wg=lwg|dr=1339|std=C++98|before=the location of the first element following<br>the filling range was not returned|after=returned}}
+{{dr list item|wg=lwg|dr=2433|std=C++11|before=this algorithm might be hijacked by overloaded {{c|operator&}}|after=uses {{lc|std::addressof}}}}
+{{dr list item|wg=lwg|dr=3870|std=C++20|before=this algorithm might create objects on a {{c|const}} storage|after=kept disallowed}}
+{{dr list end}}
 ===See also===
-{{dcl list begin}}
+{{dsc begin}}
-{{dcl list template | cpp/memory/dcl list uninitialized_fill}}
+{{dsc inc|cpp/memory/dsc uninitialized_fill}}
-{{dcl list end}}
+{{dsc inc|cpp/memory/ranges/dsc uninitialized_fill_n}}
+{{dsc end}}
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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

Latest revision as of 21:48, 24 July 2023

Contents

[edit] Parameters