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

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< cpp‎ | memory
m (Text replace - "dsc inc | cpp/experimental/parallelism/dsc algorithm" to "subst:User:T. Canens/null")
m (Missing "std::" on iterator_traits)
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for (; n--; ++first)
 
for (; n--; ++first)
 
   ::new (static_cast<void*>(std::addressof(*first)))
 
   ::new (static_cast<void*>(std::addressof(*first)))
−
     typename iterator_traits<ForwardIt>::value_type(x);
+
     typename std::iterator_traits<ForwardIt>::value_type(x);
 
}}
 
}}
  

Revision as of 01:36, 21 February 2017

 
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>
(1)
template< class ForwardIt, class Size, class T >
void uninitialized_fill_n( ForwardIt first, Size count, const T& value );
 (until C++11)
template< class ForwardIt, class Size, class T >
ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value );
 (since C++11)
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 (static_cast<void*>(std::addressof(*first)))
     typename std::iterator_traits<ForwardIt>::value_type(x);
If an exception is thrown during the initialization, the function has no effects.
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

Parameters

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

Template:par req concept

-
No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.

Return value

(none) (until C++11)

Iterator to the element past the last element copied.

(since C++11)

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

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

See also

 copies an object to an uninitialized area of memory, defined by a range
(function template) [edit]
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