Namespaces
Variants
Views
Actions

Difference between revisions of "cpp/memory/ranges/uninitialized default construct"

From cppreference.com
< cpp‎ | memory
(LWG3870)
m ({{c}})
Line 26: Line 26:
 
@@ If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
 
@@ If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
  
−
@2@ Same as {{v|1}}, but uses {{tt|r}} as the range, as if using {{c|ranges::begin(r)}} as {{c|first}}, and {{c|ranges::end(r)}} as {{c|last}}.
+
@2@ Same as {{v|1}}, but uses {{c|r}} as the range, as if using {{c|ranges::begin(r)}} as {{c|first}}, and {{c|ranges::end(r)}} as {{c|last}}.
  
 
{{cpp/ranges/niebloid}}
 
{{cpp/ranges/niebloid}}
Line 88: Line 88:
  
 
===Example===
 
===Example===
−
{{example|code=
+
{{example
 +
|code=
 
#include <cstring>
 
#include <cstring>
 
#include <iostream>
 
#include <iostream>
Line 96: Line 97:
 
int main()
 
int main()
 
{
 
{
−
     struct S { std::string m{ "▄▀▄▀▄▀▄▀" }; };
+
     struct S { std::string m{"▄▀▄▀▄▀▄▀"}; };
  
 
     constexpr int n{4};
 
     constexpr int n{4};

Revision as of 23:46, 6 October 2023

 
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>
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S >

requires std::default_initializable<std::iter_value_t<I>>

I uninitialized_default_construct( I first, S last );
 (1) (since C++20)
template< no-throw-forward-range R >

requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>

uninitialized_default_construct( R&& r );
 (2) (since C++20)
1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage designated by the range [first, last) by default-initialization, as if by 
for (; first != last; ++first)
    ::new (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<I>>;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

Parameters

first, last - iterator-sentinel pair denoting the range of the elements to initialize
r - the range of the elements to initialize

Return value

An iterator equal to last.

Complexity

Linear in the distance between first and last.

Exceptions

The exception thrown on construction of the elements in the destination range, if any.

Notes

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible_v.

Possible implementation

struct uninitialized_default_construct_fn
{
    template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S>
    requires std::default_initializable<std::iter_value_t<I>>
    I operator()(I first, S last) const
    {
        using ValueType = std::remove_reference_t<std::iter_reference_t<I>>;
        if constexpr (std::is_trivially_default_constructible_v<ValueType>)
            return ranges::next(first, last); // skip initialization
        I rollback{first};
        try
        {
            for (; !(first == last); ++first)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*first)))) ValueType;
            return first;
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
 
    template<no-throw-forward-range R>
    requires std::default_initializable<ranges::range_value_t<R>>
    ranges::borrowed_iterator_t<R>
    operator()(R&& r) const
    {
        return (*this)(ranges::begin(r), ranges::end(r));
    }
};
 
inline constexpr uninitialized_default_construct_fn uninitialized_default_construct{};

Example

Run this code
#include <cstring>
#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    struct S { std::string m{"▄▀▄▀▄▀▄▀"}; };
 
    constexpr int n{4};
    alignas(alignof(S)) char out[n * sizeof(S)];
 
    try
    {
        auto first{reinterpret_cast<S*>(out)};
        auto last{first + n};
 
        std::ranges::uninitialized_default_construct(first, last);
 
        auto count{1};
        for (auto it{first}; it != last; ++it)
            std::cout << count++ << ' ' << it->m << '\n';
 
        std::ranges::destroy(first, last);
    }
    catch (...) { std::cout << "Exception!\n"; }
 
    // Notice that for "trivial types" the uninitialized_default_construct
    // generally does not zero-fill the given uninitialized memory area.
    constexpr char etalon[]{'A', 'B', 'C', 'D', '\n'};
    char v[]{'A', 'B', 'C', 'D', '\n'};
    std::ranges::uninitialized_default_construct(std::begin(v), std::end(v));
    if (std::memcmp(v, etalon, sizeof(v)) == 0)
    {
        std::cout << "  ";
        // Maybe undefined behavior, pending CWG 1997:
        // for (const char c : v) { std::cout << c << ' '; }
        for (const char c : etalon)
            std::cout << c << ' ';
    }
    else
        std::cout << "Unspecified\n";
}

Possible output: 

1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
  A B C D

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

See also

(C++20)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count
(niebloid)[edit]
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(niebloid)[edit]
(C++20)
 constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(niebloid)[edit]
(C++17)
 constructs objects by default-initialization in an uninitialized area of memory, defined by a range
(function template) [edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/ranges/uninitialized_default_construct&oldid=160250"