allocator.construct 循环是否等于 std::uninitialized_copy？

发布于 2024-11-07 21:45:59 字数 877 浏览 4 评论 0原文

在此上下文中，T 是某种类型，allocator 是该类型的分配器对象。默认情况下它是 std::allocator但这不一定是真的。

我有一块由 allocator.allocate(n) 获取的内存。我还有一个包含 T 对象的容器 con（例如，std::vector）。我想用 T 对象初始化该内存块。

内存块的位置存储在T* data中。

这两个代码示例总是相同吗？

#include <memory>

// example 1
std::uninitialized_copy(con.begin(), con.end(), data)

// example 2
std::vector<T>::const_iterator in = con.begin();
for (T* out = data; in != con.end(); ++out, ++in) {
    allocator.construct(out, *in);
}

而对于这两个人呢？

#include <memory>

T val = T(); // could be any T value

// example 3
std::uninitialized_fill(data, data + n, val)

// example 4
for (T* out = data; out != (data + n); ++out) {
    allocator.construct(out, val);
}

原文

In this context T is a certain type and allocator is an allocator object for that type. By default it is std::allocator<T> but this is not necessarily true.

I have a chunk of memory acquired by allocator.allocate(n). I also have a container con of T objects (say, a std::vector<T>). I want to initialize that chunk of memory with the T object(s).

The location of the chunk of memory is stored in T* data.

Are these two code examples always identical?

#include <memory>

// example 1
std::uninitialized_copy(con.begin(), con.end(), data)

// example 2
std::vector<T>::const_iterator in = con.begin();
for (T* out = data; in != con.end(); ++out, ++in) {
    allocator.construct(out, *in);
}

And for these two?

#include <memory>

T val = T(); // could be any T value

// example 3
std::uninitialized_fill(data, data + n, val)

// example 4
for (T* out = data; out != (data + n); ++out) {
    allocator.construct(out, val);
}

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三人与歌 2024-11-14 21:45:59

根据这个解释他们应该做同样的事情，如 allocator::construct 据说构造了对象，而 std::uninitialized... 也构造了对象。但我不知道，在实现您自己的 allocator::construct 时，标准到底说了些什么以及您有什么自由。

编辑：好的，C++03 标准在第 20.1.5 §2 表 32 中规定，construct(p,t) 应该具有与 new ((void*)p) T(t) （对于任何符合标准的分配器，不仅是 std::allocator）。在 20.4.4.1 §1 中，uninitialized_copy 应该与

for (; first != last; ++result, ++first)
    new (static_cast<void*>(&*result))
            typename iterator_traits<ForwardIterator>::value_type(*first);

20.4.4.2 §1 中的 uninitialized_fill 具有相同的效果，

for (; first != last; ++first)
    new (static_cast<void*>(&*first))
            typename iterator_traits<ForwardIterator>::value_type(x);

所以我认为这不会不要给他们留下任何不同行为的空间。所以回答你的问题：是的，确实如此。

According to this explanations They should do the same, as allocator::construct is said to construct the object and std::uninitialized... also constructs the objects. But I do not know, what exactly the standard says and what freedom you have, when implementing your own allocator::construct.

EDIT: Ok, the C++03 standard states in section 20.1.5 §2 table 32, that construct(p,t) should have the same effect as new ((void*)p) T(t) (for any standard compliant allocator, not only std::allocator). And in 20.4.4.1 §1, that uninitialized_copy should have the same effect as

for (; first != last; ++result, ++first)
    new (static_cast<void*>(&*result))
            typename iterator_traits<ForwardIterator>::value_type(*first);

and in 20.4.4.2 §1, that uninitialized_fill has an effect of

for (; first != last; ++first)
    new (static_cast<void*>(&*first))
            typename iterator_traits<ForwardIterator>::value_type(x);

So I think that doesn't leave any room for them to behave differently. So to answer your question: yes, it does.

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始终不够 2024-11-14 21:45:59

C++ 入门第 5 章 §13.6.2：

.....
uninitialized_copy 对输入中的每个元素调用 construct
将该元素“复制”到目标的序列。那个算法
使用迭代器解引用运算符从
输入序列。因为我们传递了移动迭代器，所以取消引用
运算符产生一个右值引用，这意味着 construct 将使用
用于构造元素的移动构造函数。
......