可能吗？ std :: vector< double> my_vec（sz）;已分配但未初始化或填充的 [英] Is it possible? std::vector<double> my_vec(sz); which is allocated but not initialized or filled
问题描述
在[ C ++ 11和std :: vector构造函数，Channel72问，
At [Value-Initialized Objects in C++11 and std::vector constructor, Channel72 asks,
问题：我的理解在这里正确吗？如果T是POD，显式std :: vector（size_type count）是否提供未初始化的数组（类似于malloc）？
答案是否定的。
我的问题是，好吧，那是什么？
My question is, "Okay then, what does?"
其中之一是内文（Nevin）暗示要回答我的问题。为了澄清，我的问题是，是否可以使用std :: vector< double>而不用零或其他东西无偿地填充分配的内存？
One of the responses, by Nevin, hints at answering my question. To clarify, my question is, Is there a way to use std::vector<double> without it gratuitously filling allocated memory with zeros or whatever?
我不要求解决方法，例如以零大小启动向量并使用push_back（）。这并非总是可能的，此外，在这一点上，我想弄清楚它的原因除了我想弄清楚之外。
I am not asking for workarounds, like starting the vector at zero size and using push_back(). That is not always possible, and besides, at this point I want to get it figured out for no other reason than I want to get it figured out.
Nevin的建议，一个自定义分配器，进行编译。 VC ++ 2017rc（Dinkum）以其通常难以理解的方式抱怨。关于std :: _ Wrap_alloc的信息。 Nevin的代码不完整，我可能不知道如何完成它。在看到他之前，我写了我自己的自定义分配器，该分配器似乎可以正常工作，但是我对自己的理解不够自信，无法发誓。
I cannot get Nevin's suggestion, a custom allocator, to compile. VC++ 2017rc (Dinkum) complains in its usual inscrutable way. Something about std::_Wrap_alloc. Nevin's code is incomplete, and I probably do not know how to complete it. Before I saw his, I wrote my own custom allocator which seems to work, but I am not confident in my understanding enough to swear by it.
花了很多钱，我本来可以写出条条框框的std :: vector替代品，再加上《伟大的美国小说》的几章。
For the time I have spent puzzling over this, I could have written a less dogmatic replacement for std::vector, plus several chapters of the Great American Novel.
推荐答案
万岁！理查德·克里特（Richard Critten）营救！他对这个问题的评论直接引出了答案。
HOORAY! Richard Critten to the rescue! His comment under the question leads directly to the answer.
零位元凶是默认的分配器模板，即std :: allocator。因此，我们替换它，或使用分配器适配器对其进行修改。
The zero-spewing culprit is the default allocator template, namely std::allocator. So we replace it, or modify it with an allocator adapter.
我整理了一下代码，并扩展了注释。 Bill，请随时发布更全面的答案。但是下面的代码很好地完成了这个技巧。
I tidied up the code a little, and expanded the comments. Bill, please feel free to post a more comprehensive answer. But the following does the trick very nicely.
// Allocator adapter
// Given an allocator A, (std::allocator by default), this adapter
// will, when feasible, override A::construct() with a version that
// employs default construction rather than value-initialization.
// "Feasible" means the object (U *ptr) is default-constructable and
// the default constructor cannot throw exceptions.
//
// Thus it thwarts gratuitous initializations to zeros or whatever.
template <typename T, typename A = std::allocator<T>>
class default_init_allocator : public A {
typedef std::allocator_traits<A> a_t;
public:
// http://en.cppreference.com/w/cpp/language/using_declaration
using A::A; // Inherit constructors from A
template <typename U> struct rebind {
using other =
default_init_allocator
< U, typename a_t::template rebind_alloc<U> >;
};
template <typename U>
void construct(U* ptr)
noexcept(std::is_nothrow_default_constructible<U>::value) {
::new(static_cast<void*>(ptr)) U;
}
template <typename U, typename...Args>
void construct(U* ptr, Args&&... args) {
a_t::construct(static_cast<A&>(*this),
ptr, std::forward<Args>(args)...);
}
};
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