新的展示位置如何知道要创建哪个布局? [英] How does placement new know which layout to create?

问题描述

#include <iostream>
#include <typeinfo>
struct A { int a; };
struct B : virtual A { int b; };
struct C : virtual A  { int c; };
struct D : B,C { int d; };
int main()
{
  D complete;
  B contiguous;
  B & separate = complete;
  B * p[2] = {&separate, &contiguous};
  // two possible layouts for B: 
  std::cout<< (int)((char*)(void*) &p[0]->a -(char*)(void*)&p[0]->b)<<" "<< sizeof(*p[0])<< "\n";
  std::cout<< (int)((char*)(void*) &p[1]->a -(char*)(void*)&p[1]->b)<<" "<< sizeof(*p[1])<< "\n";

  alignas(B) char buff[sizeof(B)];
  void * storage = static_cast<void*>(buff);
  // new expression skips allocation function:
  auto pointer= new (storage) B;        // Which layout to create?
  std::cout << typeid(pointer).name()<<"\n"; 
  pointer->~B();    // Destructor knows layout through typed pointer.
}
// sample output (Debian 8, amd64):
// 24 16
// 4 16
// P1B

C ++ 14标准中是否有一个部分要求新建"以创建特定的布局?是否有保证 新创建的布局适合大小为sizeof(B)且偏移量为零的缓冲区吗?

Is there a section in the C++14 standard that reqires 'new' to create a particular layout? Is there guarantee that the layout created by new fits into a buffer of size sizeof(B) and with offset zero?

您能使用grep友好的术语还是提供参考?我在问题中添加了对标准的引用.

edit: Could you please use grep-friendly terminology or provide references? I added a reference to the standard to the question.

请考虑以上示例输出:数字24告诉您什么?缓冲区的大小是多少?

Take into consideration the sample output above: What does the number 24 tell you? What is the size of the buffer?

标准中可能有一个说法，即派生程度最高的对象始终是对象表示形式的直接且连续的副本， 但我还没有找到.

There might be a statement in the standard that a most derived object is always a straightforward, contiguous copy of the object representation, but I haven't found this one.

我们对new的了解是，它将与完整的对象类型一起使用. [expr.new]

The thing we know about new is that it shall be used with a complete object type. [expr.new]

在[class.dtor]§12.4(14)中有一个带有放置选项的new表达式示例.但是，该示例可能仅因为其中的类是标准布局而起作用.

There is an example for a new-expression with the placement option in [class.dtor] §12.4 (14). However, the example might work simply because the class therein is standard-layout.

推荐答案

哪里可以保证新创建的布局适合大小为sizeof(B)且偏移量为零的缓冲区

从new中命名的类型开始，其参数为B.正在生成B，而不是D.类型B关于D一无所知". D的声明对B没有影响；可以将B声明放入不会出现D的翻译单元中，但是程序中的每个地方都会对B的大小和布局达成一致，无论是否也知道D在那些地方.

From the the type being named in new as its argument being B. A B is being made, not an D. The type B "knows nothing" about D. The declaration of D has no influence on B; the B declaration can be put into translation units in which D doesn't appear, yet everywhere in the program there will be agreement about the size of B and is layout, regardless of whether D is also known in those places or not.

类型为T的C ++对象的大小为sizeof T.这意味着它适合sizeof T个字节；表示(sizeof T) + k字节不是必需的，其中k > 0.

A C++ object of type T has a size sizeof T. This means that it fits into sizeof T bytes; it cannot be the case that (sizeof T) + k bytes are required for its representation, where k > 0.

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