C ++ 11中的类型的字节字节副本？ [英] Byte-for-byte copies of types in C++11?

问题描述

C ++ 11标准保证逐字节复制对POD类型始终有效。

The C++11 standard guarantees that byte-for-byte copies are always valid for POD types. But what about certain trivial types?

Here's an example:

struct trivial
{
int x;
int y;
trivial（int i）：x（2 * i）{std :: cout< 建。 << std :: endl; }
};

struct trivial { int x; int y; trivial(int i) : x(2 * i) { std::cout << "Constructed." << std::endl; } };

如果我复制这个struct，byte-for-byte，即使它在技术上不是POD？

If I were to copy this struct, byte-for-byte, is it guaranteed to copy properly, even though it isn't technically a POD? When is the line drawn as to when it's not okay to byte-copy an object?

推荐答案

是什么意思？是的，保证正确复制。

Yes, it is guaranteed to copy properly.

引用FDIS，§3.9/ 2：

Quoting the FDIS, §3.9/2:

对于 trivially可复制类型 c c> T 的任何对象（基类子对象除外），无论对象是否包含类型 T 的有效值，组成对象的基础字节可以复制到 char 或 unsigned char 。如果将 char 或 unsigned char 数组的内容复制回对象，则对象将随后保留原始值。

For any object (other than a base-class subobject) of trivially copyable type T, whether or not the object holds a valid value of type T, the underlying bytes making up the object can be copied into an array of char or unsigned char. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value.

和§3.9/ 3：

对于任何平凡可复制类型 T ，如果两个指向 T to distinct T objects obj1 和 obj2 code> obj1 也不是 obj2 是一个基类子对象，如果底层字节组成 obj1 将被复制到 obj2 ， obj2 obj1 。

For any trivially copyable type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the underlying bytes making up obj1 are copied into obj2, obj2 shall subsequently hold the same value as obj1.

因此，你要求的要求是§3.9/ 9：

So the requirements you're asking about are, §3.9/9:

算术类型，枚举类型，指针类型，指向成员类型的指针， std :: nullptr_t ，这些类型的cv限定版本统称为标量类型。标量类型，POD类，这种类型的数组和这些类型的cv限定版本被统称为 POD类型。 标量类型，简单可复制类类型，此类类型的数组和这些类型的cv限定版本统称为平凡可复制类型 。

Arithmetic types, enumeration types, pointer types, pointer to member types, std::nullptr_t, and cv-qualified versions of these types are collectively called scalar types. Scalar types, POD classes, arrays of such types and cv-qualified versions of these types are collectively called POD types. Scalar types, trivially copyable class types, arrays of such types, and cv-qualified versions of these types are collectively called trivially copyable types.

和§9/ 6：

是一个类：

• 没有非平凡的复制构造函数，


• 没有非平凡的移动构造函数，


• 没有非平凡的复制赋值运算符，


• 没有非平凡的移动赋值运算符， / li>
  
• 有一个简单的析构函数。

• has no non-trivial copy constructors,
• has no non-trivial move constructors,
• has no non-trivial copy assignment operators,
• has no non-trivial move assignment operators, and
• has a trivial destructor.

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