memcpy 是否保留了一个普通对象的有效性？

Question

如果有一个普通类型的有效对象（在这种情况下，普通类型满足普通 move/copy 可构造的概念），并且 memcpy 将其放入未初始化内存区域, 复制的内存区域是有效对象吗？

根据我读到的假设：一个对象只有在它的构造函数被调用时才有效。

Answer 1

将具有 std::memcpy 的普通类型的对象复制到适当大小和对齐的存储中，将隐式地在该位置开始新对象的生命周期。

有一类名为 implicit-lifetime type 的类型，其要求是：

• a scalar type, or
• an array type, or
• an aggregate class type, or
• a class type that has
  • at least one trivial eligible constructor, and
  • a trivial, non-deleted destructor,
• or a cv-qualified version of one of above types.

Trivial class types满足这些要求。

implicit-lifetime 类型的对象具有 属性，their lifetime can be started implicitly 是多个函数或操作：

• operations that begin lifetime of an array of type char, unsigned char, or std::byte, (since C++17) in which case such objects are created in the array,
• call to following allocating functions, in which case such objects are
  • created in the allocated storage:
  • operator new
  • operator new[]
  • std::malloc
  • std::calloc
  • std::realloc
  • std::aligned_alloc (since C++17)
• call to following object representation copying functions, in which case such objects are created in the destination region of storage or the result:
  • std::memcpy
  • std::memmove
  • std::bit_cast (since C++20)