If one has a valid object of a trivial type (in this context, a trivial type satisfies the trivially move/copy constructible concepts), and one memcpys it to a region of uninitialised memory, is the copied region of memory a valid object?

Assumption from what I've read: An object is only valid if it's constructor has been called.

CodePudding user response：

Copying an object of a trivial type with std::memcpy into properly sized and aligned storage will implicitly begin the lifetime of a new object at that location.

There is a category of types called implicit-lifetime type whose requirements are :

• 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 meet these requiements.

Objects of implicit-lifetime type have the property that their lifetime can be started implicitly be several functions or operations :

• 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)