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.
>Solution :
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, orstd::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)