When is a type in c++11 allowed to be memcpyed?

Question

My question is the following:

If I want to copy a class type, memcpy can do it very fast. This is allowed in some situations.

We have some type traits:

Answer 1

Objects with trivial copy constructors, trivial copy assignment operators and trivial destructors can be copied with memcpy or memmove

The requirements for a special member function of a class T to be trivial are

Copy constructors (cc) and copy assignment operators (ca)

• Not being user-provided (meaning, it is implicitly-defined or defaulted), and if it is defaulted, its signature is the same as implicitly-defined
• T has no virtual member functions
• T has no virtual base classes
• The cc/ca selected for every direct base of T is trivial
• The cc/ca selected for every non-static class type (or array of class type) memeber of T is trivial
• T has no non-static data members of volatile-qualified type ^{(since C++14)}

Destructors

• Not being user-provided (meaning, it is implicitly-defined or defaulted)
• Not being virtual (that is, the base class destructor is not virtual)
• All direct base classes have trivial destructors
• All non-static data members of class type (or array of class type) have trivial destructors

Just declaring the function as = default doesn’t make it trivial (it will only be trivial if the class also supports all the other criteria for the corresponding function to be trivial) but explicitly writing the function in user code does prevent it from being trivial. Also all data types compatible with the C language (POD types) are trivially copyable.

^{Source : C++ Concurrency in action and cppreference.com}

Answer 2

You can copy an object of type T using memcpy when is_trivially_copyable<T>::value is true. There is no particular need for the type to be a standard layout type. The definition of 'trivially copyable' is essentially that it's safe to do this.

An example of a class that is safe to copy with memcpy but which is not standard layout:

struct T {
  int i;
private:
  int j;
};

Because this class uses different access control for different non-static data members it is not standard layout, but it is still trivially copyable.

Answer 3

From http://en.cppreference.com/w/cpp/types/is_trivially_copyable:

Objects of trivially-copyable types are the only C++ objects that may be safely copied with std::memcpy or serialized to/from binary files with std::ofstream::write()/std::ifstream::read(). In general, a trivially copyable type is any type for which the underlying bytes can be copied to an array of char or unsigned char and into a new object of the same type, and the resulting object would have the same value as the original.

Answer 4

If is_trivally_copyable<T>::value (or in C++14 is_trivially_copyable<T>(), or in C++17 is_trivially_copyable_v<T>) is not zero, the type is copyable using memcpy.

Per the C++ standard, a type being trivially copyable means:

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.

However, it is important to realise that pointers are trivially copyable types, too. Whenever there are pointer inside the data structures you will be copying, you have to brainually make sure that copying them around is proper.

Examples where hazard may be caused by just relying on the object being trivially copyable:

• A tree-structure implementation where your data is placed in a contiguous region of memory, but with nodes storing absolute addresses to child nodes
• Creating multiple instances of some data for sake of multithreading performance (in order to reduce cache crashes), with owning pointers inside, pointing anywhere
• You have a flat object without pointers, but with an embedded third party structure inside. The third party structure at some point in the future includes a pointer that should not exist twice or more.

So whenever memcopying, keep in mind to check whether pointers could be copied in that specific case, and if that would be okay.

Realise that is_trivially_copyable is only the "Syntax Check", not the "Semantic Test", in compiler parlance.

Answer 5

What I understood is

• An Object should have default constructor /destructor.
• Default Copy and Move Operations.
• No static and Virtual Functions has multiple
• access specifiers for non-static data members prevents important
• layout optimizations has a non-static member or a base that is not standard layout.

you can test if given type is pod (Plain Old Data) by using standard function is_pod::value

Reference: The C++ programming Language 4th edition