c++ - Which rules determine whether an object is trivially copyable

Question

With the introduction of c++11, trivially copyableness has gotten quite relevant. Most notably in the use of 'std::atomic'. The basics are quite simple. A class foo is trivially copyable if:

foo* src = new foo();
foo* dest = malloc(sizeof(foo));
memcpy(dest, src, sizeof(foo));

Has the same effect as:

foo* src = new foo();
foo* dest = new foo(src);

So an object where copying the memory will have the same effect as a copy constructor. However there, of course, is a catch. There's not only copy constructors. But also move constructors, move assignment operators. Etc.

std::is_trivially_copyable can be used to test whether an object is trivially copyable. So with trial and error it is possible to make an object trivially copyable.

But of course a well defined set of rules would be a bit nicer:). So hereby my request.

Answer 1

The most well-defined set of rules would come directly from the standard. Here are the relevant entries from standard draft N4296:

Trivially-copyable types are defined in [basic.types]/9

Cv-unqualified scalar types, trivially copyable class types, arrays of such types, and nonvolatile const-qualified versions of these types are collectively called trivially copyable types.

Trivially-copyable classes are defined in [class]/6

A trivially copyable class is a class that: 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.

Copy/move constructors in [class.copy]/12

A copy/move constructor for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if class X has no virtual functions and no virtual base classes, and class X has no non-static data members of volatile-qualified type, and the constructor selected to copy/move each direct base class subobject is trivial, and for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is trivial; otherwise the copy/move constructor is non-trivial.

Copy/move assignment operators in [class.copy]/25

A copy/move assignment operator for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if class X has no virtual functions and no virtual base classes, and class X has no non-static data members of volatile-qualified type, and the assignment operator selected to copy/move each direct base class subobject is trivial, and for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial; otherwise the copy/move assignment operator is non-trivial.

Destructors in [class.dtor]/5

A destructor is trivial if it is not user-provided and if: the destructor is not virtual, all of the direct base classes of its class have trivial destructors, for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor. Otherwise, the destructor is non-trivial.

User-provided constructors in [dcl.fct.def.default]/5

Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them (12.1 12.4, 12.8), which might mean defining them as deleted. A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed.

The short answer is that the short answer is sometimes more helpful than the long answer.