Safety of casting between pointers of two identical classes?

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C++11 added a concept called layout-compatible which applies here.

Two standard-layout struct (Clause 9) types are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in declaration order) have layout-compatible types (3.9).

where

A standard-layout class is a class that:

  • has no non-static data members of type non-standard-layout class (or array of such types) or reference,
  • has no virtual functions (10.3) and no virtual base classes (10.1),
  • has the same access control (Clause 11) for all non-static data members,
  • has no non-standard-layout base classes,
  • either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and
  • has no base classes of the same type as the first non-static data member.

A standard-layout struct is a standard-layout class defined with the class-key struct or the class-key class.

A standard-layout union is a standard-layout class defined with the class-key union.

Finally

Pointers to cv-qualified and cv-unqualified versions (3.9.3) of layout-compatible
types shall have the same value representation and alignment requirements (3.11).

Which guarantees that reinterpret_cast can turn a pointer to one type into a pointer to any layout-compatible type.

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