Standard-layout types can be used to communicate with code written in other programming languages, as the layout of the type is strictly specified. The C++ Standard, [class], paragraph 7 [ISO/IEC 14882-2014], defines a standard-layout classes class as a class that
- Does does not have virtual functions,
- Has has the same access control for all nonstatic data members,
- Has has no base classes of the same type as the first nonstatic data member,
- Has has nonstatic data members declared in only one class within the class hierarchy, and
- Recursivelyrecursively, does not have nonstatic data members of nonstandard-layout type.
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This noncompliant code example assumes that there is a library whose header is library.h, and an application (represented by application.cpp), and that the library and application are not ABI-compatible. Therefore, the contents of library.h constitute an execution boundary. A nonstandard-layout type object S is passed across this execution boundary. The application creates an instance of an object of this type, then passes a reference to the object to a function defined by the library, crossing the execution boundary. Because the layout is not guaranteed to be compatible across the boundary, this results in unexpected behavior.
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