AMDG Simonson, Lucanus J <lucanus.j.simonson <at> intel.com> writes:
The reason it is categorized as undefined is because Derived may extend Base by adding data members, inheriting from another base, or adding a virtual pointer.
Here are some other things which could break your code: // this is POD struct Base { char c; }; // This is not POD struct Derived : Base { int hidden_variable_added_by_the_compiler; }; Now the alignment of Derived is stricter than that of Base. The standard explicitly states that using a reinterpret_cast from Base to Derived is not guaranteed to be reversible. It also states that the result of a static_cast from Base to Derived is unspecified. Another example // This is POD struct Base { }; class Derived { thread_id_t hidden_variable_added_by_compiler; void f() { // code added by compiler // Try to determine whether the class Derived // may need to be thread safe. Note that more // sophisticated tests are possible, such as checking // whether some mutex is locked. if(hidden_variable_added_by_compiler != get_current_thread_id()) { if(hidden_variable_added_by_compiler == 0) { hidden_variable_added_by_compiler = get_current_thread_id(); } else { add_shared_type(typeid(Derived)); } } //use Base only } }; Now even though f doesn't appear to access any derived class members it really does. In Christ, Steven Watanabe