Link compatibility of enums and enum classes

Question

Suppose there is a C++11 API that uses enum classes:

// api.hpp
enum class E {A, B, C};
void f(E);
...

// api.cpp
void f(E e)
{
    if (e == E::A)
       ...
}

Now suppose I would like to use this API, but I don't have a C++11 compiler. So I:

• Modify api.hpp and change the enum class to be just a regular enum.
• Write some code code that includes the modified api.hpp and uses the API normally (e.g. calls f).
• Compile this code with my non-C++11 compiler and link it to the API implementation which was compiled with a C++11 compiler (using the unmodified api.hpp).

This seems to work with GCC, but is it safe in general, or am I playing with fire (ODR violations and such)?

Assume the two compilers are otherwise link-compatible, it is only the enum vs. enum class that is at issue.

Answer 1

Like ildjarn is saying, this is undefined behavior. And the reason that this actually can fail on real implementations is that normal C++03 enums don't have a fixed underlying type. While your enum-class type always has "int" as its underlying type, a corresponding C++03 enum could have "short" as its underlying type, making the code not be layout compatible.

Answer 2

You would be violating the One Definition Rule (§3.2/5). Result: undefined behavior.