Is it safe to cast to a class that has the same data member layout, but a different implementation?

Question

The first class will be used for private inheritance in order to ensure the exact same layout. This should make casting safe.

#include 
#include          


        

Answer 1

From [expr.reinterpret.cast]/11 in the language spec, you can cast a reference from one type to another (if you can cast a pointer to one to the other).

With your class layouts, both types have a common base class that holds all the data. The two derived types do not add any data members, nor do they add any virtual functions, so the object layout for both classes will be the same.

So the usage is safe if you use reinterpret_cast.

In this case, this is similar to casting to a reference the base class, then casting that reference to the other derived class.

Answer 2

It's more or less what's described here, the so called boost mutant idiom.

There it is said that (emphasis mine):

Boost mutant idiom makes use of reinterpret_cast and depends heavily on assumption that the memory layouts of two different structures with identical data members (types and order) are interchangeable. Although the C++ standard does not guarantee this property, virtually all the compilers satisfy it. Moreover, the mutant idiom is standard if only POD types are used.

---

Note: that page is pretty outdated, I don't know if the most recent revisions changed something about the guarantees above mentioned.