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.

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Answer 1

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.

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Note: that page is pretty outdated, I don't know if the most recent revisions changed something about the guarantees above mentioned.