Converting C source to C++

Question

How would you go about converting a reasonably large (>300K), fairly mature C codebase to C++?

The kind of C I have in mind is split into files roughly corresponding to

Answer 1

What about:

1. Compiling everything in C++'s C subset and get that working, and
2. Implementing a set of facades leaving the C code unaltered?

Why is "translation into C++ mandatory"? You can wrap the C code without the pain of converting it into huge classes and so on.