In rare cases valid C code is not valid C++ code.
The first example are keywords that have a special purpose in C++: the following is legal in C, but not C++.
int class = 5
These errors are easy to fix: just rename the variable.
In C, pointers can be cast to a
void*, which needs an explicit cast in C++. The following is illegal in C++, but legal in C:
void* ptr; int* intptr = ptr;
Adding an explicit cast makes this work, but can cause further issues.
In C++, you may not skip initialization with
switch. The following is valid in C, but not C++:
goto foo; int skipped = 1; foo;
These bugs may require redesign.