Using C-style casts.
C++ allows you to independently choose whether to allow casts between unrelated types, and whether to allow changes to const and volatile qualifiers, giving considerable improvements to compile-time type safety compared with C. It also offers completely safe casts at the cost of a runtime check.
C-style casts, unchecked conversions between just about any types, allow whole classes of error that could be easily identified by more restrictive casts. Their syntax also makes them very difficult to search for, if you want to audit buggy code for dubious conversions.
