casting.md

<<< Table of contents

15. Casting

1. The best cast is a cast that is avoided.

2. If you have to cast, do not use any form of C style casts. Use static_cast<>, dynamic_cast<>, const_cast<> or reinterpret_cast<> instead, or a combination thereof.

   • The names casts provide much better type checking as well as clarity on what’s the intent of the case. And we can easily search for the occurrence of a cast. Casts should be avoided if possible, so it is good if they look a bit ugly.

3. For basic types, use the initializer form, if necessary: int(a), instead of (int)a.

   However, prefer initializing basic type variables with numbers using literals: auto a = 10l; or unsigned long x = 42ul.

4. Use of dynamic_cast<>, const_cast<> or reinterpret_cast<> should be avoided, unless there is no other alternative. Sometimes these are necessary; these occurrences should be safely hidden away inside an implementation. To elaborate:

   • dynamic_cast<> is easily overused, and frequently querying the type of an object at run-time (especially if in the form of a decision tree) points to a design problem. Consider virtual functions, potentially in combination with the Visitor pattern, as alternatives before resorting to RTTI.

   • Use of const_cast<> is strictly disallowed, unless interfacing with non-const correct legacy APIs that cannot be changed. Even then, breaking this contract between implementer and compiler should only be done after careful consideration.

   • reinterpret_cast<> is rightfully considered non-portable and should be mostly avoided. The cast can sometimes be useful if packed objects or arrays are to be viewed as a byte-stream, e.g. for serialization or strided access.