Cstyle Casts

This page was prompted by the following quote taken from TestTypesInsteadOfDispatch:

A trivial note: Instead of "(SubclassA*)" - C syntax to cast a pointer, I would use "reinterpret_cast<SubclassA*>". (Books will tell you to use "dynamic_cast<>", which is "right," but you're trying to avoid the cost of the dynamic dispatch.) -- JeffGrigg

I cannot disagree more strongly. The C-style cast in C++ is incredibly broken, and in the context above, switching to reinterpret_cast fixes nothing. (Note that I advocate static_cast in the above context.) Let's take the following as an example:

B * b = make_new_b();

A * a = (A*) b; //line 1

A * a = static_cast<A*>(b); //line 2

A * a = reinterpret_cast<A*>(b); //line 3

Here are some possible scenarios:

Lines 1, 2, and 3 will compile and will compile to the same code. (Answer: single inheritance and the full definitions of the types are in scope.)

Lines 1, 2, and 3 will compile. 1 and 2 will compile to the same code, but 3 will compile differently. (Answer: multiple inheritance, the full definitions of the types are in scope, and luck as sometimes 3 will be the same as 1 and 2, sometimes it won't.)

Lines 1 and 3 will compile and compile to the same code. Line 2 will not compile. (Answer: The types are forward declared. The best part about this is that if later the full definitions come into scope, like if someone added a header to another header which your cpp includes, the behavior of the c-style cast can silently change. THIS IS WHY YOU NEVER USE THE C-STYLE CAST IN C++ CODE.)

Line 3 will compile. Lines 1 and 2 will not compile. (Answer 1: It's an ambiguous cast from derived to base, multiple inheritance of the same base class, and not virtually.) (Answer 2: It's a down cast going across virtual inheritance.)

For all those reading:

1- Never ever use C-style casts in C++, ever. Ever. I mean it.

2- Use reinterpret_casts only if you know what you're doing, specifically if you understand the StrictAliasingRule? of C and C++ and how casts are not no-ops when working with multiple or virtual inheritance.

The C-style cast in C++ is defined as: If one of the types is a base class of the other type, then its effect is that of a static_cast. Otherwise its effect is that of a reinterpret_cast. This is why the C-style cast is evil in C++. Its behavior may silently change depending on if it sees the full definitions of the types or just forward declarations of the types.