Why is override optional in C++?

Question

I understand the purpose of the C++ override, however am a bit confused by it's implementation compared to other higher level languages, where its use is required by default.

The C++11 wiki page describes it as a "technically identifier for declarator attribute" but does not elaborate as to why it is not simply a keyword for the language.

Answer 1

It is optional to maintain backwards compatibility with C++03. Making it non-optional would have broken all the code^*.

Similarly, making override a keyword would have broken any code that used the name override.

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^{OK, not literally all the code, but a lot of it.}

Answer 2

Technically, C++11 does not behave much differently from Java here (which is a typical example for one of the "other higher level languages" which you mention). A wrong override will be a compilation error, just like a wrong @Override in Java. A missing override will not be a compilation error, just like a missing @Override will not be a compilation error in Java.

The only real difference I can see is that Java tools have traditionally had better support to detect a missing @Override, and that Java users are traditionally encouraged to treat the corresponding warning as an error, whereas C++ compilers have been quite slow so far at adding warning options for missing overrides.

But we're getting there; Clang now has -Winconsistent-missing-override, and newer GCCs have -Wsuggest-override. All you have to do is enable those warnings and treat them as errors, either forcibly by the compiler or simply by convention.

As for why it's not simply a keyword: backward compatibility with older code.