Why must I re-declare a virtual function from an inherited class?

Question

I\'m working on a simple C++ program and am having a difficult time understanding a compiler error I was getting. The issue was caused by me attempting to create a derived class

Answer 1

A very non-intuitive reason why overridden virtual methods must be derived in the base class stems from the fact that C++ allows different parts of the class to be placed into different files, into different translation units.

With some other languages (I'm looking in Java's direction), a single class must be placed in a single file. This is not true with C++. It is perfectly legal for a class to have some of its methods declared in one translation unit, and other methods declared in another translation unit, which could be in a file in some different directory altogether.

Each such file gets compiled separately and individually. When compiling one translation, the C++ compiler has no knowledge of any other translation unit, any other file, that might contain other pieces of the same class.

Now let's say that you are allowed to omit an overriden virtual method from the class declaration. This creates an immediate problem: when compiling the class's constructor, it is necessary for the compiler to know whether the class overrides any virtual methods from any of the superclasses, in order to correctly assemble the virtual table dispatch for the class being constructed. Without an explicit declaration, the compiler has no way of knowing whether or not some other translation unit might define an overridden virtual method.

And that's why overridden virtual methods must be explicitly included in the class's declaration. In conclusion: because C++ formally specifies phase 9, the linkage phase, with the actual compilation carried on in earlier phases, overridden methods must be explicitly declared.