Where C++ Templates Fall Down

There are many articals out there detailing comparisons between C++ Templates and Java Generics (Google around and you’ll quickly find loads of them). Almost universally they view the erasure of Java generics as a fail-point. Even the ones written by “Java Gurus” seem to favour C++ templates as being more flexable. I recently stumbled upon a simple case that I’ve been doing in Java for ages, and while trying to implement the same concept in C++ discovered that it’s impossible.

The case is a generic Visitor model:

public interface Visitor<R, P> {
    R visitFirstNodeType(FirstNodeType node, P parameter);
    R visitSecondNodeType(SecondNodeType node, P parameter);
}

public abstract class AbstractNodeType {
    public abstract <R, P> R accept(Visitor<R, P> visitor, P parameter);
}

Looks simple enough, but try and implement it in C++ and you discover that you cannot use templates on a virtual method. At first this doesn’t make sense (even to a friend of mine who lives in C++). The simple reason is the way templates are handled: they are code generation shortcuts. Therefore the compiler needs to know the exact method to run, but by declaring the method virtual you are telling the compiler to deferre that descision until runtime. In Java the generics are “erased” (although they are kept in the signature), resolving to a single method implementation. This means that there is no problem trying to decide which method to run.

C++ can create the same structure without templates, but then you loose the type-safety contract that you defined in Java. It’s not really a fail-point on C++ (rather just a product of the implementation), but it is something that seems to be overlooked on the Java side of the fence. Generic erasure buys you fairly cool compile-time contracts.

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