Comparison of generic in C# Java and C++

Comparing C# and Java Generics

Bruce Eckel: How do C# generics compare with Java generics?

Anders Hejlsberg: Java’s generics implementation was based on a project originally called Pizza, which was done by Martin Odersky and others. Pizza was renamed GJ, then it turned into a JSR and ended up being adopted into the Java language. And this particular generics proposal had as a key design goal that it could run on an unmodified VM [Virtual Machine]. It is, of course, great that you don’t have to modify your VM, but it also brings about a whole bunch of odd limitations. The limitations are not necessarily directly apparent, but you very quickly go, “Hmm, that’s strange.”

For example, with Java generics, you don’t actually get any of the execution efficiency that I talked about, because when you compile a generic class in Java, the compiler takes away the type parameter and substitutes Objecteverywhere. So the compiled image for List<T> is like a List where you use the type Object everywhere. Of course, if you now try to make a List<int>, you get boxing of all the ints. So there’s a bunch of overhead there. Furthermore, to keep the VM happy, the compiler actually has to insert all of the type casts you didn’t write. If it’s a List of Object and you’re trying to treat those Objects as Customers, at some point the Objects must be cast toCustomers to keep the verifier happy. And really all they’re doing in their implementation is automatically inserting those type casts for you. So you get the syntactic sugar, or some of it at least, but you don’t get any of the execution efficiency. So that’s issue number one I have with Java’s solution.

Issue number two, and I think this is probably an even bigger issue, is that because Java’s generics implementation relies on erasure of the type parameter, when you get to runtime, you don’t actually have a faithful representation of what you had at compile time. When you apply reflection to a generic List in Java, you can’t tell what the List is a List of. It’s just a List. Because you’ve lost the type information, any type of dynamic code-generation scenario, or reflection-based scenario, simply doesn’t work. If there’s one trend that’s pretty clear to me, it’s that there’s more and more of that. And it just doesn’t work, because you’ve lost the type information. Whereas in our implementation, all of that information is available. You can use reflection to get the System.Type for object List<T>. You cannot actually create an instance of it yet, because you don’t know what T is. But then you can use reflection to get the System.Type for int. You can then ask reflection to please put these two together and create a List<int>, and you get another System.Type for List<int>. So representationally, anything you can do at compile time you can also do at runtime.

Comparing C# Generics to C++ Templates

Bruce Eckel: How do C# generics compare with C++ templates?

Anders Hejlsberg: To me the best way to understand the distinction between C# generics and C++ templates is this: C# generics are really just like classes, except they have a type parameter. C++ templates are really just like macros, except they look like classes.

The big difference between C# generics and C++ templates shows up in when the type checking occurs and how the instantiation occurs. First of all, C# does the instantiation at runtime. C++ does it at compile time, or perhaps at link time. But regardless, the instantiation happens in C++ before the program runs. That’s difference number one. Difference number two is C# does strong type checking when you compile the generic type. For an unconstrained type parameter, like List<T>, the only methods available on values of type T are those that are found on type Object, because those are the only methods we can generally guarantee will exist. So in C# generics, we guarantee that any operation you do on a type parameter will succeed.

C++ is the opposite. In C++, you can do anything you damn well please on a variable of a type parameter type. But then once you instantiate it, it may not work, and you’ll get some cryptic error messages. For example, if you have a type parameter T, and variables x and y of type T, and you say x + y, well you had better have an operator+ defined for + of two Ts, or you’ll get some cryptic error message. So in a sense, C++ templates are actually untyped, or loosely typed. Whereas C# generics are strongly typed.