Template metaprogramming trick: Get the compiler to tell you what type you have

This post has been republished via RSS; it originally appeared at: Microsoft Developer Blogs.

C++ template metaprogramming is like writing a program in Prolog without a debugger. The compiler executes your metaprogram by running through a bunch of pattern-matching rules. But unlike Prolog, the C++ template metaprogramming language doesn't have a debugger. You just feed your code to the compiler, and you get a few possible results: It fails to compile. It compiles and gives you what you want. It compiles and gives you something that wasn't what you want. Only if you're lucky do you get case 2 on the first try. There's no way to single-step through your metaprogram, and there's no print-debugging either. All you can do is see what the compiler says. Here's a trick I use to get something. It's not great, but it's still handy. This is a forward declaration of a function that takes an arbitrary number of type arguments. I can drop a call to this function at various points in my template metaprogram to see how the compiler deduced a type: When I instantiate f, a call to whatis<T> is made, among all the other stuff. I can look at the compiler output or the linker's "unresolved external" error message to see what T ended up being. Aha, in this instantiation of f, the type T was deduced to be double&. It's not a super-awesome trick, but with template metaprogramming, every little bit helps.