A couple years ago I wrote a blog post Has C++ jumped the shark? I wondered how many people would care about the new C++ standard by the time it came out. I doubted that it would matter much to me personally.

… if something is hard to do in C++, I just don’t use C++. I don’t see a new version of C++ changing my decisions about what language to use for various tasks.

The new standard is out now, and I find it more interesting that I thought I would have. I’d still rather not use C++ for tasks that are easier in another language I know, but sometimes I don’t have that option.

When I heard about plans to add lambdas and closures to C++ I commented here that “it seems odd to add these features to C++.” Now I think lambdas (anonymous functions) and closures may be the most important new features for my use.

One of my favorite features in Python is the ability to define functions on the fly. If I’m writing a function and need to create a new function to pass as an argument, then I can define that function on the spot. By contrast, C++ has not let you define a function inside a function. Now you can.

(You still cannot define a named function inside a C++ function. However, you can create anonymous functions inside another function. If you save that anonymous function to a variable, then you’ve effectively created a named function.)

Here are a couple examples of how it can be very convenient to define little functions on the fly. First, optimization software typically provides methods for minimizing functions. If you want to maximize a f(x), you define a new function g(x) = -f(x) and minimize g. Second, root-finding software typically solves equations of the form f(x) = 0. If you want to solve f(x) = c, you create a new function g(x) = f(x) – c and find where g is zero. In both cases, the function g is so trivial that there’s no need to give it a name. And more importantly, you’d rather define this auxiliary function right where you need it than interrupt your context by defining it outside.

Anonymous functions can also have associated data obtained from the context where they’re defined. That’s called a closure because it encloses some context with the function. This is very often necessary in mathematical software development. For example, say you have a function of two variables f(x, y) and you want to integrate f with respect to x, holding y constant. You might then think of f as a function of one variable with one constant, but the compiler sees it as a function of two variables and will not let you pass it to an integration routine expecting a function of one variable. One way to solve this problem is to use function objects. This isn’t difficult, but it requires a lot of ceremony compared to using closures.

For specifics of how to use lambdas and closures in C++, see Ajay Vijayvargiya’s article Explicating the new C++ standard (C++0x), and its implementation in VC10.

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