The Range-Based for Loop (C++11)

The range-based for loop, mentioned in Chapter 5, “Loops and Relational Expressions,” is designed to work with the STL. To review, here’s the first example from Chapter 5:

double prices[5] = {4.99, 10.99, 6.87, 7.99, 8.49};
for (double x : prices)
    cout << x << std::endl;

The contents of the parentheses for the for loop declare a variable of the type stored in a container and then the name of the container. Next, the body of the loop uses the named variable to access each container element in turn. Consider, for instance, this statement from Listing 16.9:

for_each(books.begin(), books.end(), ShowReview);

It can be replaced with the following range-based for loop:

for (auto x : books) ShowReview(x);

The compiler will use the type of books, which is vector<Review>, to deduce that x is type Review, and the loop will pass each Review object in books to ShowReview() in turn.

Unlike for_each(), the range-based for can alter the contents of a container. The trick is to specify a reference parameter. For example, suppose we have this function:

void InflateReview(Review &r){r.rating++;}

You could apply this function to each element in books with the following loop:

for (auto & x : books) InflateReview(x);

Generic Programming

Now that you have some experience using the STL, let’s look at the underlying philosophy. The STL is an example of generic programming. Object-oriented programming concentrates on the data aspect of programming, whereas generic programming concentrates on algorithms. The main things the two approaches have in common are abstraction and the creation of reusable code, but the philosophies are quite different.

A goal of generic programming is to write code that is independent of data types. Templates are the C++ tools for creating generic programs. Templates, of course, let you define a function or class in terms of a generic type. The STL goes further by providing a generic representation of algorithms. Templates make this possible, but not without the added element of careful and conscious design. To see how this mixture of templates and design works, let’s look at why iterators are needed.