Use Traits Classes for Information About Types

The following is an excerpt from Scott Meyers’ new book, Effective C++, Third Edition: 55 Specific Ways to Improve Your Programs and Designs.

          

Item 47: Use traits classes for information about types.

The STL is primarily made up of templates for containers, iterators, and algorithms, but it also has a few utility templates. One of these is called advance. advance moves a specified iterator a specified distance:

template<typename IterT, typename DistT>    // move iter d units
void advance(IterT& iter, DistT d);         // forward; if d < 0,
// move iter backward

Conceptually, advance just does iter += d, but advance can’t be implemented that way, because only random access iterators support the += operation. Less powerful iterator types have to implement advance by iteratively applying ++ or -- d times.

Um, you don’t remember your STL iterator categories? No problem, we’ll do a mini-review. There are five categories of iterators, corresponding to the operations they support. Input iterators can move only forward, can move only one step at a time, can only read what they point to, and can read what they’re pointing to only once. They’re modeled on the read pointer into an input file; the C++ library’s istream_iterators are representative of this category. Output iterators are analogous, but for output: they move only forward, move only one step at a time, can only write what they point to, and can write it only once. They’re modeled on the write pointer into an output file; ostream_iterators epitomize this category. These are the two least powerful iterator categories. Because input and output iterators can move only forward and can read or write what they point to at most once, they are suitable only for one-pass algorithms.

A more powerful iterator category consists of forward iterators. Such iterators can do everything input and output iterators can do, plus they can read or write what they point to more than once. This makes them viable for multi-pass algorithms. The STL offers no singly linked list, but some libraries offer one (usually called slist), and iterators into such containers are forward iterators. Iterators into TR1’s hashed containers (see Item 54) may also be in the forward category.

Bidirectional iterators add to forward iterators the ability to move backward as well as forward. Iterators for the STL’s list are in this category, as are iterators for set, multiset, map, and multimap.

The most powerful iterator category is that of random access iterators. These kinds of iterators add to bidirectional iterators the ability to perform “iterator arithmetic,” i.e., to jump forward or backward an arbitrary distance in constant time. Such arithmetic is analogous to pointer arithmetic, which is not surprising, because random access iterators are modeled on built-in pointers, and built-in pointers can act as random access iterators. Iterators for vector, deque, and string are random access iterators.

For each of the five iterator categories, C++ has a “tag struct” in the standard library that serves to identify it:

struct 

input_iterator_tag

 {};
struct 

output_iterator_tag

 {};
struct 

forward_iterator_tag

: public input_iterator_tag { };
struct 

bidirectional_iterator_tag

: public forward_iterator_tag { };
struct 

random_access_iterator_tag

: public bidirectional_iterator_tag {};

The inheritance relationships among these structs are valid is-a relationships (see Item 32): it’s true that all forward iterators are also input iterators, etc. We’ll see the utility of this inheritance shortly.

But back to advance. Given the different iterator capabilities, one way to implement advance would be to use the lowest-common-denominator strategy of a loop that iteratively increments or decrements the iterator. However, that approach would take linear time. Random access iterators support constant-time iterator arithmetic, and we’d like to take advantage of that ability when it’s present.

What we really want to do is implement advance essentially like this:

template<typename IterT, typename DistT>
void advance(IterT& iter, DistT d)
{
   if (

iter is a random access iterator

) {
      iter += d;                          // use iterator arithmetic
   }                                      // for random access iters
   else {
      if (d >= 0) { while (d--) ++iter; } // use iterative calls to
   else { while (d++) - -iter; }          // ++ or - - for other
   }                                      // iterator categories
}

This requires being able to determine whether iter is a random access iterator, which in turn requires knowing whether its type, IterT, is a random access iterator type. In other words, we need to get some information about a type. That’s what traits let you do: they allow you to get information about a type during compilation.

Traits aren’t a keyword or a predefined construct in C++; they’re a technique and a convention followed by C++ programmers. One of the demands made on the technique is that it has to work as well for built-in types as it does for user-defined types. For example, if advance is called with a pointer (like a const char *) and an int, advance has to work, but that means that the traits technique must apply to built-in types like pointers.

The fact that traits must work with built-in types means that things like nesting information inside types won’t do, because there’s no way to nest information inside pointers. The traits information for a type, then, must be external to the type. The standard technique is to put it into a template and one or more specializations of that template. For iterators, the template in the standard library is named iterator_traits:

template<typename IterT>          // template for information about
struct iterator_traits;           // iterator types

As you can see, iterator_traits is a struct. By convention, traits are always implemented as structs. Another convention is that the structs used to implement traits are known as—I am not making this up—traits classes.

The way iterator_traits works is that for each type IterT, a typedef named iterator_category is declared in the struct iterator_traits<IterT>. This typedef identifies the iterator category of IterT.

iterator_traits implements this in two parts. First, it imposes the requirement that any user-defined iterator type must contain a nested typedef named iterator_category that identifies the appropriate tag struct. deque’s iterators are random access, for example, so a class for deque iterators would look something like this:

template < ... >                          // template params elided
class deque {
public:
   class iterator {
   public:
      typedef 

random_access_iterator_tag

 iterator_category;
      ...
   }:
   ...
};

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