Generic Programming Techniques

Generic Programming Techniques

This is an incomplete survey of some of the generic programming techniques used in the boost libraries.

Table of Contents

• Introduction
• The Anatomy of a Concept
• Traits
• Tag Dispatching
• Adaptors
• Type Generators
• Object Generators
• Policy Classes

Introduction

Generic programming is about generalizing software components so that they can be easily reused in a wide variety of situations. In C++, class and function templates are particularly effective mechanisms for generic programming because they make the generalization possible without sacrificing efficiency.

As a simple example of generic programming, we will look at how one might generalize the memcpy() function of the C standard library. An implementation of memcpy() might look like the following:

void* memcpy(void* region1, const void* region2, size_t n)
{
  const char* first = (const char*)region2;
  const char* last = ((const char*)region2) + n;
  char* result = (char*)region1;
  while (first != last)
    *result++ = *first++;
  return result;
}

The memcpy() function is already generalized to some extent by the use of void* so that the function can be used to copy arrays of different kinds of data. But what if the data we would like to copy is not in an array? Perhaps it is in a linked list. Can we generalize the notion of copy to any sequence of elements? Looking at the body of memcpy(), the function's minimal requirements are that it needs to traverse through the sequence using some sort of pointer, access elements pointed to, write the elements to the destination, and compare pointers to know when to stop. The C++ standard library groups requirements such as these into concepts, in this case the Input Iterator concept (for region2) and the Output Iterator concept (for region1).

If we rewrite the memcpy() as a function template, and use the Input Iterator and Output Iterator concepts to describe the requirements on the template parameters, we can implement a highly reusable copy() function in the following way:

template <typename InputIterator, typename OutputIterator>
OutputIterator
copy(InputIterator first, InputIterator last, OutputIterator result)
{
  while (first != last)
    *result++ = *first++;
  return result;
}

Using the generic copy() function, we can now copy elements from any kind of sequence, including a linked list that exports iterators such asstd::list.

#include <list>
#include <vector>
#include <iostream>

int main()
{
  const int N = 3;
  std::vector<int> region1(N);
  std::list<int> region2;

  region2.push_back(1);
  region2.push_back(0);
  region2.push_back(3);
  
  std::copy(region2.begin(), region2.end(), region1.begin());

  for (int i = 0; i < N; ++i)
    std::cout << region1[i] << " ";
  std::cout << std::endl;
}

Anatomy of a Concept

A concept is a set of requirements consisting of valid expressions, associated types, invariants, and complexity guarantees. A type that satisfies the requirements is said to model the concept. A concept can extend the requirements of another concept, which is calledrefinement.

• Valid Expressions are C++ expressions which must compile successfully for the objects involved in the expression to be considered modelsof the concept.
• Associated Types are types that are related to the modeling type in that they participate in one or more of the valid expressions. Typically associated types can be accessed either through typedefs nested within a class definition for the modeling type, or they are accessed through a traits class.
• Invariants are run-time characteristics of the objects that must always be true, that is, the functions involving the objects must preserve these characteristics. The invariants often take the form of pre-conditions and post-conditions.
• Complexity Guarantees are maximum limits on how long the execution of one of the valid expressions will take, or how much of various resources its computation will use.

The concepts used in the C++ Standard Library are documented at the SGI STL site.

Traits

A traits class provides a way of associating information with a compile-time entity (a type, integral constant, or address). For example, the class template std::iterator_traits<T> looks something like this:

template <class Iterator>
struct iterator_traits {
  typedef ... iterator_category;
  typedef ... value_type;
  typedef ... difference_type;
  typedef ... pointer;
  typedef ... reference;
};

The traits' value_type gives generic code the type which the iterator is "pointing at", while the iterator_category can be used to select more efficient algorithms depending on the iterator's capabilities.

A key feature of traits templates is that they're non-intrusive: they allow us to associate information with arbitrary types, including built-in types and types defined in third-party libraries, Normally, traits are specified for a particular type by (partially) specializing the traits template.

For an in-depth description of std::iterator_traits, see this page provided by SGI. Another very different expression of the traits idiom in the standard is std::numeric_limits<T> which provides constants describing the range and capabilities of numeric types.

Tag Dispatching

Tag dispatching is a way of using function overloading to dispatch based on properties of a type, and is often used hand in hand with traits classes. A good example of this synergy is the implementation of the std::advance() function in the C++ Standard Library, which increments an iterator n times. Depending on the kind of iterator, there are different optimizations that can be applied in the implementation. If the iterator is random access (can jump forward and backward arbitrary distances), then the advance() function can simply be implemented with i += n, and is very efficient: constant time. Other iterators must be advanced in steps, making the operation linear in n. If the iterator isbidirectional, then it makes sense for n to be negative, so we must decide whether to increment or decrement the iterator.

The relation between tag dispatching and traits classes is that the property used for dispatching (in this case the iterator_category) is often accessed through a traits class. The main advance() function uses the iterator_traits class to get the iterator_category. It then makes a call the the overloaded advance_dispatch() function. The appropriate advance_dispatch() is selected by the compiler based on whatever type theiterator_category resolves to, either input_iterator_tag, bidirectional_iterator_tag, or random_access_iterator_tag. A tag is simply a class whose only purpose is to convey some property for use in tag dispatching and similar techniques. Refer to this page for a more detailed description of iterator tags.

namespace std {
  struct input_iterator_tag { };
  struct bidirectional_iterator_tag { };
  struct random_access_iterator_tag { };

  namespace detail {
    template <class InputIterator, class Distance>
    void advance_dispatch(InputIterator& i, Distance n, input_iterator_tag) {
      while (n--) ++i;
    }

    template <class BidirectionalIterator, class Distance>
    void advance_dispatch(BidirectionalIterator& i, Distance n, 
       bidirectional_iterator_tag) {
      if (n >= 0)
        while (n--) ++i;
      else
        while (n++) --i;
    }

    template <class RandomAccessIterator, class Distance>
    void advance_dispatch(RandomAccessIterator& i, Distance n, 
       random_access_iterator_tag) {
      i += n;
    }
  }

  template <class InputIterator, class Distance>
  void advance(InputIterator& i, Distance n) {
    typename iterator_traits<InputIterator>::iterator_category category;
    detail::advance_dispatch(i, n, category);
  }
}

Adaptors

An adaptor is a class template which builds on another type or types to provide a new interface or behavioral variant. Examples of standard adaptors are std::reverse_iterator, which adapts an iterator type by reversing its motion upon increment/decrement, and std::stack, which adapts a container to provide a simple stack interface.

A more comprehensive review of the adaptors in the standard can be found here.

Type Generators

Note: The type generator concept has largely been superseded by the more refined notion of a metafunction. See C++ Template Metaprogrammingfor an in-depth discussion of metafunctions.

A type generator is a template whose only purpose is to synthesize a new type or types based on its template argument(s)[1]. The generated type is usually expressed as a nested typedef named, appropriately type. A type generator is usually used to consolidate a complicated type expression into a simple one. This example uses an old version of iterator_adaptor whose design didn't allow derived iterator types. As a result, every adapted iterator had to be a specialization of iterator_adaptor itself and generators were a convenient way to produce those types.

template <class Predicate, class Iterator, 
    class Value = complicated default,
    class Reference = complicated default,
    class Pointer = complicated default,
    class Category = complicated default,
    class Distance = complicated default
         >
struct filter_iterator_generator {
    typedef iterator_adaptor<
        
        Iterator,filter_iterator_policies<Predicate,Iterator>,
        Value,Reference,Pointer,Category,Distance> type;
};

Now, that's complicated, but producing an adapted filter iterator using the generator is much easier. You can usually just write:

boost::filter_iterator_generator<my_predicate,my_base_iterator>::type

Object Generators

An object generator is a function template whose only purpose is to construct a new object out of its arguments. Think of it as a kind of generic constructor. An object generator may be more useful than a plain constructor when the exact type to be generated is difficult or impossible to express and the result of the generator can be passed directly to a function rather than stored in a variable. Most Boost object generators are named with the prefix "make_", after std::make_pair(const T&, const U&).

For example, given:

struct widget {
  void tweak(int);
};
std::vector<widget *> widget_ptrs;

By chaining two standard object generators, std::bind2nd() and std::mem_fun(), we can easily tweak all widgets:

void tweak_all_widgets1(int arg)
{
   for_each(widget_ptrs.begin(), widget_ptrs.end(),
      bind2nd(std::mem_fun(&widget::tweak), arg));
}

Without using object generators the example above would look like this:

void tweak_all_widgets2(int arg)
{
   for_each(struct_ptrs.begin(), struct_ptrs.end(),
      std::binder2nd<std::mem_fun1_t<void, widget, int> >(
          std::mem_fun1_t<void, widget, int>(&widget::tweak), arg));
}

As expressions get more complicated the need to reduce the verbosity of type specification gets more compelling.

Policy Classes

A policy class is a template parameter used to transmit behavior. An example from the standard library is std::allocator, which supplies memory management behaviors to standard containers.

Policy classes have been explored in detail by Andrei Alexandrescu in this chapter of his book, Modern C++ Design. He writes:

In brief, policy-based class design fosters assembling a class with complex behavior out of many little classes (called policies), each of which takes care of only one behavioral or structural aspect. As the name suggests, a policy establishes an interface pertaining to a specific issue. You can implement policies in various ways as long as you respect the policy interface.

Because you can mix and match policies, you can achieve a combinatorial set of behaviors by using a small core of elementary components.

Andrei's description of policy classes suggests that their power is derived from granularity and orthogonality. Less-granular policy interfaces have been shown to work well in practice, though. This paper describes an old version of iterator_adaptor that used non-orthogonal policies. There is also precedent in the standard library: std::char_traits, despite its name, acts as a policies class that determines the behaviors ofstd::basic_string.

Notes

[1] Type generators are sometimes viewed as a workaround for the lack of ``templated typedefs'' in C++.

自己的一些理解：

概念：感觉就是类似于类的抽象。维基百科定义为是抽象的、普遍的想法、观念或充当指明实体、事件或关系的范畴或类的实体。在它们的外延中忽略事物的差异，如同它们是同一的去处理它们，所以概念是抽象的。它们等同的适用于在它们外延中的所有事物，所以它们是普遍的。概念也是命题的基本元素，如同文法是句子的基本语义元。哲学上不是说，物质在人们的意识之外独立存在又能为人的意识所反映的客观实在。就是抽象客观实体的共同性，忽略客体之间的差异性；简单一句话就是区别于其他概念的最小特征集合（minimal requirements）

refinement：哲学中的普遍与特殊的关系，反映在C++ 就是概念的requirement更多更苛刻，具体实现表现在模板的特化，类的继承；

元数据：参考罗剑锋的Boost书，就是typedef的类型数据 enum定义的数据，函数

元函数：指的是 类

泛型编程中三大逻辑：

  顺序逻辑就不用说了

主要是条件逻辑和循环逻辑：

条件逻辑：可以通过一个泛型变量或traits技巧，主模板表示else语句，特化的模板表示if语句。因为特化的模板在函数匹配的时候优先级更大。

我们来看看traits，这个在STL中有经典应用的东西。在这里首先援引Scott Mayer的一段评价：Traits are basically compile-time else-if-thens: the unspecialized template is the else clause, the specializations are the if clauses. I think traits can be used to determine whether a template parameter is a built-in type. Also whether a template parameter inherits from a given base class. The trick is to create a template generating code that won't compile, then provide specializations that *will* compile for the special cases that are allowed.

循环逻辑：主要通过模板的递归和嵌套来实现；