C++编程思想 第2卷 第10章 设计模式 适配器模式

适配器模式接受一种类型并且提供一个对其他类型的接口
给定一个库或者具有某一接口的一段代码
同时还给定另外一个库或者与前面代码的基本相同的一段代码而只是
表达方式不一致时
适配器模式 非常有用

假设有个产生斐波那契数列的发生器类

//: C10:FibonacciGenerator.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#ifndef FIBONACCIGENERATOR_H
#define FIBONACCIGENERATOR_H

class FibonacciGenerator {
  int n;
  int val[2];
public:
  FibonacciGenerator() : n(0) { val[0] = val[1] = 0; }
  int operator()() {
    int result = n > 2 ? val[0] + val[1] : n > 0 ? 1 : 0;
    ++n;
    val[0] = val[1];
    val[1] = result;
    return result;
  }
  int count() { return n; }
};
#endif // FIBONACCIGENERATOR_H ///:~

由于它是一个发生器
可以调用operator()来使用它

//: C10:FibonacciGeneratorTest.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#include 
#include "FibonacciGenerator.h"
using namespace std;

int main() {
  FibonacciGenerator f;
  for(int i =0; i < 20; i++)
    cout << f.count() << ": " << f() << endl;
  getchar();
} ///:~

输出
1: 0
2: 1
3: 1
4: 2
5: 3
6: 5
7: 8
8: 13
9: 21
10: 34
11: 55
12: 89
13: 144
14: 233
15: 377
16: 610
17: 987
18: 1597
19: 2584
20: 4181

STL算法只能使用迭代器才能工作
这就是接口不匹配的问题
解决办法就是创建一个适配器

由于数值算法值要求一个输入迭代器
适配器模式相当地直观

//: C10:FibonacciAdapter.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Adapting an interface to something you already have.
#include 
#include 
#include "FibonacciGenerator.h"
#include "../C06/PrintSequence.h"
using namespace std;

class FibonacciAdapter { // Produce an iterator
  FibonacciGenerator f;
  int length;
public:
  FibonacciAdapter(int size) : length(size) {}
  class iterator;
  friend class iterator;
  class iterator : public std::iterator<
    std::input_iterator_tag, FibonacciAdapter, ptrdiff_t> {
    FibonacciAdapter& ap;
  public:
    typedef int value_type;
    iterator(FibonacciAdapter& a) : ap(a) {}
    bool operator==(const iterator&) const {
      return ap.f.count() == ap.length;
    }
    bool operator!=(const iterator& x) const {
      return !(*this == x);
    }
    int operator*() const { return ap.f(); }
    iterator& operator++() { return *this; }
    iterator operator++(int) { return *this; }
  };
  iterator begin() { return iterator(*this); }
  iterator end() { return iterator(*this); }
};

int main() {
  const int SZ = 20;
  FibonacciAdapter a1(SZ);
  cout << "accumulate: "
    << accumulate(a1.begin(), a1.end(), 0) << endl;
  FibonacciAdapter a2(SZ), a3(SZ);
  cout << "inner product: "
    << inner_product(a2.begin(), a2.end(), a3.begin(), 0)
    << endl;
  FibonacciAdapter a4(SZ);
  int r1[SZ] = {0};
  int* end = partial_sum(a4.begin(), a4.end(), r1);
  print(r1, end, "partial_sum", " ");
  FibonacciAdapter a5(SZ);
  int r2[SZ] = {0};
  end = adjacent_difference(a5.begin(), a5.end(), r2);
  print(r2, end, "adjacent_difference", " ");
  getchar();
} ///:~

输出
accumulate: 10945
inner product: 28284465
partial_sum:  0 1 2 4 7 12 20 33 54 88 143 232 376 609 986 1596 2583 4180 6764 10945
adjacent_difference:  0 1 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

通过被告知斐波那契数列的长度来初始化FibonacciAdapter
当创建iterator时
获得一个包含FibonacciAdapter的引用
这样就能够访问FibonacciGenerator和length