1 #include <iostream> |
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2 #include <boost/utility.hpp> |
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3 |
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4 using namespace std; |
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5 using namespace boost; |
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6 |
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7 struct True { |
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8 static const bool value = true; |
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9 }; |
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10 struct False { |
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11 static const bool value = false; |
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12 }; |
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13 |
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14 // Here are some graph structures. Some of them define the type "OneTag" |
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15 // to True or False, some does not. Not defining a tag is (or at least |
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16 // should be) equivalent to defining it to "False". |
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17 struct Graph1 {}; |
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18 struct Graph2 { |
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19 typedef True OneTag; |
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20 }; |
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21 struct Graph3 { |
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22 typedef False OneTag; |
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23 }; |
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24 |
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25 |
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26 /**************** The first method to use tags ****************/ |
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27 |
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28 template <typename Graph, typename Enable = void> |
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29 struct HasOneTag { |
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30 typedef False TheTag; |
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31 }; |
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32 |
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33 // specialization for those graphs which defined the tag to be true: |
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34 template <typename Graph> |
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35 struct HasOneTag<Graph, typename enable_if<typename Graph::OneTag>::type > { |
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36 typedef True TheTag; |
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37 }; |
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38 |
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39 template <typename Graph> |
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40 int cn1(const Graph &, False) { |
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41 return 0; |
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42 } |
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43 |
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44 template <typename Graph> |
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45 int cn1(const Graph &, True) { |
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46 return 1; |
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47 } |
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48 |
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49 template <typename Graph> |
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50 int cn1(const Graph &g) { |
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51 return cn1(g, typename HasOneTag<Graph>::TheTag()); |
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52 } |
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53 |
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54 /**************** The second method ****************/ |
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55 |
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56 // An artificial type to provoke a conversion to avoid ambuguity... |
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57 template <typename T> |
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58 struct Wrap { |
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59 const T &value; |
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60 Wrap(const T &t) : value(t) {} |
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61 }; |
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62 |
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63 template <typename Graph> |
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64 typename enable_if<typename Graph::OneTag, int>::type |
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65 _cn2(const Graph &) { |
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66 return 1; |
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67 } |
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68 |
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69 template <typename Graph> |
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70 int _cn2(Wrap<Graph>) { |
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71 return 0; |
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72 } |
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73 |
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74 template <typename Graph> |
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75 int cn2(const Graph& g) { |
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76 return _cn2<Graph>(g); |
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77 } |
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78 |
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79 |
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80 int main() { |
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81 Graph1 g1; |
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82 Graph2 g2; |
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83 Graph3 g3; |
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84 |
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85 cout << "The first method:\n"; |
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86 cout << "G1: " << cn1(g1) << endl; |
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87 cout << "G2: " << cn1(g2) << endl; |
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88 cout << "G3: " << cn1(g3) << endl; |
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89 |
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90 cout << "The second method:\n"; |
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91 cout << "G1: " << cn2(g1) << endl; |
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92 cout << "G2: " << cn2(g2) << endl; |
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93 cout << "G3: " << cn2(g3) << endl; |
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94 |
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95 } |
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