lemon-0.x: src/work/klao/tag_demo.cc@520769d825f2

     1 #include <iostream>

     2 #include <boost/utility.hpp>

     4 using namespace std;

     5 using namespace boost;

     7 struct True {

     8   static const bool value = true;

     9 };

    10 struct False {

    11   static const bool value = false;

    12 };

    14 // Here are some graph structures. Some of them define the type "OneTag"

    15 // to True or False, some does not. Not defining a tag is (or at least

    16 // should be) equivalent to defining it to "False".

    17 struct Graph1 {};

    18 struct Graph2 {

    19   typedef True OneTag;

    20 };

    21 struct Graph3 {

    22   typedef False OneTag;

    23 };

    26 /**************** The first method to use tags ****************/

    28 template <typename Graph, typename Enable = void>

    29 struct HasOneTag {

    30   typedef False TheTag;

    31 };

    33 // specialization for those graphs which defined the tag to be true:

    34 template <typename Graph>

    35 struct HasOneTag<Graph, typename enable_if<typename Graph::OneTag>::type > {

    36   typedef True TheTag;

    37 };

    39 template <typename Graph>

    40 int cn1(const Graph &, False) {

    41   return 0;

    42 }

    44 template <typename Graph>

    45 int cn1(const Graph &, True) {

    46   return 1;

    47 }

    49 template <typename Graph>

    50 int cn1(const Graph &g) {

    51   return cn1(g, typename HasOneTag<Graph>::TheTag());

    52 }

    54 /**************** The second method ****************/

    56 // An artificial type to provoke a conversion to avoid ambuguity...

    57 template <typename T>

    58 struct Wrap {

    59   const T &value;

    60   Wrap(const T &t) : value(t) {}

    61 };

    63 template <typename Graph>

    64 typename enable_if<typename Graph::OneTag, int>::type

    65 _cn2(const Graph &) {

    66   return 1;

    67 }

    69 template <typename Graph>

    70 int _cn2(Wrap<Graph>) {

    71   return 0;

    72 }

    74 template <typename Graph>

    75 int cn2(const Graph& g) {

    76   return _cn2<Graph>(g);

    77 }

    80 int main() {

    81   Graph1 g1;

    82   Graph2 g2;

    83   Graph3 g3;

    85   cout << "The first method:\n";

    86   cout << "G1: " << cn1(g1) << endl;

    87   cout << "G2: " << cn1(g2) << endl;

    88   cout << "G3: " << cn1(g3) << endl;

    90   cout << "The second method:\n";

    91   cout << "G1: " << cn2(g1) << endl;

    92   cout << "G2: " << cn2(g2) << endl;

    93   cout << "G3: " << cn2(g3) << endl;

    95 }

author	klao
	Sun, 09 Jan 2005 23:21:52 +0000
changeset 1066	520769d825f2
permissions	-rw-r--r--