c/c++ 模板与STL小例子系列<三> traits

c/c++ 模板与stl小例子系列<三> traits

对这个概念，还是处于懵逼的状态，初步体会就是，为了解决类型之间的转换问题。

从一个类型为a的指针，转化到类型为b的指针，中间需要用void*来作为中介。traits好像可以解决这种问题。

下面弄了3个小例子，可以初步感受一下。

例子1：

#include <iostream>
using namespace std;

template<typename t, typename u>
class traits{
public:
  typedef t value_type1;
  typedef t& ref1;
  typedef u value_type2;
  typedef u& ref2;
};

template<typename ta, typename ua>
class a : public traits<ta, ua>
{

};

int main(){
  a<int, double>::value_type1 a = 10;
  a<int, double>::ref1 b = a;
  cout << a << endl;
  cout << b << endl;
  a<int, double>::value_type2 a1 = 10.2;
  a<int, double>::ref2 b1 = a1;
  cout << a1 << endl;
  cout << b1 << endl;
}

例子2：

#include <iostream>
using namespace std;
class test1;
class test2;

template<typename t>
class typetb1{};

template<>
class typetb1<test1>{
public:
  typedef char ret_type;
  typedef int p1_type;
  typedef double p2_type;
};
template<>
class typetb1<test2>{
public:
  typedef double ret_type;
  typedef double p1_type;
  typedef int p2_type;
};
template<typename t>
class test{
public:
  typename typetb1<t>::ret_type func(typename typetb1<t>::p1_t\
ype x,
                              typename typetb1<t>::p2_type y){
    return x;
  }
};
int main(){
  test<test1> t;
  cout << t.func(65, 6.18) << endl;
  test<test2> t2;
  cout << t2.func(6.18, 65) << endl;
}

例子3：

#include <iostream>
using namespace std;

class a{
public:
  void show(){cout << "a show" << endl;}
};
template<typename t>
class iterator_1{
public:
  typedef t value_type;
  typedef value_type* ptn;
  typedef value_type& ref;
};
template<typename t>
class iterator_2{
public:
  typedef t value_type;
  typedef value_type* ptn;
  typedef value_type& ref;
};

template<typename t>
struct traits{};

template<typename t>
struct traits<t *>{
  typedef t value_type;
  typedef value_type* ptn;
  typedef value_type& ref;
};

int main(){
  iterator_1<int>::value_type t1 = 100;
  cout << t1 << endl;
  iterator_2<double>::value_type t2 = 1.23;
  cout << t2 << endl;
  traits<double*>::value_type t3 = 4.45;
  cout << t3 << endl;

  iterator_1<a>::ptn p;
  p->show();
}