模板偏特化的威力挺强大!
我列举两项如下。
第一是作为类型萃取器，萃取出真实类型。
1 #ifndef TYPE_TRAITS_H
2 #define TYPE_TRAITS_H
3
4 template<typename T>
5 struct type_traits{
6 typedef T value_type;
7 };
8
9 template<typename T>
10 struct type_traits<T*>{
11 typedef T value_type;
12 };
13
14 template<typename T>
15 struct type_traits<const T>{
16 typedef T value_type;
17 };
18
19 template<typename T>
20 struct type_traits<const T *>{
21 typedef T value_type;
22 };
23
24 #endif
client.cpp
#include "type_traits.h"
#include <iostream>
using namespace std;
class Obj{};
/**
* 萃取相应类型
*/
int main()
{
//---------------built-in type----------------
cout << typeid(type_traits<int>::value_type).name() << endl;
cout << typeid(type_traits<int*>::value_type).name() << endl;
cout << typeid(type_traits<const int *>::value_type).name() << endl;
//----------------UAD-------------------------
cout << typeid(type_traits<Obj>::value_type).name() << endl;
cout << typeid(type_traits<Obj *>::value_type).name() << endl;
cout << typeid(type_traits<const Obj*>::value_type).name() << endl;
return 0;
}
补充stl代码
vector
1 template <class T, class Alloc = alloc>
2 class vector {
3 public:
4 typedef T value_type;
5 typedef value_type* pointer;
6 typedef value_type* iterator;
7 typedef value_type& reference;
8 typedef size_t size_type;
9 typedef ptrdiff_t difference_type;
10
11 protected:
12 //...
13
14 public:
15 iterator begin(); //用萃取出的类型表示返回类型，perfect
16 size_type size();
17 //...
18 reference front();
19 //......
20 };
第二是实现Tuple，此Tuple可以传入任意数量的参数。此例中仅仅仅仅实现传入4、3、2个参数。
1 //tuple.h
2 #ifndef TUPLE_H
3 #define TUPLE_H
4
5 /**
6 * tuple.h
7 * by Harry Sun
8 * 2011-11-28
9 *
10 */
11
12 namespace tuple{
13
14 /**
15 * Four tuples
16 */
17 template<typename A, typename B, typename C=void, typename D=void>
18 class Tuple{
19 private:
20 A a;
21 B b;
22 C c;
23 D d;
24 public:
25 Tuple();
26 Tuple(A a, B b, C c, D d){
27 this->a = a;
28 this->b = b;
29 this->c = c;
30 this->d = d;
31 }
32 };
33
34 /**
35 * Three tuples
36 */
37 template<typename A, typename B, typename C>
38 class Tuple<A, B, C, void>{
39 private:
40 A a;
41 B b;
42 C c;
43 public:
44 Tuple();
45 Tuple(A a, B b, C c){
46 this->a = a;
47 this->b = b;
48 this->c = c;
49 }
50 };
51
52 /**
53 * Two tuples
54 */
55 template<typename A, typename B>
56 class Tuple<A, B, void, void>{
57 private:
58 A a;
59 B b;
60 public:
61 Tuple();
62 Tuple(A a, B b){
63 this->a = a;
64 this->b = b;
65 }
66 };
67
68 }
69 #endif
客户端代码
1 #include "tuple.h"
2 #include<string>
3
4 int main()
5 {
6 tuple::Tuple<int, double, char, std::string> tuple4(3, 2.0, 'c', "hello");
7
8 tuple::Tuple<int, double, char> tuple3(1, 2.0, 'd');
9
10 tuple::Tuple<int, int> pair(1, 2);
11
12 return 0;
13 }
可以传入4个、3个或者2个参数。