模版特化之萃取代码

    代码先：后面想个实用的例子【当然视频里的也实用，不过想自己想下先】：

#ifndef _TRAIT_
#define _TRAIT_

/**
 * @brief 萃取 - 根据模板特化的参数匹配相应的模版类，内部萃取到本质的数据类型.
 */
namespace trait
{
    template <typename T>
    class type_addtion
    {
    public:
        typedef T valueType;
        typedef T * pointer;
        typedef T & reference;
        typedef const T * const_pointer;
        typedef const T & const_reference;
    };

    template <typename T>
    class type_addtion <T *>
    {
    public:
        typedef T valueType;
        typedef T * pointer;
        typedef T & reference;
        typedef const T * const_pointer;
        typedef const T & const_reference;
    };

    template <typename T>
    class type_addtion <const T *>
    {
    public:
        typedef T valueType;
        typedef T * pointer;
        typedef T & reference;
        typedef const T * const_pointer;
        typedef const T & const_reference;
    };

    template <typename T>
    class type_addtion <T **>
    {
    public:
        typedef T valueType;
        typedef T * pointer;
        typedef T & reference;
        typedef const T * const_pointer;
        typedef const T & const_reference;
    };

    template <typename T>
    class type_addtion <T &>
    {
    public:
        typedef T valueType;
        typedef T * pointer;
        typedef T & reference;
        typedef const T * const_pointer;
        typedef const T & const_reference;
    };

    template <typename T>
    class type_addtion <const T &>
    {
    public:
        typedef T valueType;
        typedef T * pointer;
        typedef T & reference;
        typedef const T * const_pointer;
        typedef const T & const_reference;
    };

    ///< 完全特化
    template <>
    class type_addtion <void>
    {
    public:
        typedef void valueType;
        typedef void * pointer;
        typedef const void * const_pointer;
    };
}

#endif    //_TRAIT_

trait.c

#include "trait.h"
#include <iostream>

using namespace std;

int main(int argc, char ** argv)
{
    ///< 不管是int,还是int **作为模版参数，我们都可以萃取到变量、指针、引用类型.
    trait::type_addtion<int> iData;
    trait::type_addtion<int>::valueType i;
    trait::type_addtion<int *>::valueType iso;
    trait::type_addtion<int>::pointer ip;
    trait::type_addtion<int **>::pointer ipso;

    return 0;
}