trait与policy模板应用简单示例

trait与policy模板应用简单示例

accumtraits.hpp // 累加算法模板的trait

// 累加算法模板的trait
#ifndef ACCUMTRAITS_HPP
#define ACCUMTRAITS_HPP

template <typename T>
class AccumulationTraits; // 只有声明  

template <>
class AccumulationTraits<char> // 把具体类型char映射到int，累加后就返回int  
{
public:
    typedef int AccT; // 统一的类型别名，表示返回类型 
    static AccT zero() // 关联一个缺省值，是累加时的初始缺省值  
    {
        return 0;
    }
};

template <>
class AccumulationTraits<short> // 把具体类型short映射到累加后的返回类型int  
{
public:
    typedef int AccT;
    static AccT zero() // 没有直接在类内部定义static变量并提供缺省值，而是使用了函数
    {                  // 因为类内部只能对整型和枚举类型的static变量进行初始化 // 其他类型的必须类内部声明，在外部进行初始化
        return 0;
    }
};

template <>
class AccumulationTraits<int>
{
public:
    typedef long AccT;
    static AccT zero()
    {
        return 0;
    }
};

template <>
class AccumulationTraits<unsigned int>
{
public:
    typedef unsigned long AccT;
    static AccT zero()
    {
        return 0;
    }
};

template <>
class AccumulationTraits<float>
{
public:
    typedef double AccT;
    static AccT zero()
    {
        return 0;
    }
};


#endif // !ACCUMTRAITS_HPP

policies.hpp // 累加算法模板的policy

// 累加算法模板的policy
#ifndef POLICIES_HPP
#define POLICIES_HPP

template <typename T1, typename T2>
class SumPolicy // 累加的策略
{
public:
    static void accumulate(T1 & total, T2 const & value)
    {
        total += value; // 累加
    }
};

template <typename T1, typename T2>
class MultPolicy // 累乘的策略
{
public:
    static void accumulate(T1 & total, T2 const & value)
    {
        total *= value; // 累乘
    }
};

#endif // !POLICIES_HPP

accum.hpp // 累加算法模板：实现为类模板，用模板参数来传递policy和trait

// 累加算法模板：实现为类模板，用模板参数来传递policy和trait 
// 可用一个内联函数模板作为包装器来包装这个类模板实现 
#ifndef ACCUM_HPP
#define ACCUM_HPP

#include "accumtraits.hpp"
#include "policies.hpp"
#include <iostream>

template < typename T, // 这里使用 typename 和 class 没有区别
    const int INITVAL = 0,  // INITVAL 是一个 无类型模板参数
    template <typename, typename> class Policy = SumPolicy, // 这里的必须使用class不能是typename, 因为 Policy 是类类型, 默认采用SumPolicy策略
    typename Traits = AccumulationTraits<T> > // 模板参数Traits代表要使用的trait
class Accum
{
public:
    // AccumulationTraits 是一个 standard traits class (标准特性类)
    // AccT 嵌套在 AccumulationTraits<T> 内部类型, 而且 T 是一个模版参数
    // AccT 是一个 nested dependent type name (嵌套依赖类型名), 必须被 typename 前置
    static typename Traits::AccT accum(T const * beg, T const * end)
    {
        // total 是一个与 AccT 类型所指向的类型相同的局部变量
        // zero() 嵌套在 AccumulationTraits<T> 内部函数, 而且 T 是一个模版参数
        //typename Traits::AccT total = Traits::zero(); // 获取缺省值, 返回 0 // 存在问题: 当策略为 MultPolicy 会造成结果始终为 0
        typename Traits::AccT total = INITVAL; // 把初始值当作【无类型模板参数】传递进来
        while (beg != end) // 作累积运算
        {
            Policy<Traits::AccT, T>::accumulate(total, *beg); // 使用给定的算法策略来进行累积 
            ++beg;
        }
        return total; // 返回累积起来的值
    }
};

//// 用内联的函数模板来包装, 对默认的参数，提供对应的重载函数
template <typename T, const int INITVAL, template <typename, typename> class Policy, typename Traits>
inline typename Traits::AccT accum(T const * beg, T const * end)
{
    std::cout << "<typename T, const int INITVAL, template <typename, typename> class Policy, typename Traits> 
	---> <T, INITVAL, Policy, Traits>" << std::endl; // 标记使用
    return Accum<T, INITVAL, Policy, Traits>::accum(beg, end);
}

template <typename T, const int INITVAL, template <typename, typename> class Policy>
inline typename AccumulationTraits<T>::AccT accum(T const * beg, T const * end)
{
    std::cout << "<typename T, const int INITVAL, template <typename, typename> class Policy> 
	---> <T, INITVAL, Policy, AccumulationTraits<T>>" << std::endl; // 标记使用
    return Accum<T, INITVAL, Policy, AccumulationTraits<T>>::accum(beg, end);
}

template <typename T, const int INITVAL>
inline typename AccumulationTraits<T>::AccT accum(T const * beg, T const * end)
{
    std::cout << "<typename T, const int INITVAL> 
	---> <T, INITVAL, MultPolicy, AccumulationTraits<T>>" << std::endl; // 标记使用
    return Accum<T, INITVAL, MultPolicy, AccumulationTraits<T>>::accum(beg, end);
}

template <typename T>
inline typename AccumulationTraits<T>::AccT accum(T const * beg, T const * end)
{
    std::cout << "<typename T> 
	---> <T, 1, MultPolicy, AccumulationTraits<T>>" << std::endl; // 标记使用
    return Accum<T, 1, MultPolicy, AccumulationTraits<T>>::accum(beg, end);
}

template <>
inline typename AccumulationTraits<int>::AccT accum(int const * beg, int const * end)
{
    std::cout << "<> 
	---> <int, 1, MultPolicy, AccumulationTraits<int>>" << std::endl; // 标记使用
    return Accum<int, 1, MultPolicy, AccumulationTraits<int>>::accum(beg, end);
}

#endif // !ACCUM_HPP

mytest.cpp // 使用累加算法的客户端测试代码  

// 使用累加算法的客户端测试代码  
#include "accum.hpp"
#include <iostream>

int main()
{
    int num[] = {1,2,3,4,5}; // 整型数组 
    std::cout << "============= integer array =============" << std::endl;
    std::cout << "the total value of the integer values is "
        << accum<int, 1, MultPolicy, AccumulationTraits<int>>(&num[0], &num[5]) << std::endl;
    std::cout << "the total value of the integer values is "
        << accum<int, 1, MultPolicy>(&num[0], &num[5]) << std::endl;
    std::cout << "the total value of the integer values is "
        << accum<int, 1>(&num[0], &num[5]) << std::endl;
    std::cout << "the total value of the integer values is "
        << accum<int>(&num[0], &num[5]) << std::endl;
    std::cout << "the total value of the integer values is "
        << accum<>(&num[0], &num[5]) << std::endl;
    std::cout << "the total value of the integer values is "
        << accum(&num[0], &num[5]) << std::endl;


    char name[] = "templates"; // 创建字符值数组
    int length = sizeof(name)-1;
    std::cout << "============= characters array =============" << std::endl;
    std::cout << "the total value of the characters in ""
        << name << "" is " << accum<char, 0, SumPolicy, AccumulationTraits<char>>(&name[0], &name[length]) << std::endl;
    std::cout << "the total value of the characters in ""
        << name << "" is " << accum<char, 0, SumPolicy>(&name[0], &name[length]) << std::endl;
    std::cout << "the total value of the characters in ""
        << name << "" is " << accum<char, 0>(&name[0], &name[length]) << std::endl;
    std::cout << "the total value of the characters in ""
        << name << "" is " << accum<char>(&name[0], &name[length]) << std::endl;
    std::cout << "the total value of the characters in ""
        << name << "" is " << accum<>(&name[0], &name[length]) << std::endl;
    std::cout << "the total value of the characters in ""
        << name << "" is " << accum(&name[0], &name[length]) << std::endl;

    system("pause");
    return 0;
}

输出结果：

============= integer array =============
<typename T, const int INITVAL, template <typename, typename> class Policy, typename Traits> 
	---> <T, INITVAL, Policy, Traits>
the total value of the integer values is 120
<typename T, const int INITVAL, template <typename, typename> class Policy> 
	---> <T, INITVAL, Policy, AccumulationTraits<T>>
the total value of the integer values is 120
<typename T, const int INITVAL> 
	---> <T, INITVAL, MultPolicy, AccumulationTraits<T>>
the total value of the integer values is 120
<> 
	---> <int, 1, MultPolicy, AccumulationTraits<int>>
the total value of the integer values is 120
<> 
	---> <int, 1, MultPolicy, AccumulationTraits<int>>
the total value of the integer values is 120
<> 
	---> <int, 1, MultPolicy, AccumulationTraits<int>>
the total value of the integer values is 120
============= characters array =============
<typename T, const int INITVAL, template <typename, typename> class Policy, typename Traits> 
	---> <T, INITVAL, Policy, Traits>
the total value of the characters in "templates" is 975
<typename T, const int INITVAL, template <typename, typename> class Policy> 
	---> <T, INITVAL, Policy, AccumulationTraits<T>>
the total value of the characters in "templates" is 975
<typename T, const int INITVAL> 
	---> <T, INITVAL, MultPolicy, AccumulationTraits<T>>
the total value of the characters in "templates" is 0
<typename T> 
	---> <T, 1, MultPolicy, AccumulationTraits<T>>
the total value of the characters in "templates" is 465857536
<typename T> 
	---> <T, 1, MultPolicy, AccumulationTraits<T>>
the total value of the characters in "templates" is 465857536
<typename T> 
	---> <T, 1, MultPolicy, AccumulationTraits<T>>
the total value of the characters in "templates" is 465857536
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