[TOC]

1. 基本定义

• Think of a trait as a small object whose main purpose is to carry information used by another object or algorithm to determine "policy" or "implementation details".
• In some cases the extra parameters are entirely determined by a few main paameters. 因此需要通过几个核心参数型别的定义，来设计不同的模板，满足不同需求。

2. Fixed Traits

• 主要是构造适应各种类型的函数
• Fixed主要指，一旦T在模板中被定义，则无法再调用中改变。

2.1 传统方法

• 传统方法定义的模板

#ifndef ACCUM_HPP
#define ACCUM_HPP
template <typename T>
inline
T accum (T const* beg, T const* end)
{
    T total = T();  // assume T() actually creates a zero value
    while (beg != end) {
        total += *beg;
        ++beg;
    }
    return total;
}
#endif // ACCUM_HPP

• 调用该模板

#include "accum1.hpp"
#include <iostream>
int main()
{
// create array of 5 integer values
int num[]={1,2,3,4,5};
// print average value
std::cout << "the average value of the integer values is "
          << accum(&num[0], &num[5]) / 5
          << '\n';
// create array of character values
char name[] = "templates";
int length = sizeof(name)-1;
// (try to) print average character value
std::cout << "the average value of the characters in \""
          << name << "\" is "
          << accum(&name[0], &name[length]) / length
          << '\n';
}

• 存在问题，当调用的是char时，模板自动将返回函数也定义为char，而不是期望的int。

2.2 使用Traits方法

• 模板定义

template<typename T>
class AccumulationTraits;
template<>
class AccumulationTraits<char> {
  public:
    typedef int AccT;
};
template<>
class AccumulationTraits<short> {
  public:
    typedef int AccT;
};
template<>
class AccumulationTraits<int> {
  public:
    typedef long AccT;
};
template<>
class AccumulationTraits<unsigned int> {
  public:
    typedef unsigned long AccT;
};
template<>
class AccumulationTraits<float> {
  public:
    typedef double AccT;
};
template <typename T>
inline
typename AccumulationTraits<T>::AccT accum (T const* beg,
                                            T const* end)
{
    // return type is traits of the element type
    typedef typename AccumulationTraits<T>::AccT AccT;
    AccT total = AccT();  // assume T() actually creates a zero value
    while (beg != end) {
        total += *beg;
        ++beg;
    }
    return total;
}

• 调用方法不变。

2.3 总结

3. Value Traits

• 对模板中特定数值，特别是初始话时候的操作方法
• 模板的书写，引入zero()

template<typename T>
class AccumulationTraits;
template<>
class AccumulationTraits<char> {
  public:
    typedef int AccT;
    static AccT zero() {
        return 0;
    }
};
template<>
class AccumulationTraits<short> {
  public:
    typedef int AccT;
    static AccT zero() {
        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;
    }
}; 
template <typename T>
inline
typename AccumulationTraits<T>::AccT accum (T const* beg,
                                            T const* end)
{
    // return type is traits of the element type
    typedef typename AccumulationTraits<T>::AccT AccT;
    AccT total = AccumulationTraits<T>::zero();    
    while (beg != end) {
        total += *beg;
        ++beg;
    }
    return total;
}

• 调用方法不变
• 总结

4. Parameterized Traits

• 为了增加灵活性，引入另一个参数（AT），使其来决定参数（T）的类型。
• 有两种方法，一种是基于function traits的方法

template <typename T>
inline
typename AccumulationTraits<T>::AccT accum (T const* beg,
                                            T const* end)
{
    return Accum<T>::accum(beg, end);
}
template <typename Traits, typename T>
inline
typename Traits::AccT accum (T const* beg, T const* end)
{
    return Accum<T, Traits>::accum(beg, end);
}

• 另一种是将function traits方法引入Class

template <typename T,
          typename AT = AccumulationTraits<T> >
class Accum {
  public:
    static typename AT::AccT accum (T const* beg, T const* end)
    {
        typename AT::AccT total = AT::zero();
        while (beg != end) {
            total += *beg;
            ++beg;
        }
        return total;
    }
};

• 总结

Code

• https://github.com/weiweikong/Program_Practice/blob/master/160421.Geekband_Homework8/test.h
• https://github.com/weiweikong/Program_Practice/blob/master/160421.Geekband_Homework8/test.cpp
• https://github.com/weiweikong/Program_Practice/blob/master/160421.Geekband_Homework8/test2.h
• https://github.com/weiweikong/Program_Practice/blob/master/160421.Geekband_Homework8/test2.cpp

Reference

• 项目中一次活用C++模板(traits)的经历
  http://blog.csdn.net/my_business/article/details/8098417
• Traits的一个举例
  http://www.cnblogs.com/hellogiser/p/cplusplus-traits.html
• C++ Templates - The Complete Guide - Chp. 15