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;
    }
};