模板的Traits

Traits含义就是特性，应用Trait模板参数，使得我们的程序既保持灵活性，同时减少类型参数的数量。能够使得我们对函数进行更加细粒度的控制。

#ifndef TRAIT_H_
#define TRAIT_H_
/*
template<typename T>
T accumulate( const T *begin, const T *end )
{
    T total = T();

    while ( begin != end ) {
        total += *begin;
        ++begin;
    }

    return total;
}*/

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 long AccT;
    static AccT zero() {
        return 0;
    }
};

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

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

template<typename T, typename AT = AccumulationTraits<T> >
class Accum
{
public:
    static typename AT::AccT accumulate( const T *begin, const T *end ) {
        typename AT::AccT total = AT::zero();

        while ( begin != end ) {
            total += *begin;
            ++begin;
        }

        return total;
    }
};

template<typename T>
typename AccumulationTraits<T>::AccT accumulate( const T *begin,
        const T *end )
{
    return Accum<T>::accumulate( begin, end );
}

template<typename Traits, typename T>
typename Traits::AccT accumulate( const T *begin, const T *end )
{
    return Accum<T, Traits>::accumulate( begin, end );
}

#endif

int iv[5] = {1, 2, 3, 4, 5};
double ftotal = accumulate<typename AccumulationTraits<float>, int>( iv, iv + sizeof( iv ) / sizeof( int ) );
EXPECT_EQ( 15,  ftotal );

char cv[] = {'a', 'a', 'b', 'b'};
int total = 97 * 2 + 98 * 2;
EXPECT_EQ( total , accumulate( cv, cv + sizeof( cv ) / sizeof( char ) ) );