我想实现distance(x,y,f)的是计算你要申请的次数函数f来x获得y。

例如，如果f = square再distance(2, 256, square) == 3

我在这里使用的C ++代码改编自Stepanov和McJones的《编程元素》：

DistanceType(F) distance(Domain(F) x, Domain(F) y, Square<int> f) {
    typedef DistanceType(F) N;
    // Precondition: y is reachable from x under f
    N n(0);
    while(x != y) {
        x = f(x);
        n += 1;
    }
    return n;
}

凡Domain(F)与DistanceType(F)被#defineD移动。int

我决定将函子用作函数类型，并确定此Square<T>函数模板的层次结构：

template<typename T>
class Transformation {
    public:
        Transformation() {};
        virtual T operator() (T x) = 0;
};

template<typename T>
class Square : public Transformation<T> {
    public:
        virtual T operator() (T x) { return x * x; }
};

当我尝试该distance功能时，它可以工作：

#include <iostream>
using namespace std;
int main() {
    int x = 2;
    int y = 256;
    Square<int> f = Square<int>();
    int d = distance(x, y, f);
    cout << "the distance between " << x << " and " 
        << y << " is " << d << endl;

    return 0;
}

完整说明（使用g ++编译）

我的问题：

如何制作f模板参数的类型？

我已经试过了：

template<typename F>
DistanceType(F) distance(Domain(F) x, Domain(F) y, F f) {
    typedef DistanceType(F) N;
    // Precondition: y is reachable from x under f
    N n(0);
    while(x != y) {
        x = f(x);
        n += 1;
    }
    return n;
}

并将其更改为：

typedef Square<int> F;
int d = distance<F>(x, y, f);

但是，当我编译时，出现此错误：

In file included from /usr/include/c++/5/bits/stl_algobase.h:65:0,
                from /usr/include/c++/5/bits/char_traits.h:39,
                from /usr/include/c++/5/ios:40,
                from /usr/include/c++/5/ostream:38,
                from /usr/include/c++/5/iostream:39,
                from transformations.cpp:35:
/usr/include/c++/5/bits/stl_iterator_base_types.h: In instantiation of ‘struct std::iterator_traits<Square<int> >’:
/usr/include/c++/5/bits/stl_iterator_base_funcs.h:114:5:   required by substitution of ‘template<class _InputIterator> typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator) [with _InputIterator = Square<int>]’
transformations.cpp:42:32:   required from here
/usr/include/c++/5/bits/stl_iterator_base_types.h:168:53: error: no type named ‘iterator_category’ in ‘class Square<int>’
    typedef typename _Iterator::iterator_category iterator_category;
                                                    ^
/usr/include/c++/5/bits/stl_iterator_base_types.h:169:53: error: no type named ‘value_type’ in ‘class Square<int>’
    typedef typename _Iterator::value_type        value_type;
                                                    ^
/usr/include/c++/5/bits/stl_iterator_base_types.h:170:53: error: no type named ‘difference_type’ in ‘class Square<int>’
    typedef typename _Iterator::difference_type   difference_type;
                                                    ^
/usr/include/c++/5/bits/stl_iterator_base_types.h:171:53: error: no type named ‘pointer’ in ‘class Square<int>’
    typedef typename _Iterator::pointer           pointer;
                                                    ^
/usr/include/c++/5/bits/stl_iterator_base_types.h:172:53: error: no type named ‘reference’ in ‘class Square<int>’
    typedef typename _Iterator::reference         reference;

而且我不明白这个错误。为什么不能将其Square<int>用作模板参数？