模板通用樹類的模板迭代器的C ++ Operator ++重載

Question

我已經開發了一個c ++樹模板類，我想為其編寫一組自定義的迭代器。

GenericTree類迭代器應按以下方式使用：

GenericTree<int> tree;
GenericTree<int>::iterator<PREORDER> preorder_iterator;

for(preorder_iterator = tree.begin(); preorder_iterator != tree.end(); ++preorder_iterato){
  //Do something on the node pointed by the preorder_iterator.
}

在上面的示例中，++ preorder_iterator應該為為int和FORWARD類型定義的迭代器類調用重載的operator ++（）方法，但代碼不會編譯。 我正在使用g ++（Ubuntu 4.8.2-19ubuntu1）4.8.2

這是迭代器類的代碼。 為了縮短示例，我用向量替換了GenericTree類，並且完全省略了begin（）方法的源代碼。

#include <iostream>
#include <string>
#include <vector>

using namespace std;

template<typename P> class GenericTree{
        public:
                vector<P> data;
                GenericTree() {}
                ~GenericTree(){}

        class PREORDER{
        };

        class POSTORDER{
        };

        // ----------------------------------------------------------------------------------
        template<typename IT> class my_iterator : public std::iterator<std::forward_iterator_tag, P> {
                private:
                        int index;
                protected:
                public:
                        my_iterator()           { index=0; }
                        ~my_iterator()  {}

                IT& operator++();
        };
// ----------------------------------------------------------------------------------
        template<> my_iterator<PREORDER>& my_iterator<PREORDER>::operator++(){
                cout << "Preorder visit of GenericTree of nodes containing int values." << endl;
                return (*this);
        }

        template<> my_iterator<POSTORDER>& my_iterator<POSTORDER>::operator++(){
                cout << "Postorder visit of GenericTree of nodes containing int values." << endl;
                return (*this);
        }
};


int main(int argc, char** argv){
        GenericTree<int> c_int;
        GenericTree<int>::my_iterator<GenericTree<int>::PREORDER> iterator_int;

        for(iterator_int = c_int.begin(); iterator_int != c_int.end(); ++iterator_int)
                ;

        GenericTree<string> c_string;
        GenericTree<string>::my_iterator<GenericTree<string>::POSTORDER> iterator_str;

        for(iterator_str = c_string.begin(); iterator_str != c_string.end(); ++iterator__str)
                ;


}

盡管是局部的，但先前的代碼返回的錯誤與生成完整源代碼時產生的編譯錯誤相同：

g++     prova.cpp   -o prova
prova.cpp:35:11: error: explicit specialization in non-namespace scope ‘class GenericTree<P>’
  template<> my_iterator<PREORDER>& my_iterator<PREORDER>::operator++(){
           ^
prova.cpp:35:70: error: invalid use of incomplete type ‘class GenericTree<P>::my_iterator<GenericTree<P>::PREORDER>’
  template<> my_iterator<PREORDER>& my_iterator<PREORDER>::operator++(){
                                                                      ^
prova.cpp:24:30: error: declaration of ‘class GenericTree<P>::my_iterator<GenericTree<P>::PREORDER>’
  template<typename IT> class my_iterator : public std::iterator<std::forward_iterator_tag, P> {
                              ^
prova.cpp:40:11: error: explicit specialization in non-namespace scope ‘class GenericTree<P>’
  template<> my_iterator<POSTORDER>& my_iterator<POSTORDER>::operator++(){
           ^
prova.cpp:40:72: error: invalid use of incomplete type ‘class GenericTree<P>::my_iterator<GenericTree<P>::POSTORDER>’
  template<> my_iterator<POSTORDER>& my_iterator<POSTORDER>::operator++(){
                                                                        ^
prova.cpp:24:30: error: declaration of ‘class GenericTree<P>::my_iterator<GenericTree<P>::POSTORDER>’
  template<typename IT> class my_iterator : public std::iterator<std::forward_iterator_tag, P> {
                              ^
prova.cpp: In function ‘int main(int, char**)’:
prova.cpp:51:27: error: ‘class GenericTree<int>’ has no member named ‘begin’
  for(iterator_int = c_int.begin(); iterator_int != c_int.end(); ++iterator_int)
                           ^
prova.cpp:51:58: error: ‘class GenericTree<int>’ has no member named ‘end’
  for(iterator_int = c_int.begin(); iterator_int != c_int.end(); ++iterator_int)
                                                          ^
prova.cpp:57:30: error: ‘class GenericTree<std::basic_string<char> >’ has no member named ‘begin’
  for(iterator_str = c_string.begin(); iterator_str != c_string.end(); ++iterator__str)
                              ^
prova.cpp:57:64: error: ‘class GenericTree<std::basic_string<char> >’ has no member named ‘end’
  for(iterator_str = c_string.begin(); iterator_str != c_string.end(); ++iterator__str)
                                                                ^
prova.cpp:57:73: error: ‘iterator__str’ was not declared in this scope
  for(iterator_str = c_string.begin(); iterator_str != c_string.end(); ++iterator__str)
                                                                         ^
make: *** [prova] Error 1

我找不到為PREORDER和POSTORDER類型定義operator ++（）的兩個模板專業化的正確方法。

如果我使用template<>關鍵字，則編譯器會抱怨說專業化超出了范圍，我不明白為什么。 將這兩個模板專長移出GenericTree類主體並不能解決問題。

預先感謝您的幫助！

F。

Answer 1

將迭代器類移到GenericTree之外並部分專門化struct my_iterator完成此工作。 編譯后可能會有所幫助：

namespace internal
{
    class PREORDER {};
    class POSTORDER {};

    // ------------------------------------------------------------------------------
    template<typename P, typename IT>
    class my_iterator;


    template<typename P>
    class my_iterator<P, PREORDER>
    : public std::iterator<std::forward_iterator_tag, P>
    {
    private:
        int index;
    protected:
    public:
        my_iterator () { index = 0; }

        my_iterator& operator++ () {
            std::cout << "Preorder visit of GenericTree of nodes containing int values." << std::endl;
            return (*this);
        }
    };
    // ------------------------------------------------------------------------------
    template<typename P>
    class my_iterator<P, POSTORDER>
    : public std::iterator<std::forward_iterator_tag, P>
    {
    private:
        int index;
    protected:
    public:
        my_iterator () { index = 0; }

        my_iterator& operator++ () {
            std::cout << "Postorder visit of GenericTree of nodes containing int values." << std::endl;
            return (*this);
        }
    };

}

template<typename P> class GenericTree
{
public:
    template <typename IT>
    using my_iterator = internal::my_iterator<P, IT>;
    using PREORDER = internal::PREORDER;
    using POSTORDER = internal::POSTORDER;

    std::vector<P> data;
    GenericTree () {}
    ~GenericTree () {}
};

演示版