我已经开发了一个c++树模板类，我想为它编写一组自定义迭代器。
GenericTree类迭代器应按如下方式使用:
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.
}
在上面的例子中，++preorder_iterator应该为为int和FORWARD类型定义的迭代器类调用重载操作符++()方法，但代码无法编译。我使用的是g++ (Ubuntu 4.8.2-19ubuntu1) 4.8.2
是迭代器类的代码。为了缩短示例，我用一个向量替换了GenericTree类，并且完全省略了begin()方法的源代码。
#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)
;
}
虽然是部分的，但前面的代码返回与构建完整源代码时相同的编译错误:
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
我找不到为PREORDER和POSTORDER类型定义操作符++()的两个模板特化的正确方法。
如果我使用template<>关键字，编译器会抱怨专门化超出了作用域，我不明白为什么。将两个模板特化移出GenericTree类体并不能解决问题。
提前感谢您的帮助!
f。
将迭代器类移出GenericTree部分专业化的struct my_iterator做这项工作。以下编译和可能有帮助:
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 () {}
};