关于友元的类的，一个编译错误，求解
这个是主文件

C/C++ code

#include <iostream>#include <vector>#include "Triangulate_iterator.h"using namespace std;static int arr[13] = {1,1,2,3,5,8,13,21,34,55,89,144,233};class Triangulate {public:    typedef Triangulate_iterator iterator;    friend class Triangulate_iterator;      //这里已经声明Triangulate_iterator为友元类    Triangulate();    Triangulate(int length , int beg_pos);    Triangulate(int length);    Triangulate(const Triangulate&);    Triangulate_iterator begin() {        return Triangulate_iterator(_beg_pos);    }    Triangulate_iterator end() {        return Triangulate_iterator(_beg_pos + _length);    }    int length() const { return _length; }    int beg_pos() const { return _beg_pos; }    int length() { return _length; }    int beg_pos() { return _beg_pos; }    bool next(int &x) const;    void next_reset() const;    static bool is_elem( int );    static void gen_elements( int length );    static void gen_elems_to_value( int value );    static void display( int length, int beg_pos, ostream &os = cout );    static vector<int> _elem;private:    string _name;    int _length;    int _beg_pos;    mutable int _next;    static const int _max_elems = 1024;};Triangulate::Triangulate()    : _length(1),_beg_pos(1),_next(0),_name("Triangulate") {}Triangulate::Triangulate(int l, int b) : _name("Triangulate") {    _length = l > 0 ? l : 1;    _beg_pos = b > 0 ? b : 1;    _next = _beg_pos-1;}Triangulate::Triangulate(int l) : _name("Triangulate") {    _length = l > 0 ? l : 1;    _beg_pos = 1;    _next = _beg_pos - 1;}Triangulate::Triangulate(const Triangulate &rhs) {    _length = rhs._length;    _beg_pos = rhs._beg_pos;    _next = rhs._next;    _name = rhs._name;}bool Triangulate::next(int &elem) const {    if(_next >= _length)        return false;    elem = _elem[_next];    _next++;    return true;}void Triangulate::next_reset() const {    _next = _beg_pos - 1;}vector<int> Triangulate::_elem(arr,arr+13);int sum( const Triangulate &trian ) {    if ( !trian.length())        return 0;    int sum = 0;    int val = 0;    trian.next_reset();    while ( trian.next( val ))        sum += val;    return sum;}bool Triangulate::is_elem( int value ) {    if ( !_elem.size() || _elem[_elem.size()-1] < value)        gen_elems_to_value(value);    vector<int>::iterator found;    vector<int>::iterator last = _elem.end();    found = find(_elem.begin(), last, value);    return found != last;}void Triangulate::gen_elems_to_value( int value ) {    int ix = _elem.size();    if ( !ix ) {        _elem.push_back(1);        ix = 1;    }    if ( ix == 1) {        _elem.push_back(1);        ix = 2;    }    while ( _elem[ix-1] < value && ix < _max_elems) {        _elem.push_back(_elem[ix-2] + _elem[ix-1]);        ix++;    }    if( ix == _max_elems )        cerr << " value too large " << value << " -- exceeds max size of "            << _max_elems << endl;}void Triangulate::gen_elements( int length ) {    if ( length <= 0 || length > _max_elems )        return;    int ix = _elem.size();    if ( length == 1 && ix < length ) {        _elem.push_back(1);        ix++;    }    if ( length == 2 && ix < length ) {        if( ix == 0) {            _elem.push_back(1);            ++ix;        }        _elem.push_back(1);        ++ix;    }    for (; ix < length; ix++ ) {        _elem.push_back(_elem[ix-2] + _elem[ix-1]);    }}

Triangulate_iterator.h文件

C/C++ code

#ifndef TRIANGULATE_ITERATOR_H_INCLUDED#define TRIANGULATE_ITERATOR_H_INCLUDEDclass Triangulate;class Triangulate_iterator {public:    class Triangulate;    Triangulate_iterator(int index) : _index(index-1) {}    bool operator== ( const Triangulate_iterator& ) const;    bool operator!= ( const Triangulate_iterator& ) const;    int Triangulate_iterator::operator* (const Triangulate_iterator &rhs);    Triangulate_iterator& operator++ ();    Triangulate_iterator operator++ ( int );private:    void check_integrity() const;    int _index;};inline bool Triangulate_iterator::operator == ( const Triangulate_iterator& rhs ) const {    return this->_index == rhs._index;}inline bool Triangulate_iterator::operator != ( const Triangulate_iterator& rhs ) const {    return (! (*this == rhs ));}int Triangulate_iterator::operator* (const Triangulate_iterator &rhs) {    rhs.check_integrity();    return Triangulate::_elem[_index];     //这里编译出现错误}Triangulate_iterator& Triangulate_iterator::operator++ () {    _index++;    check_integrity();    return *this;}Triangulate_iterator Triangulate_iterator::operator++ ( int ) {    Triangulate_iterator tem = *this;    _index++;    check_integrity();    return tem;}void Triangulate_iterator::check_integrity() const {   }#endif // TRIANGULATE_ITERATOR_H_INCLUDED 
 

主要是这里
int Triangulate_iterator::operator* (const Triangulate_iterator &rhs) {
rhs.check_integrity();
return Triangulate::_elem[_index]; //这里编译出现错误
}
错误提示是
D:\cpp\codblock\essentialch4\Triangulate_iterator.h|49|error: incomplete type `Triangulate' used in nested name specifier|

错误:不完全类型“Triangulate”用于嵌套名称说明符|

不懂这个错误是什么意思，要怎么改啊！

[解决办法]
你这个定义方法陷入了先有鸡还是先有蛋的问题。
你的Triangulate_iterator依赖于Triangulate //编译错误那里直接使用了本文件前面只声明，还未定义的类型。
而Triangulate又依赖于Triangulate_iterator //比如你在begin里直接返回Triangulate_iterator对象

记住一点：在使用某个类的成员，或者构造对象 前的文件中必须含有些类的完整定义 ！ 只有个声明的情况下，只能使用指针（和引用？） 。 但是不能访问其成员！

iterator里面不能使用原始类型了，得使用原始对象“里面”的类型，比如这个可以使用vector<int> _elem传进去。
代码如下，使用cc -c Triangulate.c 编译通过。

C/C++ code

#include <iostream>#include <vector>#include <algorithm>  //使用find得包含这个头#include "Triangulate_iterator.h"using namespace std;static int arr[13] = {1,1,2,3,5,8,13,21,34,55,89,144,233};class Triangulate {public:    typedef Triangulate_iterator iterator;    friend class Triangulate_iterator;      //这里已经声明Triangulate_iterator为友元类    Triangulate();    Triangulate(int length , int beg_pos);    Triangulate(int length);    Triangulate(const Triangulate&);    Triangulate_iterator begin() {           return Triangulate_iterator(_elem,_beg_pos);    //这里改下    }    Triangulate_iterator end() {        return Triangulate_iterator(_elem , _beg_pos + _length);    }    int length() const { return _length; }    int beg_pos() const { return _beg_pos; }    int length() { return _length; }    int beg_pos() { return _beg_pos; }    bool next(int &x) const;    void next_reset() const;    static bool is_elem( int );    static void gen_elements( int length );    static void gen_elems_to_value( int value );    static void display( int length, int beg_pos, ostream &os = cout );    static vector<int> _elem;private:    string _name;    int _length;    int _beg_pos;    mutable int _next;    static const int _max_elems = 1024;};Triangulate::Triangulate()    : _length(1),_beg_pos(1),_next(0),_name("Triangulate") {}Triangulate::Triangulate(int l, int b) : _name("Triangulate") {    _length = l > 0 ? l : 1;    _beg_pos = b > 0 ? b : 1;    _next = _beg_pos-1;}Triangulate::Triangulate(int l) : _name("Triangulate") {    _length = l > 0 ? l : 1;    _beg_pos = 1;    _next = _beg_pos - 1;}Triangulate::Triangulate(const Triangulate &rhs) {    _length = rhs._length;    _beg_pos = rhs._beg_pos;    _next = rhs._next;    _name = rhs._name;}bool Triangulate::next(int &elem) const {    if(_next >= _length)        return false;    elem = _elem[_next];    _next++;    return true;}void Triangulate::next_reset() const {    _next = _beg_pos - 1;}vector<int> Triangulate::_elem(arr,arr+13);int sum( const Triangulate &trian ) {    if ( !trian.length())        return 0;    int sum = 0;    int val = 0;    trian.next_reset();    while ( trian.next( val ))        sum += val;    return sum;}bool Triangulate::is_elem( int value ) {    if ( !_elem.size() || _elem[_elem.size()-1] < value)        gen_elems_to_value(value);    vector<int>::iterator found;    vector<int>::iterator last = _elem.end();    found = find(_elem.begin(), last, value);    return found != last;}void Triangulate::gen_elems_to_value( int value ) {    int ix = _elem.size();    if ( !ix ) {        _elem.push_back(1);        ix = 1;    }    if ( ix == 1) {        _elem.push_back(1);        ix = 2;    }    while ( _elem[ix-1] < value && ix < _max_elems) {        _elem.push_back(_elem[ix-2] + _elem[ix-1]);        ix++;    }    if( ix == _max_elems )        cerr << " value too large " << value << " -- exceeds max size of "            << _max_elems << endl;}void Triangulate::gen_elements( int length ) {    if ( length <= 0 || length > _max_elems )        return;    int ix = _elem.size();    if ( length == 1 && ix < length ) {        _elem.push_back(1);        ix++;    }    if ( length == 2 && ix < length ) {        if( ix == 0) {            _elem.push_back(1);            ++ix;        }        _elem.push_back(1);        ++ix;    }    for (; ix < length; ix++ ) {        _elem.push_back(_elem[ix-2] + _elem[ix-1]);    }} 
 
[解决办法]
return Triangulate::_elem[_index];     //这里编译出现错误

因为你前面只有class Triangulate
编译器只认识Triangulate 不认识_elem

把这东西丢进cpp里 在cpp的前面+ #include "Triangulate.h"
int Triangulate_iterator::operator* (const Triangulate_iterator &rhs) {
   rhs.check_integrity();
   return Triangulate::_elem[_index];     //这里编译出现错误
}