關於友元類的一些東西

對於友元類，大家應該不陌生了，友元類聲明會將整個類說明成為另一個類的友元關係，友元類的所有成員函數都可以是另一個類的友元函數；昨天在偵錯工具的時候，當我把所有程式寫在一個檔案中的時候，包括類和友元類的實現，還有main函數，編譯能通過，而且也能執行，但是當我把類的定義和實現分開來的時候，就編譯通不過了，具體代碼如下

myPoint.h

#ifndef _myPoint_H_#define _myPoint_H_//#include "Line.h"//這裡如果加上這條就會編譯通不過，去掉後就能編譯通過，也可以執行，不知道為什麼class line;class myPoint{private:float x;float y;static int count;public:myPoint();~myPoint();myPoint(myPoint& point);myPoint(float x, float y);void display();friend class Line;friend float fdis(myPoint& p1, myPoint& p2);};#endif

myPoint.cpp

#include "stdafx.h"#include <iostream>#include <math.h>#include "myPoint.h"using namespace std;int myPoint::count = 0;myPoint::myPoint(void){this->count++;}myPoint::myPoint(myPoint& p){this->x = p.x;this->y = p.y;this->count++;}myPoint::myPoint(float x, float y){this->x = x;this->y = y;this->count++;}myPoint::~myPoint(void){}void myPoint::display(){cout<<"myPoint:("<<this->x<<","<<this->y<<")"<<endl;cout<<"當前是第"<<this->count<<"座標點"<<endl;}float fdis(myPoint& p1, myPoint& p2){float f1 = p1.x - p2.x;float f2 = p2.y - p2.y;float d = pow(f1, 2) +  pow(f2, 2);d = sqrt(d);return d;}

Line.h

#ifndef _LINE_H_#define _LINE_H_#include "myPoint.h"class Line{public:Line(){}~Line(){}Line(Line& line);Line(myPoint& pp1, myPoint& pp2);float getLength();private:myPoint p1;myPoint p2;};#endif

Line.cpp

#include "stdafx.h"#include <iostream>#include <math.h>#include "Line.h"using namespace std;Line::Line(Line &line){this->p1 = line.p1;this->p2 = line.p2;}Line::Line(myPoint& pp1, myPoint& pp2){this->p1 = pp1;this->p2 = pp2;}float Line::getLength(){float f1 = this->p1.x - this->p2.x;float f2 = this->p1.y - this->p2.y;float d = pow(f1, 2) +  pow(f2, 2);d = sqrt(d);return d;}

有誰知道為什麼在myPoint.h中加上#include "Line.h"會編譯通不過？