未定義的參考`虛表的'命名空間繼承未定義的參考`所屬類別爲

Question

我有這樣的錯誤消息，在編譯時我的（簡單）的代碼：

||=== Build: Release in zad12 (compiler: GNU GCC Compiler) ===| 
obj/Release/Section.o||In function `MyFigures::Section::Section()':| 
Section.cpp|| undefined reference to `vtable for MyFigures::Figure'| 
obj/Release/Section.o||In function `MyFigures::Section::Section(Point, Point)':| 
Section.cpp|| undefined reference to `vtable for MyFigures::Figure'| 
obj/Release/Section.o||In function `MyFigures::Section::~Section()':| 
Section.cpp|| undefined reference to `vtable for MyFigures::Figure'| 
Section.cpp|| undefined reference to `vtable for MyFigures::Figure'| 
obj/Release/Section.o:(.rodata._ZTIN10MyFigures7SectionE[_ZTIN10MyFigures7SectionE]+0x10)||undefined reference to `typeinfo for MyFigures::Figure'| 
||=== Build failed: 5 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===| 

，代碼：

#ifndef _FIGURE_H_ 
#define _FIGURE_H_ 

namespace MyFigures 
{ 
class Figure 
{ 
    public: 

     Figure(){} 
     ~Figure(){} 

     virtual void draw(); 
     virtual void move(); 
     virtual void scale(double s); 
}; 
} 
#endif 

#ifndef _Section_H_ 
#define _Section_H_ 

#include "figure.h" 
#include "point.h" 
#include <exception> 

namespace MyFigures 
{ 

class Section : public Figure 
{ 
public: 
    class badsection : public std::exception 
    { 
    public: 
     const char * what() const throw() 
     { 
      return "error"; 
     } 
    }; 

    Section(); 
    Section(Point start, Point end); 
    void draw(); 
    void move(); 
    void scale(double s); 
    ~Section(); 

private: 

    Point start; 
    Point end; 
}; 

} 

#endif 

#include "section.h" 

namespace MyFigures 
{ 

Section::Section() 
{ 
} 

Section::Section(Point start, Point end) 
{ 
    if(start == end) 
     throw badsection(); 
} 

void Section::draw() {} 
void Section::move() {} 
void Section::scale(double s) {} 
Section::~Section() {} 

} 

#ifndef _Point_H_ 
#define _Point_H_ 

class Point 
{ 
    private: 

     double x,y; 

    public: 

     Point(); 
     ~Point(); 
     Point(double xx); 
     Point(double xx, double yy); 

     Point& operator=(const Point& p); 
     bool operator==(const Point& p); 

}; 

#endif 

#include "point.h" 

Point::Point() 
{ 
    x = 0; 
    y = 0; 
} 

Point::Point(double xx) 
{ 
    x = xx; 
    y = 0; 
} 

Point::Point(double xx=0, double yy=0) 
{ 
    x = xx; 
    y = yy; 
} 

Point::~Point() 
{ 
} 

Point& Point::operator=(const Point& p) 
{ 
    x = p.x; 
    y = p.y; 
    return *this; 
} 

bool Point::operator==(const Point& p) 
{ 
    return (x == p.x) && (y == p.y); 
} 

我試圖使Figure類中的析構函數成爲虛擬的，但沒有效果（錯誤仍然存​​在）。

Answer 1

問題是，您必須定義每函數您聲明。這就是編譯器抱怨的原因，您沒有爲draw，move或scale定義任何內容。

如果你想Figure是一個完整的抽象類（我認爲這是你正在嘗試做的），你可以設置這些函數爲0，所以你不必定義它們，但派生類必須執行它們。

//Now you don't need to implement them 
virtual void draw() = 0; 
virtual void move() = 0; 
virtual void scale(double) = 0;