C++ - 错误 : member access into incomplete type in base class virtual function

Question

我有一个基类Shape，它有一个虚函数intersect()。

HitRecord 是在同一 .h 文件中定义的结构。

另外，Shape 有一个子类 Triangle。我试图在 Shape::intersect() 中访问 HitRecord 的成员，但出现错误 error: member access into incomplete type in base class virtual函数

奇怪的是，我可以在子类中执行此操作，但不能在基类中执行此操作。

是不是因为它是一个虚拟函数？

注意:另一个奇怪的事情:我可以在我的 Ubuntu 16.04 上运行，但在我的 mac 上遇到这个错误。

代码

struct HitRecord;   // forward declaration

class Shape {
public:
    virtual bool intersect(Ray& r, HitRecord& rec) {
        std::cout << "Child intersect() is not implement." << std::endl;
        rec.obj = this;
        return false;
    }
}

struct HitRecord {
    float t;
    vec3f p;    // point coord
    vec3f norm;
    Shape* obj;
};

class Triangle: public Shape {
public:
    Mesh* mesh_ptr;
    unsigned int vertexIndex[3];

    Triangle() {...}

    Triangle(Mesh* m) {...}

    inline bool intersect(Ray& r, HitRecord& rec);
}

inline bool Triangle::intersect(Ray& r, HitRecord& rec) {
    vec3f n = cross(v1-v0, v2-v0);
    float t = - (dot(n, r.origin())+d) / dot(n, r.direction());
    vec3f p = r.origin() + t*r.direction();

    rec.t = t;
    rec.p = p;
    rec.norm = unit(n);
    rec.obj = this;
    return true;
}

Answer 1

这个问题被称为循环依赖。

在你的代码中..

// in shape.h

struct HitRecord;   // forward declaration

                    // this forward declaration means all you can do until 
                    // the struct is fully declared is declare a pointer
                    // or a reference to it.  No more.

class Shape {
public:
    virtual bool intersect(Ray& r, HitRecord& rec);  // <-- this is fine


    virtual bool intersect(Ray& r, HitRecord& rec) {
        //...
        rec.obj = this;   // <-- this is where you hit an error. the compiler
                          // doesn't know yet what HitRecord::obj is.
        return false;
    }
};


.. in hitrecord.h...

struct HitRecord {
    float t;
    vec3f p;    // point coord
    vec3f norm;
    Shape* obj;
};


 // this would usually reside in shape.cpp, but what's important is the order
 // in which the compiler reads the successive declarations

 #include "shape.h"
 #include "hitrecord.h"   // for example...

 bool Shape::intersect(Ray& r, HitRecord& rec) 
 {
 //...
     rec.obj = this;   // Now, the compiler knwos all about HitRecord
                       // so this will compile.
     return false;
 }