C++爲什麼相同變量的值有所不同？

Question

我有類Sphere和Triangle這兩個都是Intersectable的子類。 Intersectable有一個公共成員變量colour。請看下面的代碼片段：

float t_min = 100000.0f; 
pair<float, f3Vector> point_hit; 
Intersectable * object_hit; 
Triangle triangle; 
Sphere sphere_trans; 
bool hit = false; 

//loop through triangles 
for(unsigned int i = 0; i < mesh->tvi.size(); i++){ 
    ... 

    triangle = Triangle((fRGB)mesh->color[mesh->tci[i].c0], va.toVector3(), vb.toVector3(), vc.toVector3()); 
    point_hit = triangle.intersect(orig, dir, c_near, c_far); 

    if(point_hit.first != 0.0f && point_hit.first < t_min){ 
     object_hit = &triangle; 
     std::cout << "color1 " << object_hit->color << std::endl; 
hit = true; 
     ... 
    } 
} 

// loop through spheres 
for(unsigned int j = 0; j < spheres.size(); j++){ 
    ... 

    sphere_trans = Sphere(sphere.color, center3, sphere.getRadius()); 
    point_hit = sphere_trans.intersect(orig, dir, c_near, c_far); 

    if(point_hit.first != 0 && point_hit.first < t_min){ 
     object_hit = &sphere_trans; 
     std::cout << "color1 " << object_hit->color << std::endl; 
hit = true; 
     ... 
    } 
} 

if(hit){ 
    std::cout << "color2 " << object_hit->color << std::endl; 
} 

我期待，在如果我有各種各樣的color1 (1 0 0)和下輸出的輸出是color2 (...)的值outprinted顏色應該是相同的。但是，這不會發生。事實上，我總是得到相同的輸出爲color2 (...)。你能告訴我我做錯了什麼嗎？謝謝！

Answer 1

讓我們苗條下來一點......

Intersectable * object_hit; 
Sphere sphere_trans; 

// loop through spheres 
for(unsigned int j = 0; j < spheres.size(); j++) 
{ 
    ... 
    sphere_trans = Sphere(sphere.color, center3, sphere.getRadius()); 

    if(some condition) 
    { 
     object_hit = &sphere_trans; 
     ...  
    } 
} 

現在，當條件滿足時，在sphere_transobject_hit點。但是下一次循環，sphere_trans被分配一個新的對象。所以，當然，object_hit現在也指向新的對象，這可能不是你想要的。

最好的方法可能是使object_hit成爲一個對象而不是指針。或者只是將索引保存到數組中。

Answer 2

在此聲明：

object_hit = &sphere_trans; 

你分配object_hit到本地地址（for循環）變量。一旦您離開for循環，此pointer不再有效，並且取消引用pointer是未定義的行爲。