C++類函數無法更改變量

Question

當調用buildingname.damagebuilding（34）時，當前健康狀況只會暫時降低，第一次打印會打印100第二次打印66，但是在再次調用damagebuilding之後，它會再次重置爲100。 currenthealth不是在其他地方設置的，而是在構造函數中。

類：

class building { 
public: 
    int maxhealth; 
    int currenthealth; 
    int cost; 
    int level; 
    int Posx; 
    int Posy; 
    building() {} 
    building(int h, int c, int x, int y) { 
     maxhealth = h; 
     currenthealth = h; 
     cost = c; 
     level = 1; 
     Posx = x; 
     Posy = y; 
    } 

    void damageBuilding(int h) { 
     if (currenthealth - h > 0) { 
      std::cout << " Before "+std::to_string(currenthealth); 
      currenthealth=currenthealth-h; 
      std::cout << "\n After " + std::to_string(currenthealth)+"\n"; 
     } 
     else { 
      std::cout << "\n Building destroyed "; 
     } 
    } 

}; 

當它被稱爲：

void fire(int x,int y) { 
    if (activeplayer == 1 && bulletsP1 > 0 && sgrid2[x][y] != 0) { 
     if (sgrid2[x][y] > 0 && sgrid2[x][y] < 9) { 
      for (std::list<cannon>::iterator it = cannonsP1.begin(); it != cannonsP1.end(); ++it) { 
       if (it->ID == bulletsP1) { 
        findbuilding(x, y).damageBuilding(it->damage); 
       } 
      } 
     } 
     bulletsP1--; 
     sgrid2[x][y] = grid2[x][y]; 
     vgrid2[x][y] = 3; 
    } 
} 

Findbuilding：

building & findbuilding(int x, int y) { 
    if (activeplayer == 1) { 

     for each (building b in bgrid2) 
     { 
      if (b.Posx == x && b.Posy == y) { 
       return b; 
      } 
     } 

    } 
    //else if (activeplayer == 2) { 
    else { 
     for each (building b in bgrid) 
     { 
      if (b.Posx == x && b.Posy == y) { 
       return b; 
      } 
     } 
    } 
} 

Answer 1

功能

findbuilding(x, y) 

按值返回類building的實例，這意味着您只修改實例的副本。您必須返回pointer（*）或reference（&）才能生效。

---

building findbuilding(int x, int y) {} 

到

building & findbuilding(int x, int y) {} 

---

還有就是你用currenthealth = 100內存

+-----+-----+-----+-----+-----+ 
|  |  | i |  |  | 
|  |  | 100 |  |  | 
+-----+-----+-----+-----+-----+ 

如果按價值copy返回它的實例i，這種細胞是警察滅蠅燈在內存中另一個地方，iCpy

+-----+-----+-----+-----+------+ 
|  |  | i |  | iCpy | 
|  |  | 100 |  | 66 | 
+-----+-----+-----+-----+------+ 
      ^  ^
       |   --------------- You are modifying this copy 
       --------------------------- The real instance stays unchanged 

傳遞一個引用到這個實例

+-----+-----+-----+-----+-----+ 
|  |  | i |  |  | 
|  |  | 66 |  |  | 
+-----+-----+-----+-----+-----+ 
      ^   
       -------------------------- You are modifying real instance