“无效使用不完全类型的”

Question

所以我目前的任务是做一个基本的猎人与猎物的模拟，以及具有其它几个问题后，我的教授建议，我只是把一切都放在main.cpp实际得到的现在工作的东西。

我目前的问题是，在Creature::Find()函数声称Grid类是不完整的，即使它是预先声明在文件的顶部。我最初的想法是在Creature之前放置Grid类，但这会导致更多或相同的错误（指Creature不完整），因为Grid本质上是指针的二维数组。下面是代码的相关位，整个文件可以在Dropbox here上找到。

---

class Grid; //*** error: forward declaration of 'class Grid' 

//Other stuff... 

class Creature 
{ 
public: 
    Grid* theGrid; 
    Coords position; 
    int stepBreed; 
    int stepHunger; 
    char face; 
    bool hasMoved; 

    Creature(Grid* _grid, Coords _position, char _face) //Constructor 
    { 
     theGrid = _grid; 
     position = _position; 
     face = _face; 
     stepBreed = stepHunger = 0; 
     hasMoved = false; 
    } 

    vector<Coords> Find(char thisFace) //Returns a list of coords with prey on them 
    { 
     vector<Coords> result; 
     for(int i = position.x-1; i <= position.x+1; i++) 
      for(int j = position.y-1; j <= position.y+1; j++) 
      { 
       Coords temp(i,j); 
       if(theGrid->Occupant(temp) == thisFace) //*** error: invalid use of incomplete type 'class Grid' 
        result.push_back(temp); 
      } 
     return result; 
    } 

    virtual Coords Move() = 0; //Allows the creature type to define it's own movement 
    virtual Coords Breed() = 0; //Allows the creature type to define it's own breeding 
    virtual bool Starve() = 0; //Allows the creature type to starve of its own accord 
}; 

class Grid 
{ 
public: 
    Creature* grid[MAX_X][MAX_Y]; 

    Grid() //Initalizes the grid and spawns random creatures 
    { 
     cout<<endl<<"grid init"<<endl; 
     for(int i = 0; i < MAX_X; i++) 
      for(int j = 0; j < MAX_Y; j++) 
       grid[i][j] = NULL; 
    } 

    void Move() //Tells each creature on the grid to move 
    { 
     cout<<endl<<"--- Grid::Move() TOP ---"<<endl<<endl; 
     ResetMoved(); 

     for(int i = 0; i < MAX_X; i++) 
      for(int j = 0; j < MAX_Y; j++) 
       if(grid[i][j]) 
        grid[i][j]->Move(); 
     cout<<endl<<"--- Grid::Move() BOTTOM ---"<<endl<<endl; 
    } 

    void Breed() //Tells each creature to breed (if it can) 
    { 

    } 

    void Kill() //Tells each creature to die (if it's old) 
    { 

    } 

    char** Snapshot() //Creates a char array "snapshot" of the board 
    { 
     char** result = new char*[MAX_X]; 
     for(int i = 0; i < MAX_X; i++) 
     { 
      result[i] = new char[MAX_Y]; 
      for(int j = 0; j < MAX_Y; j++) 
      { 
       result[i][j] = Occupant(Coords(i, j)); 
      } 
     } 
     return result; 
    } 

    Creature* Get(Coords here) //Returns a pointer to the object at the specified position 
    { 
     return grid[here.x][here.y]; 
    } 

    char Occupant(Coords here) //Returns the character of the specified position 
    { 
     if(!Get(here)) 
      return FACE_EMPTY; 
     return Get(here)->face; 
    } 

    void Clear(Coords here) //Deletes the object at the specified position 
    { 
     cout<<endl<<"--- Grid::Clear() TOP ---"<<endl<<endl; 

     if(!Get(here)) 
     { 
      cout<<" inside if"<<endl; 
      delete Get(here); 
     } 
     cout<<" outside if"<<endl; 
     grid[here.x][here.y] = NULL; 

     cout<<endl<<"--- Grid::Clear() BOTTOM ---"<<endl<<endl; 
    } 

    void ResetMoved() 
    { 
     for(int i = 0; i < MAX_X; i++) 
      for(int j = 0; j < MAX_Y; j++) 
       if(grid[i][j]) 
        grid[i][j]->hasMoved = false; 
    } 
}; 

编辑：预览和标记工具栏不工作的一些原因。

Answer 1

你有一个循环依赖（我好像记得你以前有这个问题）。把东西放在同一个文件中并不能真正解决问题（尽管也许它可以帮助你更清楚地看到问题）。你必须做的是正确地排列事物，以便每个函数都在它需要的类之后定义。

在这种情况下，我会做这样的

class Grid; 

class Creature 
{ 
public: 
    Grid* theGrid; 
    ... 
    vector<Coords> Find(char thisFace); 
    ... 
}; 

class Grid 
{ 
public: 
    Creature* grid[MAX_X][MAX_Y]; 
    ... 
}; 

vector<Coords> Creature::Find(char thisFace) 
{ 
    ... 
} 

Creature::Find需要两个Creature类（明显）和Grid类，所以后两个类已经完全确定它莫属。

如果最终将Creature::Find定义放在头文件中，那么您必须添加inline关键字，否则您将获得多个定义。

Answer 2

远期声明是罚款声明的指针。然而，编译器不知道你的转发声明类包含了什么。所以你不能指望它能够像

class Grid; 
// 
theGrid->Occupant(temp) 

---

您需要可以包括其中包含电网的整个定义的文件。或您可以将Creature::find函数的定义移动到包含Grid.h的另一个文件中。例如，在Grid.cpp