不能用通告包含转发声明

Question

我一直在寻找这个问题的解决方案，这里的每一个答案都有一些关于不包括头文件的变体，但我似乎已经被愚弄了，无法弄清楚。 ..

我有基本的 C++ class creature，应该在 ladybird 和 aphid 中实现，但我不断收到 expected class-name before ‘{’ token 错误。

creature.h

#ifndef CREATURE_H_
#define CREATURE_H_

#include <iostream>
#include "board.h"
#include "cell.h"

class Creature {
  private:
    int x, y;
  protected:
    Creature(int x, int y);
  public:
    int getX();
    int getY();
    //virtual bool willMove() = 0;
    //virtual void doMove(Board *board) = 0;
    //std::ostream& operator<<(std::ostream &out, Creature &cCreature);
};

#endif

aphid.h:

#ifndef APHID_H_
#define APHID_H_

#include "creature.h"

class Aphid : public Creature {
  public:
    Aphid(int x, int y);
    static float moveProbability;
    static float killProbabllity;
    static float killModifier;
    static float matingProbability;
    //virtual bool willMove();
    //virtual void doMove(Board *board);
};

#endif

ladybird.h:

#ifndef LADYBIRD_H_
#define LADYBIRD_H_

#include "creature.h"

class Ladybird : public Creature {
  public:
    Ladybird(int x, int y);
    static float moveProbability;
    static float directionProbability;
    static float killProbabllity;
    static float matingProbability;
    //virtual bool willMove();
    //virtual void doMove(Board *board);
};

#endif

完整堆栈跟踪

g++ src/*.cpp -o out/main.o -Wall -std=c++11 && ./out/main.o
In file included from src/cell.h:8:0,
                 from src/board.h:4,
                 from src/creature.h:5,
                 from src/aphid.h:4,
                 from src/aphid.cpp:2:
src/ladybird.h:6:34: error: expected class-name before ‘{’ token
 class Ladybird : public Creature {
                                  ^
src/cell.cpp: In member function ‘void Cell::simulateMove(Board*)’:
src/cell.cpp:44:19: error: ‘class Aphid’ has no member named ‘willMove’
         if((*it)->willMove()){
                   ^
src/cell.cpp:45:18: error: ‘class Aphid’ has no member named ‘doMove’
           (*it)->doMove(newBoard);
                  ^
In file included from src/cell.h:7:0,
                 from src/board.h:4,
                 from src/creature.h:5,
                 from src/creature.cpp:1:
src/aphid.h:6:31: error: expected class-name before ‘{’ token
 class Aphid : public Creature {
                               ^
In file included from src/cell.h:8:0,
                 from src/board.h:4,
                 from src/creature.h:5,
                 from src/creature.cpp:1:
src/ladybird.h:6:34: error: expected class-name before ‘{’ token
 class Ladybird : public Creature {
                                  ^
In file included from src/cell.h:7:0,
                 from src/board.h:4,
                 from src/creature.h:5,
                 from src/ladybird.h:4,
                 from src/ladybird.cpp:2:
src/aphid.h:6:31: error: expected class-name before ‘{’ token
 class Aphid : public Creature {
                               ^

我已经尝试向前声明 Creature class，但后来我得到 invalid use of incomplete type ‘class Creature’

board.h:

#ifndef BOARD_H_
#define BOARD_H_
#include "cell.h"
#include "creature.h"

class Board{
  private:
    Cell ***board; // A 2D ARRAY OF POINTERS, "what if i just add more stars??!" -- Owen G
    int width;
    int height;
  public:
    Board(int width, int height);
    void populate();
    Cell* getCell(int x, int y);
    void print();
    void simulateMove(Board *board);
};

#endif

cell.h:

#ifndef CELL_H_
#define CELL_H_

#include <iostream>
#include <list>
#include "aphid.h"
#include "ladybird.h"
#include "board.h"

class Board;
class Aphid;
class Ladybird;

class Cell{
  private:
    int x, y;
    std::list<Aphid*> aphids;
    std::list<Ladybird*> ladybirds;

  public:
    Cell(int x, int y);
    void addCreature(Aphid *creature);
    void addCreature(Ladybird *creature);
    int getAphidCount();
    int getLadybirdCount();
    void simulateMove(Board *nextGenerationBoard);
};

#endif

Answer 1

您的文件中有循环 #include 依赖项。
假设您首先在一些 .cpp 文件中包含 "aphid.h"，以下是您要按顺序包含的文件：

creature.h（定义包含保护符号CREATURE_H_）
board.h（来自creature.h）
cell.h（来自board.h）
aphid.h（来自 cell.h，因为包含保护而跳过）
ladybird.h（来自cell.h）
creature.h（来自 ladybird.h， 因包含防护 而跳过）

-> ladybird.h 将无法解析，因为当 Ladybird class 声明发生时 Creature 仍未定义：
```
class Ladybird : public Creature { ... };
```

为了解决这个问题，您应该从文件 cell.h 中删除以下行：

#include "aphid.h"
#include "ladybird.h"

这可以很好地编译，因为您在 cell.h 中使用这些 classes 的唯一方法是使用指向它们的指针，这不需要完整的声明，并且会破坏循环包含。

出于同样的原因，您可能还应该从该文件中删除 #include "board.h"。当你这样做时，你还可以从 board.h 文件中删除 #include "cell.h"。

Answer 2

在 cell.h 中，您有：

#include "aphid.h"
#include "ladybird.h"
#include "board.h"

class Board;
class Aphid;
class Ladybird;

这没有意义。您首先包含整个 class 定义，然后再次声明 classes。只需删除 #include 行：

class Board;
class Aphid;
class Ladybird;

在 board.h 中，您有：

#include "cell.h"
#include "creature.h"

然后是下面的class定义：

class Board{
  private:
    Cell ***board; // A 2D ARRAY OF POINTERS, "what if i just add more stars??!" -- Owen G
    int width;
    int height;
  public:
    Board(int width, int height);
    void populate();
    Cell* getCell(int x, int y);
    void print();
    void simulateMove(Board *board);
};

Creature不在任何地方使用，Cell仅用于指针。您也可以安全地删除这些 #include 行，并用 Cell.

的前向声明替换它们

顺便说一句：你应该尽快摆脱你的 Cell*** 并改用 std::vector。

不能用通告包含转发声明

cannot forward declare with circular includes

c++

oop