c++ - 错误 C2512 : 'LayerTwoTree' : no appropriate default constructor available

Question

我有两个像这样的头文件:

#ifndef LAYER_ONE_TREE_H
#define LAYER_ONE_TREE_H

#include "Utils.h"
#include "LayerTwoTree.h"

class LayerOneTreeNode{
public:
    friend class LayerOneTree;
    friend class LayerTwoTree;

    .
    .
    .
    LayerTwoTree* S_U1;// A pointer to the root of a layerTwoTree

    LayerOneTreeNode(){
        S_U1 = new LayerTwoTree; //here is the error Error  1   erro C2512: 'LayerTwoTree' : no appropriate default constructor available
        S_U1->init();
    }
};

class LayerOneTree{
public:
    LayerOneTree(){
    }
    .
    .
    .

private:
    .
    .
    .
};
#endif

和第二个标题:

#ifndef LAYER_TWO_TREE_H
#define LAYER_TWO_TREE_H

#include "Utils.h"
#include "LayerOneTree.h"


class LayerTwoTreeNode{
public:
    friend class LayerTwoTree;
    friend class LayerOneTree;

    .
    .
    .

    //constructor
    LayerTwoTreeNode(Point v = Point(), LayerTwoTreeNode *l = nullptr,
        LayerTwoTreeNode *r = nullptr, NodeColor c = Black)
        : key(v), color(c), left(l), right(r)
    {}
};

class LayerTwoTree{
public:
    friend class LayerOneTree;
    friend class LayerOneTreeNode;
    .
    .
    .
    LayerTwoTree(){
    }

    LayerOneTreeNode*     fatherNode;     //the father node of this tree

};

#endif

我不知道为什么当我尝试在我的 LayerOneTree 中使用 LayerTwoTree 时出现“没有合适的默认构造函数可用错误”。我认为问题是因为我想在 LayerOneTree 中有一个 LayerTwoTree 并且在我的 LayerTwoTree 中有一个 LayerOneTree。有什么办法可以解决这个问题吗？如果您需要了解有关代码的更多详细信息，请发表评论。

Answer 1

分析:

假设一些文件包含LayerTwoTree.h，相关行是:

#ifndef LAYER_TWO_TREE_H
#define LAYER_TWO_TREE_H
#include "LayerOneTree.h"

至此，LayerOneTree.h的内容被包含在翻译单元中:

#ifndef LAYER_ONE_TREE_H
#define LAYER_ONE_TREE_H
#include "LayerTwoTree.h"

此时LayerTwoTree.h的内容又被包含在了翻译单元中:

#ifndef LAYER_TWO_TREE_H
#endif

请注意，包含守卫之间的所有内容都已被跳过，因为宏已被定义!所以，回到 LayerOneTree.h:

class LayerOneTreeNode{
public:
    friend class LayerOneTree;
    friend class LayerTwoTree;

至此，两个树类已经声明，但不完整。

    LayerTwoTree* S_U1;// A pointer to the root of a layerTwoTree

创建指向不完整类型的指针是可以的，所以这可行...

    LayerOneTreeNode(){
        S_U1 = new LayerTwoTree; //here is the error Error  1   erro C2512: 'LayerTwoTree' : no appropriate default constructor available
        S_U1->init();
    }

...但此时您正在尝试创建这个不完整类的实例，MSC 对此提示并带有稍微误导性的错误消息。

解决方案:

使用这个模式:

// declare classes
class Foo;
class Bar;
// define classes
class Foo {
    Bar* p;
public:
    // declare ctor
    Foo();
};
class Bar {
    Foo* p;
public:
    // declare ctor
    Bar();
};
// define ctor
Foo::Foo() {
    p = new Bar();
}
Bar::Bar() {
    p = new Foo();
}

或者，搜索“循环依赖 C++”，您会找到更多解释。