类不同headers不认识对方？

Question

即使 visual studio pre-compiler 或其他名称将 Graph 识别为来自不同 header 的 class，但在构建后我得到最荒谬的错误作为如果我以前从未提到过其他 header。首先我没有向前声明两个 classes 并且下面的第一组错误来自于此，但是后来我尝试向前声明并且有与 classes 本身的结构相关的类似错误。使用来自另一个 class 的函数生成它们，这表明 header 文件什么都不做。他们不知道彼此的功能，我也不知道为什么。

Vertex.h :

#pragma once
#include "Graph.h"
#include <vector>

class Graph;
class Vertex
{
    int unique_id;
    int longestChain = 0;
    int chainComponent_id;
    std::vector<int> edges;
    Graph* master;
public:
    int get_id()
    {
        return unique_id;
    }

    int getChainComponent_id()
    {
        return chainComponent_id;
    }

    void setChainComponent_id(int id)
    {
        chainComponent_id = id;
    }

    int DFS(int, int);

    Vertex(int id, std::vector<int> _edges, Graph* _master)
    {
        unique_id = id;
        edges = _edges;
        master = _master;
        longestChain = 0;
        chainComponent_id = -1;
    }
};

Graph.h :

#pragma once
#include "Vertex.h"
#include <vector>
#include <iostream>
class Vertex;
class Graph
{
    std::vector<Vertex*> vertex;
    int amountOfChainComponents = 0;
public:
    Vertex* getVertex(int id)
    {
        if(id<0 || id>vertex.size())
        {
            return nullptr; //shouldn't be possible with proper input
        }
        return vertex[id];
    }
    int getAmountOfChainComponents()
    {
        return amountOfChainComponents;
    }
    int longestChain()
    {
        int longest = 0;
        for(int i = 0; i < vertex.size(); i++)
        {
            if(vertex[i]->getChainComponent_id() == -1)
            {
                int tmp = vertex[i]->DFS(0, amountOfChainComponents);
                amountOfChainComponents++;
                if(tmp > longest)
                {
                    longest = tmp;
                }
            }
        }
        if(longest == -1)
        {
            std::cout << "There is a chain for every positive integer" << std::endl;
            return -1;
        }
        if(longest < 2)
        {
            std::cout << "There is no chain" << std::endl;
            return 0;
        }

        return longest;

    }
    Graph(std::vector<std::vector<int>> vertices)
    {
        amountOfChainComponents = 0;
        for(int i = 0; i < vertices.size(); i++)
        {
            Vertex* tmp = new Vertex(i, vertices[i], this);
            vertex.push_back(tmp);
        }
    }
    ~Graph()
    {
        while(!vertex.empty())
        {
            delete vertex[vertex.size() - 1];
            vertex.pop_back();
        }
    }
};

Line Severity Description File 11 Error syntax error: missing ';' before '*' c:\users\bico\source\repos\longestchaingraph\longestchaingraph\vertex.h 34 Error '_master': undeclared identifier c:\users\bico\source\repos\longestchaingraph\longestchaingraph\vertex.h 11 Error missing type specifier - int assumed. Note: C++ does not support default-int c:\users\bico\source\repos\longestchaingraph\longestchaingraph\vertex.h 11 Error unexpected token(s) preceding ';' c:\users\bico\source\repos\longestchaingraph\longestchaingraph\vertex.h 30 Error syntax error: identifier 'Graph' c:\users\bico\source\repos\longestchaingraph\longestchaingraph\vertex.h 34 Error 'master': undeclared identifier c:\users\bico\source\repos\longestchaingraph\longestchaingraph\vertex.h Line Severity Description File 8 Error 'Vertex': undeclared identifier c:\users\bico\source\repos\longestchaingraph\longestchaingraph\graph.h

前向声明后出现的错误：

Line Severity Description File 28 Error use of undefined type 'Vertex' c:\users\bico\source\repos\longestchaingraph\longestchaingraph\graph.h 28 Error left of '->getChainComponent_id' must point to class/struct/union/generic type c:\users\bico\source\repos\longestchaingraph\longestchaingraph\graph.h 30 Error use of undefined type 'Vertex' c:\users\bico\source\repos\longestchaingraph\longestchaingraph\graph.h 30 Error left of '->DFS' must point to class/struct/union/generic type c:\users\bico\source\repos\longestchaingraph\longestchaingraph\graph.h 57 Error use of undefined type 'Vertex' c:\users\bico\source\repos\longestchaingraph\longestchaingraph\graph.h

Answer 1

这是一个循环依赖问题；这两个头文件相互包含。

对于这两种情况，只有 forward declaration will be enough; declare a pointer to class doesn't need the class to be complete type.

Vertex.h

#pragma once
#include <vector>

class Graph;
class Vertex
{
    int unique_id;
    int longestChain = 0;
    int chainComponent_id;
    std::vector<int> edges;
    Graph* master;
};

Graph.h

#pragma once
#include <vector>
#include <iostream>

class Vertex;
class Graph
{
    std::vector<Vertex*> vertex;
    int amountOfChainComponents = 0;
};

编辑

将成员函数的实现移动到实现文件中。例如

Vertex.h

#pragma once
#include <vector>

class Graph;
class Vertex
{
    int unique_id;
    int longestChain = 0;
    int chainComponent_id;
    std::vector<int> edges;
    Graph* master;
public:
    int get_id();
    ...
};

Vertex.cpp

#pragma once
#include "Vertex.h"
#include "Graph.h"

int Vertex::get_id()
{
    return unique_id;
}
...

Graph.h

#pragma once
#include <vector>
#include <iostream>

class Vertex;
class Graph
{
    std::vector<Vertex*> vertex;
    int amountOfChainComponents = 0;
public:
    Vertex* getVertex(int id);
    ...
};

Graph.cpp

#pragma once
#include "Vertex.h"
#include "Graph.h"

Vertex* Graph::getVertex(int id)
{
    if(id<0 || id>vertex.size())
    {
        return nullptr; //shouldn't be possible with proper input
    }
    return vertex[id];
}
...

编辑2

正如@M.M 所指出的，前向声明对于 Vertex.h 中的 class Graph 就足够了。所以你可以只删除 Vertex.h 中的 #include "Graph.h"，并保留 Graph.h 中的 #include "Vertex.h"。

类不同headers不认识对方？

Classes in different headers do not recognize each other?

c++

circular-dependency

header-files

visual-studio-2017

编辑

编辑2