分配空间到list< int>使用malloc的指针不起作用 [英] allocating space to list<int> pointers using malloc not working

问题描述

class graph
{

    int v;
    list<int> *adj;
    void dfsutil(int v,bool visited []);
    public:
    graph(int v)
    {
        this->v=v;
        //adj = new list<int>[v];
        adj = (list<int> *)malloc(v*sizeof(list<int>));
    }
    void addedge(int v,int w);
    void dfs(int v);
};


void graph::addedge(int v,int w)
{
    adj[v].push_back(w);
}

void graph::dfsutil(int v,bool visited[])
{
    list<int>::iterator i;
    cout<<v<<" ";
    visited[v]=true;

    for(i=adj[v].begin();i!=adj[v].end();i++)
    {
        if(!visited[*i])
            dfsutil(*i,visited);
    }
}

void graph::dfs(int v)
{
    int i=0;
    bool visited[this->v];
    for(i=0;i<this->v;i++)
        visited[i]=false;
    dfsutil(v,visited);
    for(i=0;i<v;i++)//this loop is required if there are multiple component of the graph
        if(!visited[i])
            dfsutil(i,visited);
}

int main()
{
    // Create a graph given in the above diagram
    graph g(4);
    g.addedge(0, 1);
    g.addedge(0, 2);
    g.addedge(1, 2);
    g.addedge(2, 0);
    g.addedge(2, 3);
    g.addedge(3, 3);

    cout << "Following is Depth First Traversal (starting from vertex 2) \n";
    g.dfs(2);

    return 0;
}

在上面的代码中，如果尝试使用上面编写的malloc为列表* adj分配空间，则无法正常工作，而如果我们使用new，则如上面的注释部分所述，效果很好，我无法弄清楚为什么

in the above code if try to allocate space for list *adj using malloc as written above,it does not work fine whereas if we use new,it works fine as its written in commented part above,i cant figure out why

推荐答案

使用malloc时没有创建std::list 对象的数组. malloc所做的只是从堆中分配内存-不创建任何对象.因此，尝试像正确创建std::list那样使用std::list会导致不确定的行为.

You did not create an array of std::list objects when you used malloc. All malloc does is allocate memory from the heap -- no objects are created. Thus attempting to use your std::list's as if they are created correctly will result in undefined behavior.

您应该使用诸如std::vector之类的容器来存储列表对象:

You should use a container such as std::vector to store your list objects:

#include <vector>
#include <list>

class graph
{
    int v;
    std::vector<std::list<int>> adj;
    void dfsutil(int v,bool visited []);
    public:
        graph(int num) : v(num), adj(num) {}
    void addedge(int v,int w);
    void dfs(int v);
};

请注意，无需分配内存.您的其余代码应保持不变，因为vector具有重载的operator []来访问项目.

Note there is no need to allocate memory. The rest of your code should stay the same, since vector has an overloaded operator [] to access the items.

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