c++ - 如何使用智能指针构建树？

Question

假设我有一个员工树，每个员工都有同事，同事有同事.....我设置了两个类employee用来存储员工的信息和eTree 表示树架构。

每个员工都有一个 vector 存储智能指针指向他们的同事。看下面的代码。为什么智能指针指向的地址最后变成了00000000。我已将指针返回到函数 buildChilds 之外。

#include <iostream>
#include <string>
#include <vector>
#include <memory>

class employee
{
    std::string NAME;
    int NUM;
    int Enum = 0;
public:
    std::vector<std::shared_ptr<employee>> childs;
    employee(std::string name, int num) :NAME(name), NUM(num)
    {
        childs.resize(num);
    }
    std::string getName()
    {
        return NAME;
    }
    int getNum()
    {
        return NUM;
    }
    void setEnum(int num)
    {
        Enum = num;
    }
    int getEnum()
    {
        return Enum;
    }
};

class eTree
{
private:
    std::shared_ptr<employee> proot;
public:
    eTree(employee node) { proot = std::make_shared<employee> (node); }
    void buildTree(std::shared_ptr<employee> parent);
    std::shared_ptr<employee> getP() { return proot; }
    std::shared_ptr<employee> buildChilds(employee &parent, int i);
};
void eTree::buildTree(std::shared_ptr<employee> parent)
{
    int num = parent->getNum();
    if (num != 0)
    {
        for (int i = 0; i < num; i++)
        {
            (*parent).childs[i] = buildChilds(*parent, i);
            std::cout << (*parent).childs[i] << std::endl;
            buildTree((*parent).childs[i]);
        }
        int eNum = (*parent).getNum();
        for (int j = 0; j < num; j++)
        {
            eNum += (*((*parent).childs[j])).getEnum();
        }
        (*parent).setEnum(eNum);
    }
}
std::shared_ptr<employee> eTree::buildChilds(employee &parent, int i)
{
    std::string name;
    int num;
    std::cout << "Enter the name of " << parent.getName() << "'s " << i + 1 << " child:" << std::endl;
    std::cin >> name;
    std::cout << "How many employee work for " << parent.getName() << "'s " << i + 1 << " child:" << std::endl;
    std::cin >> num;
//  employee child(name, num);
    std::shared_ptr<employee> pchild (new employee(name, num));
//  std::shared_ptr<employee> pchild = std::make_shared<employee>(child);

    return pchild;
}

void more3(employee parent);
void noChild(employee parent);

int main()
{
    using namespace std;
    std::string name;
    cout << "Enter the employee's name: ";
    cin >> name;
    int num;
    cout << "How many employees work for him/her: ";
    cin >> num;
    employee root(name, num);
    shared_ptr<employee> proot = make_shared<employee> (root);
    eTree tree(root);
    tree.buildTree(proot);

    cout << tree.getP()->childs[0] << endl;
    cout << tree.getP()->childs[1] << endl;

    return 0;
}

可以输入a->2->b->0->c->0，然后查看结果；谢谢!

Answer 1

问题是您正在创建根员工的许多拷贝并在不同的地方使用这些拷贝。

在您的主函数中，尝试以下操作:

shared_ptr<employee> proot = make_shared<employee> (name, num);
eTree tree(proot);
tree.buildTree(proot);

然后更新你的eTree构造函数:

eTree(std::shared_ptr<employee> node) { proot = node; }

---

我的推理:

在您的代码中，您似乎创建了一名新员工:

employee root(name, num);

然后你制作一个拷贝但是一个智能指针:

shared_ptr<employee> proot = make_shared<employee> (root);

然后将原件提供给 eTree:

eTree tree(root);

并复制到 buildTree 函数:

tree.buildTree(proot);

因此，构建树为另一个员工拷贝构建树

编辑:您的 eTree 构造函数还制作了另一个拷贝作为智能指针，因此最终您得到了原始员工的 3 个实例。

如下删除employee的拷贝构造函数，你会看到会发生什么:

employee(const employee&) = delete;

我的代码会编译，你的代码不会...

---

编辑:显示 std::make_shared<>() 的示例代码构造一个新对象:

#include <memory>
#include <iostream>

class MyClass {
public:
    MyClass() { std::cout << "    -> MyClass Constructor called\n"; }
    MyClass(const MyClass& other) { std::cout << "    -> MyClass Copy Constructor called\n"; }
};

int main(int argc, char *argv[])
{
    std::cout << "[1] Making MyClass\n";
    MyClass myClass;
    std::cout << "[2] Making MyClass Pointer\n";
    MyClass* myClassPtr = new MyClass();
    std::cout << "[3] Making MyClass Shared using New\n";
    std::shared_ptr<MyClass> myClassSmart1 = std::shared_ptr<MyClass>(new MyClass());
    std::cout << "[4] Making MyClass Shared\n";
    std::shared_ptr<MyClass> myClassSmart2 = std::make_shared<MyClass>();
    std::cout << "[5] Making MyClass Shared from first MyClass\n";
    std::shared_ptr<MyClass> myClassSmart3 = std::make_shared<MyClass>(myClass);
    std::cout << "Done\n";
}

以上代码证明std::make_shared<T>()构造一个新对象，而不是仅仅为一个已经存在的对象创建一个共享指针。另外，std::make_shared<Class>(class)使用复制构造函数创建新对象。

以上在我的电脑上的输出是:

[1] Making MyClass
    -> MyClass Constructor called
[2] Making MyClass Pointer
    -> MyClass Constructor called
[3] Making MyClass Shared using New
    -> MyClass Constructor called
[4] Making MyClass Shared
    -> MyClass Constructor called
[5] Making MyClass Shared from first MyClass
    -> MyClass Copy Constructor called
Done