谈谈为何需要将类的成员函数声明为private

1 析构函数声明为私有， 有些资源必须要在析构前释放掉，则将析构函数声明为私有，然后另外再定义一个公有的destroy函数，先做释放资源操作，再调用析构函数。 delete this.

2 当类成员中有文件描述符，锁之类的系统资源时，因为这些资源不具备可复制性。所以要防止使用者复制使用它们。此时的做法是将copy构造函数、赋值操作符声明为private

3 析构函数声明为私有，则在创建该类的对象时必须是在堆上创建，如果是在栈上创建会导致编译不过。原因：编译器可见性，在栈上创建对象时必须是析构函数可见，而声明为private则不可见。

#include <iostream>
#include <string>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>

using namespace std;

template<typename T>
class DeviceManager {
public:
    DeviceManager(const T a, string filePath) : m_deviceNumber(a), m_filePath(filePath) {

    }

    void CreateDevice() {
       m_file = fopen(m_filePath.c_str(), "w");
    }

    void DestroyDevice() {
        if (m_file != nullptr) {
            cout << "clean the resource" << endl;
            fclose(m_file);
            m_file = nullptr;
        }

        delete this;
    }

private:
    // private destructure.
    virtual ~DeviceManager() {
        cout << "~DeviceManager()" << endl;
    }

    // private assignment.
    DeviceManager& operator=(const DeviceManager& deviceManager)
    {
        if (&deviceManager != this) {
            m_deviceNumber = deviceManager.m_deviceNumber;
            m_file = deviceManager.m_file;
            m_filePath = deviceManager.m_filePath;
        }
        return *this;
    }

    // private copy constructure.
    DeviceManager(const DeviceManager& deviceManager)
    {
        cout << "copy constructure" << endl;
        m_deviceNumber = deviceManager.m_deviceNumber;
        m_file = deviceManager.m_file;
        m_filePath = deviceManager.m_filePath;
    }

private:
    string m_filePath;
    T m_deviceNumber;
    FILE *m_file;
};

int main()
{
    string deviceNumber = "ssd";
    //DeviceManager<string> deviceManager(deviceNumber, "~/my_project");
    DeviceManager<string> *p = new DeviceManager<string>(deviceNumber, "~/my_project/aa.log");
    p->CreateDevice();
    p->DestroyDevice();

    return 0;
}