why the internal class is't constructed? C++

Question

This is my source code.

#include <iostream>
using namespace std;

class MySingleton;

template<class T>
class CSingleton
{
public:
    CSingleton()
    {
        cout << "constructor" << endl;
    }

    static T* GetInstance()
    {
        return m_Instance;
    }

private:
    static T* m_Instance;

    // This is important
    class CGarbageCollection
    {
    public:
        CGarbageCollection()
        {
            cout << "CGarbageCollection init\r\n";
        }

        ~CGarbageCollection()
        {
            // We can destory all the resouce here, eg:db connector, file handle and so on
            if (m_Instance != NULL)
            {
                cout << "Here is the test\r\n";
                delete m_Instance;
                m_Instance = NULL;
            }
        }
    };

    static CGarbageCollection gc;
};

template<class T>
typename CSingleton<T>::CGarbageCollection CSingleton<T>::gc;

template<class T>
T* CSingleton<T>::m_Instance = new T();

class MySingleton : public CSingleton<MySingleton>
{
public:
    MySingleton(){}
    ~MySingleton(){}
};

int main()
{
    MySingleton *pMySingleton = MySingleton::GetInstance();

    return 0;
}

When I build the project, the internal class CGarbageCollection is not constructed? Why? Because this is used with template? When I delete the template, It's ok; but now, I cannot get the message.

Oh, you are not creating a garbage collector, just something to clean up the singleton. This is not a good way to create a singleton. Create a static unique_ptr. In the get function, check if the pointer is null, and if so, allocate for it. — Neil Kirk, Aug 14 '14 at 16:03
If you insist on using the singleton pattern, which [should be considered harmful](http://stackoverflow.com/a/138012/1007504), you should at least know about/use the [Meyers singleton](http://stackoverflow.com/questions/17712001/how-is-meyers-implementation-of-a-singleton-actually-a-singleton). — Robert Allan Hennigan Leahy, Aug 14 '14 at 16:04
@NeilKirk Even better, make the `unique_ptr` a `static` local variable of the function. With C++11 thread-safe statics, you get thread safety for it without extra effort that way. — Angew is no longer proud of SO, Aug 14 '14 at 16:05
@Angew: Why is this something you need free store allocation for? — Robert Allan Hennigan Leahy, Aug 14 '14 at 16:05
@RobertAllanHenniganLeahy Of course you don't. I was just commenting on the "static `unique_ptr`" part. — Angew is no longer proud of SO, Aug 14 '14 at 16:06
I didn't know local statics are only constructed when the function enters. — Neil Kirk, Aug 14 '14 at 16:10
@NeilKirk Ok, Thank you very much. I got it. But I am weak in this. But, now, I know how to do. Thank you. — vipygd, Aug 14 '14 at 17:07

score 1 · Accepted Answer · edited May 23 '17 at 12:29

So OP's question is, why gc is not instantiated in the template case because if it is not a template it is instantiated as OP expects. The reason is explained in Point of Instantiation of Static Data Members and in the answers of this question C++ Static member initalization (template fun inside).

Long story short, you need to use or reference gc in your code to instantiate. For example,

static T* GetInstance()
{
    gc;
    return m_Instance;
}

will print out the result expected.

why the internal class is't constructed? C++

1 Answers1