CRTP - Counting Individual Classes and Total Count

问题

I've recently learned about CRTP and have been practicing some of the better documented code examples out there to understand it better. Currently the block of code I have is able to count the instances of different derived classes but what if I want to find the total of all instances regardless of class? Here is the code I have:

template <typename CountedType>
class countable {
    static unsigned count_;
public:
    countable() { ++count_; }
    countable(countable const&) { ++count_; }
    ~countable() { --count_; }

    static unsigned n_alive() { return count_; }

};
template <typename CountedType>
unsigned countable<CountedType>::count_ = 0;

template <typename CharT>
class BasicMyString : public basic_string < CharT >, public countable<BasicMyString<CharT>> { };


typedef BasicMyString<char> MyString;
typedef BasicMyString<wchar_t> WMyString;

template <typename T>
class MyVector : public vector<T>, public countable<MyVector<T>> {};

int main() {
    MyString s1, s2;
    WMyString wms;
    MyVector<double> v;

    cout << "Number of MyString: " << MyString::n_alive() << endl;
    cout << "Number of WMyString: " << WMyString::n_alive() << endl;
    cout << "Number of MyVector: " << MyVector<double>::n_alive() << endl;
   //cout << "Total number of objects: " << countable::n_alive() << endl;
}

The last line that is currently commented out is the way I want to be able to call to see how many total objects there currently are. I know I can find the total out another way by creating a global counter and creating a new function within the class to return that global count but I want to be able to call it like this, without defining a templated argument. Do I have to make an Adapter sort of interface or is there an easier way of implementing this?

Thanks.

回答1:

Solution 1

Make countable a derived class of a non-template class, such as countable_base, and move all the counting code to the base class.

class countable_base {
    static unsigned count_;
public:
    countable_base() { ++count_; }
    countable_base(countable_base const&) { ++count_; }
    ~countable_base() { --count_; }

    static unsigned n_alive() { return count_; }

};
unsigned countable_base::count_ = 0;

template <typename CountedType>
class countable : public countable_base {};

Solution 2

Make countable a regular class.

class countable {
    static unsigned count_;
public:
    countable() { ++count_; }
    countable(countable const&) { ++count_; }
    ~countable() { --count_; }

    static unsigned n_alive() { return count_; }

};
unsigned countable::count_ = 0;

回答2:

You may create an extra class to do the count which will be the base of your current template countable class, something like:

class countable_total {
    static unsigned count_;
protected:
    countable_total() { ++count_; }
    countable_total(countable_total const&) { ++count_; }
    ~countable_total() { --count_; }
public:
    static unsigned n_alive() { return count_; }
};

template <typename CountedType>
class countable : public countable_total {
    static unsigned count_;
public:
    countable() { ++count_; }
    countable(countable const&) { ++count_; }
    ~countable() { --count_; }

    static unsigned n_alive() { return count_; }
};

template <typename CountedType>
unsigned countable<CountedType>::count_ = 0;

// in cpp
unsigned countable_total::count_ = 0;

来源：https://stackoverflow.com/questions/25066469/crtp-counting-individual-classes-and-total-count

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crtp