Best way to for C++ types to self register in a list?

Question

Suppose I have some per-class data: (AandB.h)

class A
{
public:
   static Persister* getPersister();
}

class B
{
public:
   static Persister* getPersister()         


        

Answer 1

You can execute something before main once if a instantiation of a template is made. The trick is to put a static data member into a class template, and reference that from outside. The side effect that static data member triggers can be used to call the register function:

template<typename D>
struct automatic_register {
private:
    struct exec_register {
        exec_register() {
            persistenceSystem::registerPersistableType(
                D::getPersister()
            );
        }
    };
    // will force instantiation of definition of static member
    template<exec_register&> struct ref_it { };

    static exec_register register_object;
    static ref_it<register_object> referrer;
};

template<typename D> typename automatic_register<D>::exec_register 
    automatic_register<D>::register_object;

Derive the class you want to be auto-registered from automatic_register<YourClass> . The register function will be called before main, when the declaration of referrer is instantiated (which happens when that class is derived from, which will implicitly instantiate that class from the template).

Having some test program (instead of the register function, a function do_it is called):

struct foo : automatic_register<foo> {    
    static void do_it() {
        std::cout << " doit "; 
    } 
}; 

int main() { 
    std::cout << " main "; 
}

Yields this output (as expected):

doit main

Answer 2

Use file-level static blocks to perform the different registrations

A static block? What's that?

A static block is a block of code (i.e. code between curly braces, which defines a scope) that gets executed sometime before main() runs. Java has this feature, and C++ has it too-

Whatch'a talkin' bout, Willis? C++ don't have no static blocks!

No, really, C++ has static blocks. You just need to, uh, shall we say, "expose" their existence.

Hmm. Curious. And how do the static blocks help my registration problem?

It's really very simple. Right after you define class A, you register it like so:

class A { /* ... whatever ... */ };

static_block {
    persistenceSystem::registerPersistableType(A::getPersister());
}

There is one caveat, though: static blocks can be a part of the static initialization order fiasco together with any statically-initialized part of your persistence system; so you need to make sure it's ok for these static blocks to run before (most) other statics; and that it's ok for the different classes' Persister's to be registered in arbitrary order.

Answer 3

The Registerable solution is a neat idea, but has a couple of issues. Ideally, I'd like to not add code to the constructor:

1. Because it relies on calling the constructor in order to register the type, it's a little haphazard about what gets registered and what doesn't. For things like persistence, I may never call the constructor of a particular type before using the list, but I may need the type's data in the list in order to know how to un-persist an object in a file.

2. There's runtime cost during the constructor call. I'd like to front load the time cost and not pay the cost many times. If I had a vector of these objects and resized the vector I'd pay the time-cost each time the copy constructor was called.

Answer 4

Register each template at run-time in the constructor. Use a static variable per template to check if the type has already been registered. The following is a quickly hacked together example:

#include <iostream>
#include <vector>

using namespace std;

class Registerable {
    static vector<Registerable *> registry_;

public:
    static void registerFoo(Registerable *p)
    {
        registry_.push_back(p);
    }

    static void printAll()
    {
        for (vector<Registerable *>::iterator it = registry_.begin();
             it != registry_.end(); ++it)
            (*it)->print();
    }

    virtual void print() = 0;
};

vector<Registerable *> Registerable::registry_;

template <typename T>
class Foo : public Registerable {
    static bool registered_;

public:
    Foo()
    {
        if (!registered_) {
            registerFoo(this);
            registered_ = true;
        }
    }

    void print()
    {
        cout << sizeof (T) << endl;
    }
};

template <typename T> bool Foo<T>::registered_ = false;

int
main(int argc, char *argv[])
{
    Foo<char> fooChar;
    Foo<short> fooShort;
    Foo<int> fooInt;

    Registerable::printAll();

    return 0;
}

It should output the size of each template parameter in the order the classes were instantiated:

1
2
4

---

This version removes the registration code from each constructor and puts it in a base class.

#include <iostream>
#include <vector>

using namespace std;

class Registerable {
    static vector<Registerable *> registry_;

public:
    static void registerFoo(Registerable *p)
    {
        registry_.push_back(p);
    }

    static void printAll()
    {
        for (vector<Registerable *>::iterator it = registry_.begin();
             it != registry_.end(); ++it)
            (*it)->print();
    }

    virtual void print() = 0;
};

vector<Registerable *> Registerable::registry_;

template <typename T>
class Registerer : public Registerable {
    static bool registered_;

public:
    Registerer(T *self)
    {
        if (!registered_) {
            registerFoo(self);
            registered_ = true;
        }
    }
};

template <typename T> bool Registerer<T>::registered_ = false;

template <typename T>
class Foo : public Registerer<Foo<T> > {
public:
    Foo() : Registerer<Foo<T> >(this) { }

    void print()
    {
        cout << sizeof (T) << endl;
    }
};

int
main(int argc, char *argv[])
{
    Foo<char> fooChar;
    Foo<short> fooShort;
    Foo<int> fooInt;

    Registerable::printAll();

    return 0;
}

I added an example of another non-template class using the registry. So, the final output would be:

foo: 1
foo: 2
foo: 4
bar