Constructor of class in template

Question

I have a object cache class like this:

#include "boost/thread/mutex.hpp"
#include "boost/unordered_map.hpp"

template <typename type1, typename type2>
class objectCache
{
public:
    objectCache()
    {
     IDCounter = 0;
    }
    ~objectCache()
    {
     for ( it=free_objects.begin() ; it != free_objects.end(); it++ )
      delete (*it).second;
     for ( it=busy_objects.begin() ; it != busy_objects.end(); it++ )
      delete (*it).second;
    }
    type1* objectCache::Get()
    {
     boost::mutex::scoped_lock(io_mutex);
     if(free_objects.size() > 0)
     {
      it = free_objects.begin();
      type1 *temp = (*it).second;
      busy_objects[(*it).first] = temp;
      free_objects.erase(free_objects.begin());
      return temp;
     }
     type1 * temp = new type2;
     ++IDCounter;
     busy_objects[IDCounter] = temp;
     return temp;
    }
    void objectCache::Pushback(type1)
    {
     boost::mutex::scoped_lock(io_mutex);
     free_objects[ID] = socket;
     it = busy_objects.find(ID);
     busy_objects.erase(it);
    }
protected:
private:
    boost::mutex io_mutex;
    long long IDCounter;
    boost::unordered_map<long long, type1*> free_objects;
    boost::unordered_map<long long, type1*> busy_objects;
    typename boost::unordered_map<long long, type1*>::iterator it;
};

class A{
public:
    A(int num){
     number = num;
    }
    int number;
};
int main(int argc, char* argv[])
{
    objectCache<a, a(1)> intcache;
    A* temp = intcache.Get();
    cout <<temp->number <<endl;
    system("pause");
    return 0;
}

I known that "typename type2" was unnecessary but I need a way to pass a class object that have a constructor with parameter like class A to the template. or their was another way to do this ? please help.

Answer 1

You can have the template parameter on the method:

template <typename type2>
type1* objectCache::Get()

Answer 2

Do you want something like this?

template <typename type1>
class objectCache
{

// ...

template<typename type2>
type1* Get(type2 value)
{
    boost::mutex::scoped_lock(io_mutex);
    if(free_objects.size() > 0)
    {
        it = free_objects.begin();
        type1 *temp = (*it).second;
        busy_objects[(*it).first] = temp;
        free_objects.erase(free_objects.begin());
        return temp;
    }
    type1 * temp = new type1(value);
    ++IDCounter;
    busy_objects[IDCounter] = temp;
    return temp;
}

// ...

};

int main(int argc, char* argv[])
{
    objectCache<A> intcache;
    A* temp = intcache.Get(1);
    cout << temp->number << endl;
    system("pause");
    return 0;
}

Answer 3

Rather than passing an explicit value, pass in an object that creates your instance for you:

template <typename type1>
struct DefaultInstanceCreator {
  type1 * operator ()() const {
    return new type1;
  }
};

template < typename type1
         , typename InstanceCreator = DefaultInstanceCreator<type1> >
class objectCache {
public:
  objectCache (InstanceCreator const & instCreator)
    : instCreator_ (instCreator)  {
  }
  type1* Get() {
    type1 * temp = instCreator_ ();
  }
private:
  InstanceCreator instCreator_;
};

Then your object can have it's own specific creator:

class A {
public:
    A(int num){
        number = num;
    }
    int number;

public:
  struct CreateInstance  {
    CreateInstance (int value)
      : value_ (value) {
    }
    A * operator ()() const {
      return new A(value_);
    }
    int value_;
  };
};

int main(int argc, char* argv[]) {
  objectCache<A, A::CreateInstance> intcache ( A::CreateInstance (1) );
  A* temp = intcache.Get();
  return 0;
}

The advantage of this approach is that you can use your cache with objects that have different numbers of arguments to their constructors.

Answer 4

int main(int argc, char* argv[])
{    
    objectCache<a, a(1)> intcache;
    ...
}

is wrong. You are supposed to indicate the types used by the objectCache class. It should be something like

objectCache<A, A> intcache;

In the objectCache class when you instantiate objects of A, you will have to pass a parameter to its constructor.

For example:

...
type1 * temp = new type1(1);
...