Call constructor of template parameter

问题

Given a templated factory method, I would like to call different constructors based on the constructors the template parameter provides:

template<typename T>
T* Factory::create()
{
  if (hasOnlyDefaultConstructor<T>())
    return new T();
  else
    return new T(this);
}

Two problems:

If T does not have a constructor T(Factory*), then there are compilation problems.
How to write hasOnlyDefaultConstructor()?

In general I would like the following:

template<typename T>
T* Factory::create()
{
  if (hasDefaultConstructor<T>())
    return new T();
  else if (hasUnaryConstructor<T>())
    return new T(create());
  else if (hasBinaryConstructor<T>())
    return new T(create(), create());
  else ....
}

Is there a way to achieve this in C++? I understand the problems if there are multiple constructors for the compiler to choose, but let's say that we only pass types T which have exactly one public constructor.

class A 
{
  A(B* b);
}

class B 
{
  B(C* c, D* d);
}  

A* a = Factory::create<A>(); // same as A* a = new A(new B());
B* b = Factory::create<B>(); // same as B* b = new B(new C(), new D());

Shouldn't it be possible to write a generic function create(), which could instantiate both B and A?

回答1:

You may want this:

struct X
{
  enum {TYPE = 0;}// has default constructor
  X() {}
}；


struct A
{
  enum {TYPE = 1;}
  typedef B P;
  A(P* p) {}
}；

struct B
{
  enum {TYPE = 2;}
  typedef C P1;
  typedef D P2;
  B(P1* p1, P2* p2) {}
}；

template<T, type> //type default = 0
struct FactoryDetail<T>
{
  static T* create(){return new T(); } 
};

template<T>
struct FactoryDetail<T, 1>
{
  static T* create(){return new T(new typename T::P()); } 
};

template<T>
struct FactoryDetail<T, 2>
{
  static T* create(){return new T(new typename T::P1()， new typename T::P2()); } 
};

//final Factory
template<T>
struct Factory
{
  static T* create(){return FactoryDetail<T, T::TYPE>::create(); } 
};

I don't have dev environment now, the above codes describing the basic idea.

回答2:

Your example is a bit strange...

I would guess that you want something like:

template<typename T>
T* Factory::create()
{
    return new T();
}

template<typename T, typename P0>
T* Factory::create(P0&& p0)
{
    return new T(std::forward<P0>(p0));
}

template<typename T, typename P0, typename P1>
T* Factory::create(P0&& p0, P1&& p1)
{
    return new T(std::forward<P0>(p0), std::forward<P1>(p1));
}

or with variadic templates:

template<typename T, typename... Args>
T* Factory::create(Args&&... args)
{
    return new T(std::forward<Args>(args)...);
}

EDIT:

Based on comment below... still wierd from a memory management view.

template<typename T>
struct factory;

template<>
struct factory<A>
{
   static A* create(){return new A(new B());}
}

template<>
struct factory<B>
{
   static B* create(){return new B(new C(), new D());}
}

int main()
{
     A* a = factory<A>::create();
     B* b = factory<B>::create();
     return 0;
}

来源：https://stackoverflow.com/questions/8711542/call-constructor-of-template-parameter

标签

c++

templates

factory-pattern