Suppose we have a class

struct X
{
  X ( int, _std__string_ ) ;
};

And a container for it which supports an empty state. That is, a container which can contain zero objects:

struct C
{
   C() : contained_(0) {}
  ~C() { delete contained_ ; }
  X* contained_ ;
};

A container designed to support an empty state typically does not require the contained type to be DefaultConstructible, but it typically requires it to be CopyConstructible as a mechanism to initialize the object to store:

struct C
{
   C() : contained_(0) {}
   C ( X const& v ) : contained_ ( new X(v) ) {}
  ~C() { delete contained_ ; }
  X* contained_ ;
};

There is a subtle problem with this: since the mechanism used to initialize the stored object is copy construction, there must exist a previously constructed source object to copy from. This object is likely to be temporary and serve no purpose besides being the source:

void foo()
{
  // Temporary object created.
  C c( X(123,"hello") ) ;
}

A solution to this problem is to support direct construction of the contained object right in the container's storage.

In this scheme, the user supplies the arguments for the X constructor directly to the container:

struct C
{
   C() : contained_(0) {}
   C ( X const& v ) : contained_ ( new X(v) ) {}
   C ( int a0, std::string a1 ) : contained_ ( new X(a0,a1) ) {}
  ~C() { delete contained_ ; }
  X* contained_ ;
};

void foo()
{
  // Wrapped object constructed in-place
  // No temporary created.
  C c(123,"hello");
}

Clearly, this solution does not scale well since the container must duplicate all the constructor overloads from the contained type, or at least all those which are to be supported directly in the container.

This library proposes a framework to allow some containers to directly construct contained objects in-place without requiring the entire set of constructor overloads from the contained type. It also allows the container to remove the CopyConstructible requirement from the contained type since objects can be directly constructed in-place without need of a copy.

The only requirement on the container is that it must provide proper storage. That is, the container should be correctly aligned and sized. Naturally, the container will typically support uninitialized storage to avoid the in-place construction to override a fully-constructed object, as this would defeat the purpose of in-place construction.

For this purpose, the framework provides two concepts called: InPlaceFactories and TypedInPlaceFactories. Helpers to declare these classes are declared in <boost/utility/in_place_factory.hpp> and <boost/utility/typed_in_place_factory.hpp>.

Essentially, these classes hold a sequence of actual parameters and a method to construct an object in place using these parameters. Each member of the family differs only in the number and type of the parameter list. The first family takes the type of the object to construct directly in method provided for that purpose, whereas the second family incorporates that type in the factory class itself. From the container point of view, using the framework amounts to calling the factory's method to contruct the object in place. From the user point of view, it amounts to creating the right factory object to hold the parameters and pass it to the container.

The following simplified example shows the basic idea. A complete example follows the formal specification of the framework:

struct C
{
   template <class InPlaceFactory>
   C ( InPlaceFactory const& aFactory )
    :
    contained_ ( uninitialized_storage() )
   {
     aFactory.template apply<X>(contained_);
   }

  ~C()
  {
    contained_ -> X::~X();
    delete[] contained_ ;
  }

  char* uninitialized_storage() { return new char[sizeof(X)] ; }

  char* contained_ ;
};

void foo()
{
  C c( in_place(123,"hello") ) ;
}

The following is the first member of the family of InPlaceFactory classes, along with its corresponding helper template function. The rest of the family varies only in the number and type of template and constructor parameters.

namespace boost {

struct in_place_factory_base {};

template<class A0>
class in_place_factory : public in_place_factory_base
{
  public:
    in_place_factory ( A0 const& a0 ) : m_a0(a0) {}

    template< class T >
    void apply ( void* address ) const
    {
      new (address) T(m_a0);
    }

  private:
      A0 const& m_a0 ;
};

template<class A0>
in_place_factory<A0> in_place ( A0 const& a0 )
{
  return in_place_factory<A0>(a0);
}

}

Similarly, the following is the first member of the family of typed_in_place_factory classes, along with its corresponding helper template function. The rest of the family varies only in the number and type of template and constructor parameters.

namespace boost {

struct typed_in_place_factory_base {};

template<class T, class A0>
class typed_in_place_factory : public typed_in_place_factory_base
{
  public:
    typed_in_place_factory ( A0 const& a0 ) : m_a0(a0) {}

    void apply ( void* address ) const
    {
      new (address) T(m_a0);
    }

  private:
    A0 const& m_a0 ;
};

template<class T, class A0>
typed_in_place_factory<A0> in_place ( A0 const& a0 )
{
  return typed_in_place_factory<T,A0>(a0);
}
}

As you can see, the in_place_factory and typed_in_place_factory template classes vary only in the way they specify the target type: in the first family, the type is given as a template argument to the apply member function while in the second it is given directly as part of the factory class.

When the container holds a unique non-polymorphic type, such as the case of Boost.Optional, it knows the exact dynamic-type of the contained object and can pass it to the apply() method of a non-typed factory. In this case, end users can use an in_place_factory instance which can be constructed without the type of the object to construct.

However, if the container holds heterogeneous or polymorphic objects, such as the case of Boost.Variant, the dynamic-type of the object to be constructed must be known by the factory. In this case, end users must use a typed_in_place_factory instead.

As shown in the introductory simplified example, the container class must contain methods that accept an instance of these factories and pass the object's storage to the factory's apply method.

However, the type of the factory class cannot be completely specified in the container class because that would defeat the whole purpose of the factories which is to allow the container to accept a variadic argument list for the constructor of its contained object.

The correct function overload must be based on the only distinctive and common characteristic of all the classes in each family: the base class.

Depending on the container class, you can use enable_if to generate the right overload, or use the following dispatch technique, which is used in the Boost.Optional class:

struct C
{
   C() : contained_(0) {}
   C ( X const& v ) : contained_ ( new X(v) ) {}

   template <class Expr>
   C ( Expr const& expr )
    :
    contained_ ( uninitialized_storage() )
   {
    construct(expr,&expr);
   }

  ~C() { delete contained_ ; }

  template<class InPlaceFactory>
  void construct ( InPlaceFactory const& aFactory, boost::in_place_factory_base* )
  {
    aFactory.template apply<X>(contained_);
  }

  template<class TypedInPlaceFactory>
  void construct ( TypedInPlaceFactory const& aFactory, boost::typed_in_place_factory_base* )
  {
    aFactory.apply(contained_);
  }

  X* uninitialized_storage() { return static_cast<X*>(new char[sizeof(X)]) ; }

  X* contained_ ;
};

End users pass to the container an instance of a factory object holding the actual parameters needed to construct the contained object directly within the container. For this, the helper template function in_place is used.

The call in_place(a0,a1,a2,...,an) constructs a (non-typed) in_place_factory instance with the given argument list.

The call in_place<T>(a0,a1,a2,...,an) constructs a typed_in_place_factory instance with the given argument list for the type T.

void foo()
{
  C a( in_place(123, "hello") ) ;    // in_place_factory passed
  C b( in_place<X>(456, "world") ) ; // typed_in_place_factory passed
}

Copyright Fernando Luis Cacciola Carballal, 2004