TAOVar Out type

TAOVar Out type

By WalterXia walter.xia@gmail.com

 

In our project, TAO, ACE are used widelyand they are crossing platforms. Especially, the CORBA based on them is highweight IPC infrastructure and used ininfra-card, inter-card, intra-process, and inter-process (different threads).

The Var, Out type implemented in TAO are employedto encapsulate defined IDL structures. In brief, the Var type is a smartpointer class and wrap native pointer and provides following member functions:

Supposed native type is T and its pointeris T*, T* & is a reference of native pointer.

Fucntion	Return type	Comment
in	const T &	Used as input parameter
out	T *&	The implementation is as followed:   delete this->ptr_; //destruct the original pointed content   this->ptr_ = 0; //set empty to the pointer   return this->ptr_;
inout	T &
_retn()	T *	The implementation is as followed:   T * tmp = this->ptr_;   this->ptr_ = 0;  //original pointer will be set empty   return tmp;  //original pointer content will be returned
ptr	T *	Return native pointer

 

While Out type is a reference of native pointer.

PracticeI:

If you want to convert the Var type to Out,such as XXX_Var to XXX_Out

XXX_Var à XXX_Out  //invoke XXX_Var’s _retn() member functionfor converting

 

Func(XXX_Out xxx_out)

{

  XXX_Var tmpVar;

   ….

   xxx_out= tmpVar. _retn();

}

 

If want to use as following:

XXX_Var xxx_var;

……//changing the xxx_var

Func(xxx_var)  //implicitly convert to Out type for the Vartype using following construct function TAO_Out_T (T_var &)

(OR) Func(xxx_var.out())

The xxx_var contents will be erased when invokingthe Func.

Make sure the Func the statement of “ xxx_out= tmpVar. _retn();” will be called and there will be issues (xxx_var will benull and accessing its member functions is forbidden) if exception is thrown orreturned before the statement is executed.

The construct function’s implementation isas followed:

template<typename T, typename T_var>

ACE_INLINE

TAO_Out_T<T,T_var>::TAO_Out_T (T_var& p)

  :ptr_ (p.out ())

{

  delete this->ptr_;  //original pointed content will be erased

 this->ptr_ = 0;    

}

 

PracticeII:

Passing native pointer as input argumentand doesn’t change the content

Using in member function of Var is recommended.

 

PracticeIII:

A function has an argument Var and want tochange the content in the function body. Just considering the Var ordinaryclass and its reference is recommend.

 

 

 

References:

ACE_wrappers/TAO/tao/VarOut_T.h  //header files for

ACE_wrappers/TAO/tao/VarOut_T.inl  //inline functions implementations

http://www.dre.vanderbilt.edu/Doxygen/5.5.1/html/tao/VarOut__T_8h.html