Re: C++ Conversion functions for pointers
From: B. v Ingen Schenau (bart_at_ingen.ddns.info)
Date: 04/30/04
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Date: Fri, 30 Apr 2004 11:22:31 +0200
Paul wrote:
> Hi group,
> I have worked out a way to return different types by subscript from a
> Variant class with the prerequesites that UDT's
> a)inherit from a Variant class and..
> b)they oveload the assignment operator to take an argument of type
> Variant&.
The only flaw is that it does not work, because you are invoking undefined
behaviour. :-(
>
> However I have stumbled upon a problem when I try to convert an
> integer pointer to a Variant pointer. I can think other ways to
> achieve my goal but I would like to build the Variant class in such a
> way that is can implicitly convert basic pointer types such as integer
> pointers into a Variant pointer.
You might want to take a look at boost::Any for an example how a proper
variant class can be created.
>
> Here is some code stripped down to only have conversion for integers:
> #include <iostream>
>
> class Variant{
> public:
> Variant(){std::cout<<"Construcing Variant(Default).\n";}
>
> /*Conversions for basic types*/
> Variant(int x):intData(x){std::cout<<"Construcing Variant(int).\n";}
> operator int(){return intData;}
> int& operator=(const int& rhs){ intData = rhs ; return intData;}
>
> virtual Variant& operator=(Variant&){return *this; }
> virtual ~Variant(){std::cout<<"Destrucing Variant.\n";}
> public:
> /* list of basic types*/
> int intData;
> };
>
>
> /* A UDT must derive form Variant */
> /* Also must overload the assignment operator to take a Variant&*/
> class ObjA:public Variant{
> public:
> ObjA():itsData(0){std::cout<<"Construcing ObjA(Default).\n";}
>
> ObjA& operator=(Variant& rhs){
> if(this == &rhs)
> return *this;
> itsData = static_cast<ObjA&>(rhs).itsData;
You might want to use a dynamic_cast<> here. That immediately catches any
attempt to assign an object that is not really of type ObjA.
> return *this;
> }
>
> ObjA& operator=(ObjA& rhs){
> if(this==&rhs)
> return *this;
> itsData = rhs.itsData;
> return *this;
> }
>
> void setData(int x){itsData = x;};
> ~ObjA(){std::cout<<"Destrucing ObjA.\n";}
> private:
> int itsData;
> };
>
>
> template<class DEFAULT_TYPE=Variant>
> class ObjArray{
> public:
> ObjArray():itsSize(0){std::cout<<"Construcing(Default ObjArray).\n";}
> ObjArray(int size):itsSize(size){
> std::cout<<"Construcing ObjArray.\n";
> itsArray = new DEFAULT_TYPE[size];
Because itsArray always has the type Variant*, this will only work if
DEFAULT_TYPE happens to be Variant.
For any other type, you will either get a compile error, or you invoke UB
later down the line.
> }
> ~ObjArray(){std::cout<<"Destrucing ObjArray.\n"; delete [] itsArray;}
You can not use the delete[] operator if there is a mismatch between the
static and dynamic types of its operand.
So, in normal language, you can not use 'delete[] itsArray' if itsArray does
not point at an array of objects of type Variant.
This is because otherwise the compiler does not know the size of each
object, which makes it impossible to call the destructors for the objects
with indices 1 and higher.
The same situation arises when you try to index the array.
<snip>
>
> AHA TIA
> Paul.
>
Bart v Ingen Schenau
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