C++ Conversion functions for pointers
From: Paul (paulmmm01_at_hotmail.com)
Date: 04/30/04
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Date: 29 Apr 2004 20:21:38 -0700
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&.
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.
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;
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];
}
~ObjArray(){std::cout<<"Destrucing ObjArray.\n"; delete [] itsArray;}
ObjArray(const ObjArray& rhs){std::cout<<"Copying ObjArray.\n";
itsSize = rhs.itsSize;
itsArray = new Variant[itsSize];
for(std::size_t i=0; i<itsSize; i++)
itsArray[i]= rhs.itsArray[i];
}
ObjArray& operator=(const ObjArray& rhs){std::cout<<"Assigning
ObjArray.\n";
if(this == &rhs)
return *this;
/*will need to delete and reallocate*/
/* but working on a more efficient solution*/
for(std::size_t i=0; i<itsSize; i++)
itsArray[i]= rhs.itsArray[i];
return *this;
}
Variant& operator[](std::size_t index){
if(index< itsSize){return (Variant&)itsArray[index];}
}
private:
std::size_t itsSize;
Variant* itsArray;
};
int main()
{
ObjArray<ObjA> objArr1(3);
/* uncomment to see error */
/*ObjArray<int> objArr2(3);*/
}
Ok the idea is to have an array of different types all convertable to
Variant types, which works fine as tested using individual types
withing the class i.e: An integer will convert to a Varaint.
The problem is ..how do I convert an integer pointer to a Variant
pointer, so I can initialise dynamically? As the error shows when line
3 in main is uncommented.
AHA TIA
Paul.
Note: In case your wondering I haven't worked out yet the best way for
changing a particular indexes type, probably have a few different
methods i.e: template functions , non template functions and
constructors. So for example you can construct an Array of mixed types
with a template arguments list i.e:
ObjArray<int, double , MyObject, bool> myVariableArray(10);
Where the last 6 elements would be default types.
And functions like....
ChangeTypeAtIndex<MyNumberObj>( 0 );
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