what's the equivilend of a VB property in C++?

ie if I have a Simple class which does nohing but hold a number, something like this

class cNumber
{
public:

long Value; //Stored Data

void cNumber(void); //Constructor

};

void cNumber::cNumber(void){}


how would I put validation code for Value without changing the interface, ie so I can still go

cNumber objMyInstace;

objMyInstance = 10;

thnx in advance