C++ runtime field access - c++

Question

class SuperClass{
/* ==================== METHODS ======================================= */
void
setValue (
std::string name,
int i ) {
MemberMapIterator it = memberMap_.find ( name );
if ( it != memberMap_.end ( ) ) {
void* ptr = ( *it ).second;
long long classPtr = reinterpret_cast< long long > ( this );
long long memberPtr = reinterpret_cast< long long > ( ptr );
int* value = reinterpret_cast< int* > ( classPtr + memberPtr );
( *value ) = i;
}
} // setValue
int
getValue (
std::string name ) {
MemberMapIterator it = memberMap_.find ( name );
if ( it != memberMap_.end ( ) ) {
void* ptr = ( *it ).second;
long long classPtr = reinterpret_cast< long long > ( this );
long long memberPtr = reinterpret_cast< long long > ( ptr );
int* value = reinterpret_cast< int* > ( classPtr + memberPtr );
return *value;
}
return -234234;
} // getValue
protected:
/* ==================== METHODS ======================================= */
void
Build ( ) {
configure ( );
} // Build
void
AddMember (
std::string name,
void* ptr ) {
memberMap_.insert ( MemberMapPair ( name, ptr ) );
} // AddMember
/* ==================== STATIC METHODS======================================= */
virtual void
configure ( ) = 0;
private:
/* ==================== METHODS ======================================= */
/* ==================== DATA MEMBERS ======================================= */
MemberMap memberMap_;
};
class SubClass: public SuperClass {
public:
/* ==================== LIFECYCLE ======================================= */
SubClass( ) : age_ ( 0 ) {
Build ( );
} /* constructor */
~SubClass( ) /* destructor */
{ }
protected:
/* ==================== STATIC METHODS======================================= */
void
configure ( ) {
long long classPtr = reinterpret_cast< long long > ( this );
long long agePtr = reinterpret_cast< long long > ( &this->age_ );
void* ptr = reinterpret_cast< void* > ( agePtr - classPtr );
this->AddMember ( "age", ptr );
} // configure
private:
/* ==================== DATA MEMBERS ======================================= */
int age_;
}
In SubClass, I add the offset of the private class field (thinking about the class as a C structure) the Super class map using string name as a key.I will make to execute configure only once and then I want to use this offset for every Person instance to access to its private fields at runtime (this + offset = field). Will be this safe? I tested this code and its work it is doing what I want. But should I expect any memory violations or something else (assuming that it won't be intentional violation (programmer errors))?

Answer 1

First of all, it's worth knowing that C++ classes are not like C structs. The compiler puts extra things in classes like the vtable pointer, which could be in the beginning, the end, or somewhere else depending on the compiler. There is one kind of class that behaves like a C struct (i.e. a bag of bits) and they're called Plain Old Data types (POD). You can find plenty on them on StackOverflow. Since you're using inheritance, you're not using a POD.
If you're trying to forcefully access private members, you probably need to rework your design. You should ask yourself why they're private in the first place. Based on what it looks like you're going for in the code, I can think of more straightforward approaches:
Cast the base class to the subclass, then set the private member with a setter function.
You could make setValue and getValue virtual and override it in the subclass.
Use friendship.