Pointer strangeness

31 Mar 2003

      I'm now trying to really get a handel on pointers.  I must admit this stuff 
has always been confusing to me.  The code below produces an unexpected 
result when it is run.  The last line of output is "HelloClass::memberIntPtr 
= -1073747936".  I expected it to be "HelloClass::memberIntPtr = 1848", as it 
is in the previous invocation of sayHello().  My guess as to what is 
happening is that my use of a member variable of type string, and then 
assigning to that variable is resulting in and over write of the memory 
pointed to by HelloClass::memberPtr.

Is my understanding of what's happening correct?  What's the proper remedy for 
this situation?

#include <iostream> // cout, cin
#include <string>
using namespace std;

class HelloClass {

public:
	HelloClass::HelloClass();
	void sayHello();

	/** Accessors for HelloClass::message */
	void setMessage(string messageStr="This is the default setMessage message.");
	string getMessage();

	/** Accessors for HelloClass::messagePtr */
	void setMessagePtr(string messageStr="This is the default setMessagePtr 
message.");
	string getMessagePtr();

	/** Accessors for HelloClass::memberInt */
	void setMemberInt(int memberInt=2112);
	int getMemberInt();

	/** Accessors for HelloClass::memberIntPtr */
	void setMemberIntPtr(int memberIntPtr=1848);
	int getMemberIntPtr();

private:
	string message;
	string *messagePtr;
	int memberInt;
	int *memberIntPtr;
};

/** Constructor: initialize all member variables. */
HelloClass::HelloClass(){
	this->message = "c++bean message. Assigned in constructor.";
	this->messagePtr = (new string("c++bean messagePtr. Assigned in 
constructor."));
 	this->memberInt = 42;
	int temp = 100760;
 	this->memberIntPtr = &temp;
}

/** Accessors for HelloClass::message */
void HelloClass::setMessage(string messageStr){
	this->message = messageStr;
}
string HelloClass::getMessage(){
	return this->message;
}

/** Accessors for HelloClass::messagePtr */
void HelloClass::setMessagePtr(string messageStr){
	this->messagePtr = new string(messageStr);
}
string HelloClass::getMessagePtr(){
	return *this->messagePtr;
}

/** Accessors for HelloClass::memberInt */
void HelloClass::setMemberInt(int memberInt){
	this->memberInt = memberInt;
}
int HelloClass::getMemberInt(){
	return this->memberInt;
}

/** Accessors for HelloClass::memberIntPtr */
void HelloClass::setMemberIntPtr(int memberIntPtr){
	this->memberIntPtr = &memberIntPtr;
}
int HelloClass::getMemberIntPtr(){
	return *this->memberIntPtr;
}

/** Output the properties of HelloClass*/
void HelloClass::sayHello() {
	cout << "HelloClass::message      = " << this->getMessage()      << endl;
	cout << "HelloClass::messagePtr   = " << this->getMessagePtr()   << endl;
	cout << "HelloClass::memberInt    = " << this->getMemberInt()    << endl;
	cout << "HelloClass::memberIntPtr = " << this->getMemberIntPtr() << endl;
	cout << "___________________________" << endl;
}

main() {
  HelloClass h;

	h.sayHello();

	h.setMessage();
	h.setMessagePtr();
	h.setMemberInt();
	h.setMemberIntPtr();
	h.sayHello();

	h.setMessage("User defined \"Hello World\" message.");
	h.sayHello();

  return 0;
}

Pointer strangeness

Steven T. Hatton