Need help implementing a SmartPointer - c++

Question

I'm having trouble implementing a smart pointer in C++ for an assignment.
My code is supposed to create two SmartPointers. The first points to 1 and the second initially points to 3 but is changed to 10. My code should print out
intOne: 1
intTwo: 10
but instead it prints out
intOne: 10
intTwo: 4919984
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <exception>
using namespace std;
template <typename T>
class SmartPointer
{
public:
SmartPointer(T in)
{
try
{
if(in<0)
{
throw "SmartPointer does not handle negative numbers";
}
T * val = &in;
this->data = val;
}
catch(exception& e)
{
cout << "Standard exception: " << e.what() << endl;
}
}
~SmartPointer()
{
delete data;
}
T getValue();
void setValue(T val);
private:
T*data;
};
template <typename T> T SmartPointer<T>::getValue()
{
return *this->data;
}
template <typename T> void SmartPointer<T>::setValue(T val)
{
if(val<0)
{
throw "SmartPointer does not handle negative numbers";
}
this->data = &val;
}
int main()
{
SmartPointer<int> intOne(1);
SmartPointer<int> intTwo(3);
intTwo.setValue(10);
cout<<"intOne: "<<intOne.getValue()<<endl;
cout<<"intTwo: "<<intTwo.getValue()<<endl;
}

Answer 1

Your constructor takes a T by value and then attempts to set the internal pointer to the local copy of the value. It should take the address of a T instead.
Your exception stuff is unnecessary, since the class doesn't even know what type is T, so it could be some type that does not have a comparison operator with an int. Even if you do need to throw exceptions, they should be caught in main(), since nobody expects a constructor to print error messages to the screen. throw "message" is wrong, since you're trying to throw an instance of const char*. Instead, call the constructor of one of the standard exceptions like throw domain_error("message").
Using this-> does not really do anything other than make it slower to type.
In case you didn't know, using endl flushes the buffer. If you don't need it, use '\n'.
The fixed code:
//#include <stdio.h> Why?
//#include <stdlib.h>
#include <iostream>
#include <exception>
using namespace std;
template<typename T>
class SmartPointer
{
public:
SmartPointer(T *in);
~SmartPointer();
T getValue();
void setValue(const T &val);
private:
T *data;
};
template<typename T> SmartPointer<T>::SmartPointer(T *in) //Moved implementation outside of class declaration
{
//Completely unnecessary exception stuff
data = in; //this-> is unnecessary
}
template<typename T> SmartPointer<T>::~SmartPointer()
{
delete data;
}
template<typename T> T SmartPointer<T>::getValue()
{
return *data;
}
template<typename T> void SmartPointer<T>::setValue(const T &val)
{
//More unnecessary exception stuff
*data = val;
}
int main()
{
SmartPointer<int> intOne(new int); //The constructor should take a pointer for the object to store instead of an value
SmartPointer<int> intTwo(new int);
intOne.setValue(1);
intTwo.setValue(10);
cout << "intOne: " << intOne.getValue() << '\n'; //endl flushes the buffer, kind of unnecessary here
cout << "intTwo: " << intTwo.getValue() << '\n'; //since it gets flushed upon the program exiting anyways
}