code:

//Class that wraps a constant reference variable.
//useful for return values from a container find method.
#include <iostream>
using namespace std;


template <class Object>
class Cref{
    public:
        Cref() : obj(NULL){}
        explicit Cref( const Object & x): obj( &x){ }

        const Object & get() const{
            if(isNull()){
                throw NullPointerException( );
            }
            else{
                return *obj;
            }
        }
        bool isNull( ) const{
            return obj == NULL;
        }

    private:
        const Object* obj; //stores a pointer...
};

//Usage example:

class TestClass{
    int test;


    public:
        TestClass():test(10){}

        int& get(bool valid){
            if(valid){
                Cref<int> retv(test);
                return retv;
            }
            else{
                Cref<int> retv;
                return retv;

            }
        }
};



int main( ){
    TestClass temp;
    try{
        Cref<int> test = temp.get(true);
        Cref<int> test2 = temp.get(false);
    }
    catch(exception& e){
        cout<<"NULL pointer exception occurred"<<endl;
    }

}

The error message:

3_ConstRef.cpp: In member function ‘const Object& Cref<Object>::get() const’:
3_ConstRef.cpp:15:33: error: there are no arguments to ‘NullPointerException’ that depend on a template parameter, so a declaration of ‘NullPointerException’ must be available [-fpermissive]
3_ConstRef.cpp:15:33: note: (if you use ‘-fpermissive’, G++ will accept your code, but allowing the use of an undeclared name is deprecated)
3_ConstRef.cpp: In member function ‘int& TestClass::get(bool)’:
3_ConstRef.cpp:41:12: error: invalid initialization of reference of type ‘int&’ from expression of type ‘Cref<int>’
3_ConstRef.cpp:45:12: error: invalid initialization of reference of type ‘int&’ from expression of type ‘Cref<int>’
3_ConstRef.cpp: In function ‘int main()’:
3_ConstRef.cpp:56:33: error: conversion from ‘int’ to non-scalar type ‘Cref<int>’ requested
3_ConstRef.cpp:57:35: error: conversion from ‘int’ to non-scalar type ‘Cref<int>’ requested