Smart C++ pointers: shared ptr

sharedptr.cpp

struct Expression;
struct Number;
struct BinaryOperation;

struct SharedPtr
{
     explicit SharedPtr(Expression *ptr = 0){
         ptr_=ptr; count= new int; *count=1;
      
    }
    ~SharedPtr(){
        *count-=1;
        if( *count==0) {
        delete count;
        delete ptr_;          
            ptr_=0;
        }
       
    }
    SharedPtr(const SharedPtr & obj) {
        
       
        ptr_=obj.ptr_;
       
        count= obj.count;
        *count+=1;
          
     }
    SharedPtr& operator=(const SharedPtr & obj){
        if(this!=(&obj) ){    *count-=1;
        if(*count==0) {
        delete ptr_; 
        delete count;
            
       }             
             ptr_= obj.ptr_;
           
            count= obj.count;
          *count+=1;
    
                        }
        return *this;
    }
    Expression* get() const{
       
     return ptr_;   
    }
    void reset(Expression *ptr = 0){
             *count-=1;
        if(*count==0) {
        delete ptr_;  
        delete count;
       }
        
        ptr_=ptr;
       count=new int;
        *count=1;
     
    }
    Expression& operator*() const {
      
        return *ptr_;
    }
     Expression* operator->() const  {
       
     return ptr_;   
    }
    private:
    int* count;
    Expression* ptr_;
  
};