{ What's wrong with my code ? }

As we can see, I set default assignment. But the output is "Empty!"
You can try to run this code snippet to see the output.

Code:

class Shape{
public:
     virtual void setType( const std::string& str = "abc") = 0;
     virtual void getType() const;
private:
      std::string str_;
};

void Shape::getType() const { 
     if( str_.empty() ){
          std::cout<<"Empty!"<<std::endl;
     }else{
          std::cout << str_ << std::endl; 
     }
}

void Shape::setType( const std::string& str) {
     str_ = str;
}

class Square:public Shape{
public:
     void setType( const std::string& str = "abc"){
          str_=str;
     }
private:
     std::string str_;
};

int main(){
     Shape * sp = new Square;
     sp->setType();
     sp->getType();
     return 0;     
}

Works for me - VS2005.
What compiler?

Showed "Empty" on VS2003. I noticed that you have str_ twice though. Both in your base and derived classes. I removed str_ in the derived class and changed private to protected in the base class and it worked fine.

Code:

class Shape{
public:
	virtual void setType( const std::string& str = "abc") = 0;
	virtual void getType() const;
protected:
	 std::string str_;
};

class Square:public Shape{
public:
	void setType( const std::string& str = "abc"){
		str_=str;
	}
};

Don't give virtual functions default arguments. There was a thread recently about this. Just don't do it.

Thanks, but would you explain why 2 "str_" code doesn't work?
As we can see in my code the 2 "str_" are private to each class...

Originally Posted by Desolation

Showed "Empty" on VS2003. I noticed that you have str_ twice though. Both in your base and derived classes. I removed str_ in the derived class and changed private to protected in the base class and it worked fine.

Code:

class Shape{
public:
	virtual void setType( const std::string& str = "abc") = 0;
	virtual void getType() const;
protected:
	 std::string str_;
};

class Square:public Shape{
public:
	void setType( const std::string& str = "abc"){
		str_=str;
	}
};

What did you do to the code? It was indented before! It's a mess now!
The thing is, that str_ is private in each class and getType only exists in the base class, so it calls the base class's function, which prints out the base class's str_.
But since you overloaded setType for both classes and made them virtual, the compiler will call the derived setType and set the derived's str_ to the value you specify.
Thus:

Base: Empty.
Derived: Non-empty.

There's your problem.

Originally Posted by Elysia

What did you do to the code? It was indented before! It's a mess now!
The thing is, that str_ is private in each class and getType only exists in the base class, so it calls the base class's function, which prints out the base class's str_.
But since you overloaded setType for both classes and made them virtual, the compiler will call the derived setType and set the derived's str_ to the value you specify.
Thus:

Base: Empty.
Derived: Non-empty.

There's your problem, I believe.

Thank you. Sorry for the mess.

But getType() is a virtual function, the sp->getType(); should follow virtual table to the Square::getType(), shouldn't it?

There is no Square::getType().

Which is precisely the problem. The compiler can't call a getType in the derived class because there isn't one, so it calls the base class's instead.
Though I still recommend you make str_ protected and getType and setType should be non-virtual sicne there's no point in doing it this way. Of course, since this probably is an exercise, carry on.