Thread: No default constructor available

Code:

#include <iostream>
#include <string>

using namespace std;

class Point
{
	private:
			int _x,_y;

	public:
			Point(int x,int y)
			{
				_x=x;
				_y=y;
			}	
};


class Shape 
{
	private:
			char *colour;
			Point _pos;
	public:
			Shape(char *p,int x,int y)
			{
				*colour=*p;
				_pos = Point(x,y);
			}
};

int main()
{
	Shape shape1("blue",10,20); 
}

Hi guys, I'm a newbie.. In the above code, I have two classes. Point and Shape. I wanted to initialize using this "Shape shape1("blue",10,20); ". But I'm not able to do so. The compiler shows this error. error C2512: 'Point' : no appropriate default constructor available

Can someone explain to me why? Thank you!

You declared a constructor for the point class, so the compiler doesn't generate the default constructor for you.

Code:

Point _pos;

Here you are telling it to use the default constructor.

And by the way, symbols that start with underscore are reserved for implementation use. Don't use them.

Thanks for the prompt reply. I ran into another problem. I'm using inheritance now.

Code:

#include <iostream>
#include <string>

using namespace std;

class Point
{
	protected:
			int _x,_y;

	public:
			Point(int x,int y)
			{
				_x=x;
				_y=y;
			}
			int getx()
			{
				return _x;
			}

};


class Shape : public Point 
{
	protected:
			char *colour;
			
	public:
		Shape(char *p,int x,int y) : Point(x,y)
			{
				*colour=*p;		
			} 
};

int main()
{
	Shape shape1("blue",10,20); 
	cout << shape1.getx() << endl;
	return 0;
}

It compiles fine but when I run it, it says the program has stopped running and windows is finding for a solution to the problem. Is there anything wrong with the code?

Yes, there is something wrong, but you will learn much more if you figure out what's wrong yourself, using a debugger. Have you tried that?

Oh! Just tried that. Thanks cyberfish =) I changed the line "*colour=*p;" to "colour=p;". But I am not very sure why the former doesn't work and the latter works. can you explain pls

"colour" is a pointer to char, so is p.

*p gives you the first character of "blue", so 'b'.

*colour = *p; is the same as "*colour = 'b';"

which doesn't work, because you never allocated space for colour! you are trying to store 'b' to a random place in memory. That usually doesn't work.

"colour = p;" on the other hand sets colour to point to the same string p points to, and is perfectly fine.

Ah Icic! Got another burning question. When I use this constructor, it works.

Code:

               Shape(char *p,int x,int y) : Point(x,y)
	{
		colour=p;		
	}

But when I use the following constructor, it doesn't work. Why is that and What's another equivalent version of this? I tried reading my lecture notes but it doesn't use any alternative versions.. only the constructor above.

Code:

	Shape(char *p,int x,int y)
	{
		Point(x,y);
		colour=p;
	}

In the first one you are saying you want the parent's "Point(x,y)" constructor to be executed before Shape's, instead of the default "Point()" constructor.

Why do you need another version?

In the second one you are making a function call to a non-existent Point() function which doesn't make much sense.

Think you misunderstood my question. Haha.. I wanted to know whether there is another of saying the following. Another code which does the same thing as the constructor below in other words. Is there any? Tks =)

Code:

     Shape(char *p,int x,int y) : Point(x,y)
	{
	      colour=p;		
	}

Originally Posted by laserlight

More accurately, you are invoking a Point constructor to create a temporary Point object which is then destroyed.

Ah, brainfart on my part. Thanks!

Also, you cannot invoke the constructor "manually", if that's what you are asking, unlike in Java (but even in Java, "super" has to be the first statement, so it's kind of the same thing).