Getting confused with pointers

Question

I trying to reproduce the Bridge pattern from Java book, but getting stuck with pointers dilemma cause I'm novice. I threw away includes and part the of implementation, because it's just printf's over there:

// main.cpp
int main () {
    DrawApi *rc = new RedCircle();
    Circle redCircle = new Circle(rc);
    redCircle.draw();
}

// DrawApi.h
class DrawApi
{
public:
    virtual void drawCircle() = 0;
};

// Shape.h
class DrawApi;
class Shape
{
protected:
    DrawApi *drawApi;
public:
    Shape(DrawApi *mDrawApi) : drawApi(mDrawApi) {}
    virtual void draw() = 0;
};

// Circle.h
class Circle : public Shape {
public:
    Circle(DrawApi *mDrawApi) : Shape(mDrawApi) {}
    void draw();
};

After compile and run a get an error:

$ g++-5 -std=c++14 -g main.cpp RedCircle.cpp Circle.cpp && ./a.out && rm a.out
main.cpp: In function ‘int main()’:
main.cpp:7:46: error: conversion from ‘Circle*’ to non-scalar type ‘Circle’ requested
     Circle redCircle = new Circle(rc);

Shape ctor is waiting for pointer to DrawApi but seems gets an object. I thought I declared rc variable as pointer to DrawApi.

Answer 1

Circle redCircle = new Circle(rc);

If you want to initialise (copy construct) a Circle from Circle *, you need a constructor, say Circle(Circle *).

---

I guess that, in fact, you just want to initialise a Circle from rc. Since rc is DrawApi *, and Circle has a constructor which takes DrawApi * as argument, a.k.a Circle(DrawApi *mDrawApi). You can simply write:

Circle redCircle(rc);
redCircle.draw();

If I was wrong, then maybe:

Circle *redCircle = new Circle(rc);
redCircle->draw();

I prefer the first version for two reasons:

• they are all calling to Circle's draw, not it's base's.
• DrawApi has a virtual member function too, and you have already conducted a Derived-to-Base conversion.

---

BTW, your program has some serious memory management issues. Having them solved before go any further will be a good choice. (Smart pointer will help you a lot)

There are subtle differences between Type obj(x) and Type obj = x.