Question

Assume I have the following abstract class and use it as an “interface” in C++:

class IDemo
{
  public:
    virtual ~IDemo() {}
    virtual void Start() = 0;
};


class MyDemo : IDemo
{
  public:
    virtual void start()
    {
      //do stuff
    }
};

Then in the class that need to have a handle to the interface (concrete class through injection):

class Project
{
  public:
    Project(IDemo demo);

  private:
    IDemo *_demo;
};

My intention is to assign concrete Demo class through the constructor of Project. This code doesn’t compile since IDemo can’t be instantiated. Any suggestions?
Thanks in advance.

Answer 1

Try:

 Project::Project(IDemo* demo)
     : _demo(demo)
 {}

But If the demo object is never going to change for the lifetime of the project then I prefer to pass by reference:

class Project
{
    public:
        Project(IDemo& d)
          : demo(d)
        {}
    private:
        IDemo&  demo;
};

Then use it like this:

int main()
{
    MyDemo    demo;
    Project   project(demo);
}