When reducing dependencies we usually have several ways of grouping dependencies.

Introduction

This article builds on previous articles on dependency injection using C++ and illustrates a different grouping to reduce dependencies for the house example, using interfaces. We’ll group objects into a living_room and kitchen which share the same door.

Code

house.h

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

#include "kitchen_interface.h"
#include "living_room_interface.h"

class house
{
public:
  house(
    kitchen_interface & kitchen,
    living_room_interface & living_room);

  void chillax();

private:
  kitchen_interface & kitchen_;
  living_room_interface & living_room_;
};

house.cpp

1
2
3
4
5
6
7
8
9
10
11
12
13
14

#include "house.h"

house::house(
  kitchen_interface & kitchen,
  living_room_interface & living_room) :
    kitchen_{ kitchen }
    living_room_ { living_room }
{
}

void house::chillax() {
  kitchen_.exit();
  living_room_.enter();
}

Here’s how for example the kitchen is implemented:

kitchen_interface.h

1
2
3
4
5
6
7

struct kitchen_interface
{
  virtual void exit() = 0;

protected:
  ~kitchen_interface() {};
};

kitchen.h

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

#include "kitchen_interface.h"

#include "cuppa_interface.h"
#include "door_interface.h"

class kitchen :
  public kitchen_interface
{
public:
  kitchen(
    cuppa_interface & cuppa,
    door_inteface & door);

  void exit() override;

private:
  cuppa_interface & cuppa_;
  door_interface & door_;
};

kitchen.cpp

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

#include "kitchen.h"

kitchen::kitchen(
  cuppa_interface & cuppa,
  door_inteface & door) :
    cuppa_{ cuppa },
    door_{ door }
{
}

void kitchen::exit()
{
  cuppa_.finish();
  door_.open();
}

The living_room class will follow a similar pattern with a door to close() and a tv to switch_on(). Testing should be obvious by now.

The point of this article is that we can arrange if we wish for both the kitchen and the living_room to share the same door, and the place to do that is when we construct the house:

main.cpp

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

#include "cuppa.h"
#include "door.h"
#include "tv.h"

#include "kitchen.h"
#include "living_room.h"

#include "house.h"

int main() {
  cuppa c;
  door d;
  tv t;

  kitchen k{ c, d };
  living_room lr{ d, t};

  house h{ k, lr };
  h.chillax();
}