declaration of a class inside another class

Question

I have a problem with my program... I have a class A, and class B and C that inherit from class A... they live inside another class called Game, like this:

class Game {
    public:
        Game(bool something);
       //all the other functions
    private:
      A a;
}

I do it like this because I don't know beforehand if the object will be a class B or C, so I declare it like A and then:

Game(bool something) {
    if (something) { a = B(); }
    else (something) { a = C(); }
}

Now my problem: In the program I ask weather 'a' is B or C... and I want that if its B, run a function than only B has and neither A nor C have. But, of course, the compiler don´t let me do it because it thinks a is a class A object. Does anybody knows how to fix this?

You need *pointers* otherwise you get [object slicing](http://stackoverflow.com/questions/274626/what-is-object-slicing) — crashmstr, Feb 15 '16 at 19:54
Also, if there is no common ground in `A`, then have two pointers and decide what to do based on which pointer is not null. — crashmstr, Feb 15 '16 at 19:55
Use casting based on something variable. if(something) ((B)a).methodB() else ((C)a).methodC(). For example. — honzakuzel1989, Feb 15 '16 at 19:58
"because it thinks a is a class A object" - a ***is*** a class A object. — user253751, Feb 15 '16 at 20:31

oklas · Accepted Answer · 2016-02-16T07:44:30.283

2

class Game {
    public:
        Game(bool something) : a() {};
        ~Game(){ if(a) delete a; };
        // this is only example, and really need to implement 3 or 5 method
        // or use std::shared_ptr instead of pointer, more details at
        // https://en.wikipedia.org/wiki/Rule_of_three_%28C%2B%2B_programming%29
    private:
      A* a;
}

Game(bool something) {
    if (something) { a = new B(); }
    else           { a = new C(); }
}

B* b= dynamic_cast<B*>(a);
if( b ) b->method_b_only();
C* b= dynamic_cast<C*>(a);
if( c ) c->method_c_only();

Possible special object name or id to define a type manually:

class B : public A {
  public:
    B() : _name("B") {}
    const std::string& name() { return _name; }
  private:
    std:string _name;
}

B* b = a;
if( "B" == b.name() ) b->method_b_only();
C* C = a;
if( "C" == c.name() ) c->method_c_only();

edited Feb 16 '16 at 07:44

answered Feb 15 '16 at 20:01

oklas

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It should be `dynamic_cast(a)`. Also, you can't define the same constructor twice. – callyalater Feb 15 '16 at 20:04
Yes of course, thanks – oklas Feb 15 '16 at 20:06
Great!!! Thanks a lot! that compleatedly solved my problem! youre great! ^^ – J Agustin Barrachina Feb 15 '16 at 20:26
Think carefully before doing this. Consider instead defining a pure virtual `virtual void doSubclassSpecificBehaviour() = 0;` in `A` and having Game call this function. `B` and `C` both implement this function with their own behaviours. This method MUST be implemented by subclasses and removes the possibility of a whole family of logic problems because they will be caught by the compiler. – user4581301 Feb 15 '16 at 21:29
@oklas I recommend editing this answer to use [`std::unique_ptr a;`](http://en.cppreference.com/w/cpp/memory/unique_ptr) because `unique_ptr` makes everything else you may do with this Class a lot easier. For example, [your current answer violates the Rule of Three.](http://stackoverflow.com/questions/4172722/what-is-the-rule-of-three) – user4581301 Feb 15 '16 at 21:31

score 0 · Answer 2 · answered Feb 15 '16 at 20:00

The most straightforward way to achieve the functionality you want is to use pointers. ie.

class Game {
    public:
        Game(bool something);
    private:
        A *a;
}

Then you can create the object with the new keyword.

This allows you to have the polymorphic structure you are looking for. There are other ways that involve copy constructors and references, but are not as straightforward and easy to use.

declaration of a class inside another class

2 Answers2