With OOP, when you have an instance of a class as attribute of another class, how is it possible to instantiate it only in the 2nd class?

Question

Their is something I don't understand with OOP : when you have an instance of a class as attribute of another class, how is it possible to instantiate it only in the constructor of the 2nd class?

For example in this example I would like to instantiate the class Foo in Bar's constructor. How should I modify my code so it work?

Foo.h

class Foo
{
private:
    int x, y;

public:
    Foo(int a, int b);
};

Foo.cpp

#include "Foo.h"

Foo::Foo(int a, int b)
{
    x = a;
    y = b;
}

Bar.h

#include "Foo.h"

class Bar
{
private:
    Foo foo;

public:
    Bar();
};

Bar.cpp

#include "Bar.h"

Bar::Bar()
{
}

Look specifically at the part right after the word "Furthermore" in the accepted answer to the question LogicStuff suggested this question duplicates. — JSF, Jan 13 '16 at 16:56
I have some more extra questions, that's why I didn't accepted any answer right now — LukeImYourFather, Jan 13 '16 at 19:45

score 2 · Accepted Answer · answered Jan 13 '16 at 16:42

2

Since Foo needs to be constructed from 2 parameters you are either going to have to have the user pass them to Bars constructor and create it with them or just use some hard coded values. I would suggest getting them from the user but also have a default value for the parameters so it acts as a default constructor.

class Bar
{
private:
    Foo foo;

public:
    Bar(int a = 0, int b = 0) : foo(a, b) {}
};

answered Jan 13 '16 at 16:42

NathanOliver

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Thanks for your fast answer. What if I have a Foo foo[3]; ? Would it be : Bar(int a = 0, int b = 0) : foo[0](a, b), foo[1](a, b), foo[2](a, b) {} – LukeImYourFather Jan 13 '16 at 19:01
@LukeImYourFather see this example: http://coliru.stacked-crooked.com/a/d867a927236d786a – NathanOliver Jan 13 '16 at 19:19
I get then error "cannot specify explicit initializer for arrays" – LukeImYourFather Jan 13 '16 at 19:43
@LukeImYourFather You need to compile with C++11 or above for it to work. – NathanOliver Jan 13 '16 at 19:44

With OOP, when you have an instance of a class as attribute of another class, how is it possible to instantiate it only in the 2nd class?

1 Answers1