How do I reference another class into another class in C++? - c++

Question

I've been trying to reference another class inside of a class in C++ and I have no idea how.
I have created a small program to demonstrate the issue
#include <iostream>
class foo{
public:
int variable1 = 012;
};
class bar{
public:
int getFooVariable(){
return variable1; // How would I get bar to refrence foo in foobar?
}
};
class foobar{
public:
bar p1;
foo p2;
};
int main(){
foobar fb;
std::cout << fb.p1.getFooVariable(); << std::endl;
return 0;
}
How would I solve this problem?

Answer 1

You're looking for dependency injection:
class foo
{
public:
int variable1 = 012;
};
class bar
{
foo _foo;
public:
bar(foo& fooInjected) :
_foo(fooInjected)
{}
int getFooVariable() { return _foo.variable1; }
};
There is an idea of inversion of control: bar has no control over the creation of foo instance, which is created outside. It's a common way to inject for instance a service. It's also a very important way to loosely couple classes working together and to mock and to test them. But here foo is a concrete class, instead and ideally you're refering only an interface (c#) or an abstract class (c++) in bar. Which concrete class is behind is out of control of bar. I recommend M. Seeman's book Dependency Injection in .NET to understand this completely.
or inheritance:
class foo
{
public:
int variable1 = 012;
};
class bar: public foo
{
public:
int getFooVariable() { return variable1; }
};
Here, bar builds up heavily on foo. bar is a "richer version" of foo if this makes sense. This is what to choose if there is a high cohesion inside bar.
Invoking a variable through another's instance function is a most horrid idea, that goes against class cohesion:
int getFooVariable(foo* foo) { return foo->variable1; }

Answer 2

Sorry to change the formatting, here's a solution:
#include iostream
class foo
{
public:
int variable1 = 012;
};
class bar
{
public:
int getFooVariable(foo* foo)
{
return foo->variable1; // How would I get bar to refrence foo in foobar?
}
};
class foobar
{
public:
bar p1;
foo p2;
};
int main()
{
foobar fb;
std::cout << fb.p1.getFooVariable(&(fb.p2));
<< std::endl;
return 0;
}
You can add a parameter to the getFooVariable() function to take a foo* reference. The reference can be found when you call the function.