Firstly, my apologies if this is a poor question. I am new to C++.
I have a set of class headers that contain a reference type field, which is an interface (class built up from pure virtual functions). I would like to initialise my class so that by default the reference type field is set to a certain "concrete" derived class, and this is done using the default constructor (parameterless!). I would also like to be able to override this initialisation with another "concrete" derived class.
So far, I have class headers as follows:
class Foo {
public:
Foo();
Foo(IBar & bar);
protected:
/* Update 1: const field */
const IBar & bar;
...
}
But I am struggling with the implementation:
/* Is it possible to create the reference type through the initialisation list? */
/* Update 2: corrected initialisation of bar field from "BarDerivedA() bar" to "BarDerivedA()" */
Foo::Foo()
: bar(BarDerivedA())
{
}
/* Override */
Foo::Foo(IBar & bar)
: bar(bar)
{
}
Update
We found that using this potential design wasn't going to be efficient. The default for const IBar & bar will almost always be the same object for every class with the field, with the exception of unit testing - we want to be able to swap in mock classes as required.
I don't want to constantly create the same object on the stack so will work on a factory for these group of objects.
I have gone down the route of a single constructor as follows:
Foo::Foo(IBar & bar)
: bar(bar)
{
}
If someone wants to provide an appropriate answer regarding the setting of class reference fields to a temporary object in the initialisation list (i.e. can only do it for const fields, and will go out of scope outside of the constructor) I will mark it as the accepted answer. Or alternatively mark it as an appropriate duplicate.
You may add the default object in the class, something like:
class Foo {
public:
Foo() : bar(defaultBar);{}
Foo(IBar& bar) : bar(bar) {}
protected:
const BarDerivedA defaultBar; // Before the reference.
const IBar& bar;
};