Conversion between classes [closed]

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Let's say we have a class called A and another one called B. and we want to have a conversion method that converts A to B.

In the software architecture point of view, which one is preferred?

1. write A.export()
2. write B.import()
3. write a converter class, i.e. convert(A, B) or Convert(A) or ...

if the language matters, I'm using C++

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It entirely depends on how you intend to use it, but in many cases, the cleanest way to do this is to implement a converting constructor:

class B { };
class A
{
    A(const B& b) { }
};

B b;
A a = b; //< calls converting constructor

(of course, you could implement a converting constructor for converting from A to B, as well)

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First of all, decide whether a conversion between them is really natural and intuitive. In that case, you could use a constructor or a member function.

However, if the connection is not too natural, you may want to go with a separate ConversionUtils class with conversion methods or something like this. You don't want to start creating too many dependencies between classes or end up implementing n*n conversions.

It is also important to be careful with "conversion constructors", as they can be invoked without you realizing it. For example, if I have an a and a b and I write something like a==b, the presence of a constructor in A or B that takes the other could result in successful compilation with no warning, and possibly interesting results at runtime. I would strongly encourage you to make your constructor explicit in these cases.

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If we take a short look at Java (just for information, even though you are using C++):

Say that class A is String and class B is Integer (or Double, Boolean, et c). The Java library designers have put a static method in Integer ("B") called getInteger ("import") that takes a String ("A") and produces an Integer ("B"). That would be your case 2.

On the other hand, in every class (call them "A") there is a method toString ("export") that returns a String ("B"). That would be your case 1.

So even for such basic classes, they decided to use different ways. I guess that means the same thing as @James McNellis said: it depends.

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You can overload cast operators, i.e., operator T () { .. return a T value }. Whether or not you should is another matter.

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I would use your third suggestion. A separate converter class for the conversion. This way you don't directly coupled A to B or B to A. A and B will have their own responsibilities whatever it is.

The converter class C its responsibility is purely conversion. Each class will be responsible for a single thing which in my opinion is a good thing. I believe this is the S in the SOLID design principles (Single Responsibility principle).