C++ Structs with Member Functions vs. Classes with Public Variables

Question

This is really a question of good form/best practices. I use structs in C++ to form objects that are designed to basically hold data, rather than making a class with a ton of accessor methods that do nothing but get/set the values. For example:

struct Person {     std::string name;     DateObject dob;     (...) }; 

If you imagine 20 more variables there, writing this as a class with private members and 40-something accessors is a pain to manage and seems wasteful to me.

Sometimes though, I might need to also add some sort of minimal functionality to the data. In the example, say I also sometimes need the age, based on dob:

struct Person {     std::string name;     DateObject dob;     (...)     int age() {return calculated age from dob;} } 

Of course for any complex functionality I would make a class, but for just a simple functionality like this, is this "bad design"? If I do use a class, is it bad form to keep the data variables as public class members, or do I just need to accept it and make classes with a bunch of accessor methods? I understand the differences between classes and structs, I'm just asking about best practices.

Answer 1

I think there are two important design principles to consider here:

1. Hide a class's representation through an interface if there is some invariant on that class.

   A class has an invariant when there is such thing as an invalid state for that class. The class should maintain its invariant at all times.

   Consider a Point type that represents a 2D geometric point. This should just be a struct with public x and y data members. There is no such thing as an invalid point. Every combination of x and y values is perfectly fine.

   In the case of a Person, whether it has invariants depends entirely on the problem at hand. Do you consider such things as an empty name as a valid name? Can the Person have any date of birth? For your case, I think the answer is yes and your class should keep the members public.

   See: Classes Should Enforce Invariants

2. Non-friend non-member functions improve encapsulation.

   There's no reason your age function should be implemented as a member function. The result of age can be calculated using the public interface of Person, so it has no reason to be a member function. Place it in the same namespace as Person so that it is found by argument-dependent lookup. Functions found by ADL are part of the interface of that class; they just don't have access to private data.

   If you did make it a member function and one day introduced some private state to Person, you would have an unnecessary dependency. Suddenly age has more access to data than it needs.

   See: How Non-Member Functions Improve Encapsulation

So here's how I would implement it:

struct Person {   std::string name;   DateObject dob; };  int age(const Person& person) {   return calculated age from person.dob; } 

Answer 2

In C++, Structs are classes, with the only difference (that I can think of, at least) being that in Structs members are public by default, but in classes they are private. This means it is perfectly acceptable to use Structs as you are - this article explains it well.