One more thought ...

I think it's a mistake to associate C++ classes with object-orientated design or some program-wide high level design choice. A better way to think of them is just as custom types, and as a convenient packaging of data and and the functions (class methods) that works on the type/data.

In fact using OO concepts such as inheritance, and god forbid multiple inheritence, in C++ is generally a bad idea unless you have a very specific reason for doing so. You shouldn't have the mindset of "I'm using C++, so my classes should be using inheritance". In general keep your class hierarchies as flat as possible, and most of the time you don't want inheritance at all - you are just creating a packaging of data + associated methods.

Of course there is also nothing stopping you from combining classes with methods with additional global functions that work with those classes, but it would be advisable if they only did so using methods provided by the class, rather than having class data members declared public and letting things outside the class modify them.

The point* of classes - packaging data and the methods that work on the data together - is to support easy and localized future changes. If your class data members are private and only operated on by class methods, then you can change the data members however you like and users of the class will not be affected.

Used properly, C++ classes are an enabler for writing large complex projects that are easy to maintain. If using classes is impacting the design of your project, then it really means you are using them in the wrong way.

Edit:

* Well, one point. The other main point/value is the support of constructors and destructors so that your classes/structs are guaranteed to be initialized and destroyed. You don't need to remember to call an initialization function, or need to worry about code that throws exceptions and therefore has many paths where structure cleanup/destruction would be needed - the destructor will always be called when the object goes out of scope.