Using struct we can achieve all the functionalities of a class:
The only difference are the default access rights: private for class, public for struct.
Why do we need a Class then ?
IIRC when C++ was created (as C with classes), a design goal was to only ever add functionality, so the initial idea was to have a class as in C++, and a struct as in C. But then things got blurred, and in the following 20 years struct "mutated" into some kind of class.
Because it's more OO, obviously.
Note: this answer is ironic, as noted by the OP struct achieves all the functionality of class, so there is little added value and it seems more an arbitrary design choice. Quite strange from a language which advocates limiting the number of reserved words.
While struct can behave like a class, it doesn't emphasize the object-orientedness of the langauge. So I think class was added to the language to emphasize the object-orientedness of C++. It also emphasizes data-encapsulation, as everything in a class is private by default, whereas struct doesn't emphasize data-encapsulation, as everything in a struct is public by default.
Data-encapsulation is one of the principles of object-oriented programming, so I think, making default-access private is very important. It gives a sense of class-design from the very beginning. Just pretend for a moment that there is no keyword class in C++, then imagine how many people would have created topics like "why everything in C++ is public by default, as everything in struct is public by default? Is it a characteristic of a language which supports object-oriented programming". To me, it feels that C++ would be lacking the sense of class-design.
You could argue you get more readable code if you adopt a convention where you use class for encapsulated OO class types and struct for non-encpasulated structural aggregate types with public data members: this is the usage typically mandated by coding standards.
It is mostly a more intuitive syntax. It would seem strange to have no class in a class based OO language.
I think the difference is semantic, and a bit syntactic. class emphasizes that this is a class and not anything else. So by the name it tells its usage. Where as with struct we can structure some thing, like a new datatype, by grouping other basic types. but grouping datatypes and making a new one with a class is possible, but the name class in this case will be misleading.
You are correct. Ideally we don't need class. It has been introduced for historical reasons, where the inventor of C++ wanted to distinguish between struct as plain old data type (C style) constructs and class as the interface which can have functions and constructors as well. (Though, struct still held all the privileges as class).
If you remove class keyword from the C++ language, it can still work flawlessly (not the old code). Because it's normal usage is achievable by struct and its template related meaning is achievable by typename.
To me the difference between class and struct is documentation.
So technically we do not need them (like many features of the language) but they allow us to impart extra information to the user of our code by the way we use them (as long as you don't abuse them and follow the standard conventions).
Underneath they are the same thing. But I find that in different context I use them to identify different things. Thus I am able to convey extra information to the user of my types by the way they are declared.
How I use them:
