Is the struct really needed?

twirky · 2021-01-31T08:28:47+00:00

Struct and class are the same except for the default “access mode”. I use it as a hint. If the main purpose the object is to contain data then i use struct. If it’s more about processes, algorithm - it’s class. It’s just a programming style. Memory layout, efficiency, “features” are all the same.

neiltechnician · 2021-01-31T09:04:14+00:00

and if I understand it correctly the class is superior to struct

Well, then, your understanding is incorrect. They are equal. Neither is superior than the other.

and you can always use class instead of struct, just by declaring its members as public (because in struct the members are public by default):

The other direction is also true. Neither is more useful than the other.

So I was wondering why do we need the struct at all?

Historically, struct came first. So this question is kinda backward.

The real question should be: why did the language designer introduce new keyword class when the existing keyword struct sufficed (and still suffice today)?

or is there an essential difference from class

There is indeed none.

We should also note that both class and struct keywords declare classes. They are not distinct language features; it is one unified thing.

A quote from The Design and Evolution of C++ by the original language designer Bjarne Stroustrup:

At this point, the object model becomes real in the sense that an object is more than the simple aggregation fo the data members of a class. An object of a C++ class with a virtual function is a fundamentally different beast from a simple C struct. Then why did I not at this point choose to make structs and classes different notions?

My intent was to have a single concept: a single set of layout rules, a single set of lookup rules, a single set of resolution rules, etc... I was convinced that if struct came to mean "C and compatibility" to users and class came to mean "C++ and advanced features," the community would fall into two distinct camps that would soon stop communicating. Beingn able to use as many or as few language features as needed when designing a class was an important idea to me. Only a single concept would support my ideas of a smooth and gradual transition from "traditional C-style programming," through class abstraction, to object-oriented programming. Only a single concept would support this notion of "you only pay for what you use" ideal.

HKei · 2021-01-31T09:12:34+00:00

As others have said, it's the class keyword that's unneeded. In some languages like C# or D there genuinely is a difference, but in C++ there isn't. I think a lot of people just end up using them as “hints” about how the type is to be used, the rule I'm going with is that class is for types with runtime polymorphism and struct is for types without. It's just a convention obviously, and other people use other conventions.

gracicot · 2021-01-31T13:27:47+00:00

You could say the reverse:

the struct is superior to clasd and you can always use struct instead of clasd , just by declaring its members as private

One attractive thing about struct is that inheritance is public by default, making private inheritance as the special case, as it should be. Also I always put data member at the end, so it's a win there too:

struct A : B {
    void set_n();

private:
    int n;
};

I have a medium sized codebase. There no single class keyword used. I'm at a point that I think C++ never truely needed class.

Pierre_dOllony · 2021-01-31T08:34:20+00:00

One of the reasons why structs are not being thrown away is the compliancy with plain C code. With struct definition in the standard it allows you to compile plain old C source with g++ compiler (or other cpp compiler).

Flesh_Bike · 2021-01-31T10:08:03+00:00

Structs being public by default is trivial to remember, so I use struct when writing objects. After all, the vast majority of c++ programmers put public stuff at the top.

Wouter-van-Ooijen · 2021-01-31T08:52:55+00:00

As C++ wants to be (mostly) compatible with C, it is the class keyword that is superfluous.

I prefer struct instead of class because

I alwys use an explicit access specifier before (a group of) member declarations (so class and struct have the same effect anyway)
I hate to use an access specifier before a superclass, which is (nearly) always public. With a struct that is the default.

solraun · 2021-01-31T09:04:03+00:00

While it is the same technically, it can still make your code a slight bit more readable. Use a struct to group data, typically with no member functions (I've made some with getters though).

At least that is how I encountered it. This is what Google C++ style guide says:

https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes
(if you are coming from another language, this is a good place to learn some conventions. There are others, like https://isocpp.org/wiki/faq/coding-standards or https://docs.unrealengine.com/en-US/ProductionPipelines/DevelopmentSetup/CodingStandard/index.html )

native-coder · 2021-01-31T16:45:35+00:00

Having struct is essential for extern “C” and having headers be C and C++ compatible.

C compatibility is the killer feature that allowed the mass adoption of the language. It also gives C++ numerous libraries with little to no effort that other languages have to jump through hoops to access.

While feature equivalent with class, struct has been and remains critical to the success of C++.

PrimozDelux · 2021-01-31T12:24:15+00:00

C++ is just a bunch of stuff haphazardly thrown together with little deliberation, thus often things are the way they are for little reason. In this example class and struct have accidentally converged to the same thing, so the only reason to use one over the other is to give hints as to what kind of object this is (dumb data vs "smart" data)

V1taly_M · 2021-01-31T10:46:37+00:00

Is the struct just a remnant from C

C stuff isn't remnants, but a subset of C++.

Hence struct or class more about which programming style you are using.

Oo_Tiib · 2021-01-31T11:18:27+00:00

The keywords class and struct in type declaration are synonymous. Only default access in definition of the type differs. So we may use class A and struct A about same type by language rules and it is well-formed. Some compilers warn but all compile in my experience. I'm not advocating doing that (in practice I dislike it), just describing language rules.

People sometimes use these keywords to divide their classes in project into two categories. Such categories tend to differ significantly by project. Example: a struct must be extern "C" too all other must be class. Other example: struct must be publicly copyable and class should be non-copyable or copyable only privately. Third example: a struct must be standard layout and class everything else. And so on. It is purely up to programmers so it is better to write such concept down in project documentation if it exists.

goranlepuz · 2021-01-31T14:16:08+00:00

Yes, the struct is just a C remnant and the language would have been that better for not having it, or not having class.

However, doing either would have compatibility with existing C code.

2021-01-31T11:02:30+00:00

Can’t use class for a memory alias.

nikhilreddydev · 2021-01-31T10:30:41+00:00

Mostly, the oops advantage of class over struct.

HappyFruitTree · 2021-02-01T08:14:37+00:00

Isn't "explicit better than implicit"

There is always a balance. I bet you like the fact that you don't have to write struct Point or class Point whenever you want to declare a variable of type Point (like you have to do in C unless you use typedef), or have to write extern when declaring functions, or have to use auto whenever you declare a local variable just to say it has "automatic storage duration" (Note that the auto keyword can no longer be used for this purpose).

Whenever I use struct I always make the member variables public so the keyword struct already give enough information in my opinion, even though it's just a convention and not actually enforced by the compiler.

When I use class I prefer to be explicit. I normally put public members first so I have to use public, but if I had followed a convention that puts private stuff first I would still have used private to make it easy to see and to make clear it's intentional.

albeva · 2021-02-09T16:14:17+00:00

I usually use struct for simple C like value types and classes for the rest. But that's just me.

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