C++ all differences between 'struct' and 'class'?

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What are the differences between struct and class in C++

I used to think that the only differences between C++ classes were the private-by-default class member access modifiers and the laid-out-like-C guarantee.

It turns out I was wrong, because this code doesn't compile:

class { int value; } var = { 42 };

whereas this does:

struct { int value; } var = { 42 };

I can't figure out why there's a difference, but there apparently is in Visual C++ 2008:

error C2552: 'var' : non-aggregates cannot be initialized with initializer list

So, yes, I will ask a many-times-over duplicate question (hopefully without duplicate answers!):

What are all the differences between structs and classes in C++?

^{Of course, feel free to close this if you find that I've missed something in the other questions -- I certainly might have. But I didn't see this being discussed in any of the answers, so I thought I'd ask.}

The first *is* the only difference between `struct` and `class` but they are crucial to your example. Because `value` is private in the first class the first class is not an aggregate so you can't use aggregate initialization for it. The "laid-out-like-C" guarantee is not a C++ guarantee. — CB Bailey, Aug 25 '11 at 11:51
@Kerrek: "Aggregate" means "public"?! English isn't my first language... now I have a feeling may very well never be. :( — user541686, Aug 25 '11 at 11:56
@Mehrdad: Why "English"? I mean in the C++ standard, the term "aggregate" is thus defined. That has nothing to do with the meanings of "aggregate" in the English language, to my knowledge. — Kerrek SB, Aug 25 '11 at 12:00
@Kerrek: Yes, it was indeed my bad to try and use common sense when interpreting the error message, rather than getting a copy of the standard and looking it up in there. Will try and keep this in mind for next time, thanks... — user541686, Aug 25 '11 at 12:01
@Kerrek: Indeed, please do! x__x If I happen to get a chance to see it at 4 votes then I'll cast the 5th one myself. ;) — user541686, Aug 25 '11 at 12:14

score 15 · Accepted Answer · edited May 23 '17 at 11:47

15

You can use {} initializer for aggregates only¹ and the first one is not an aggregate, as it has one private data member.

The Standard says in section §8.5.1/1,

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).

^{1. Well, I meant, in C++03, you can use {} for aggregates ONLY, but in C++11, you can use {} even with non-aggregates (if the non-aggregate class is properly implemented to handle this).}

Also see this for detail answer (on {} initializer):

What is assignment via curly braces called? and can it be controlled?

edited May 23 '17 at 11:47

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answered Aug 25 '11 at 11:50

Nawaz

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WOWWWW I feel so silly now. Adding `public:` fixes the issue. +1 thanks. *facepalm* – user541686 Aug 25 '11 at 11:53
I'm now kind of wondering -- *why* are we allowed to create a class like that in the first place? There's no legal way to access `x` (not even reflection), so shouldn't the compiler give an error or something? – user541686 Aug 25 '11 at 11:59
@Mehrdad: Actually you can. You can still create instances of such class. The compiler generated *public* constructor is there, and you can use [the instances more like handles!](http://www.ideone.com/c79Gd) – Nawaz Aug 25 '11 at 12:01
Use them as handles? Don't you at least need to be able to check them for equality (and differentiate between them)? – user541686 Aug 25 '11 at 12:03
@Mehrdad: Like this : http://www.ideone.com/c79Gd . Just check the addresses. Windows API uses such handles all the time! – Nawaz Aug 25 '11 at 12:04
Ohh I guesssssss that's *possible*, though it seems like a really roundabout way of making handles. Oh well, I guess that works. – user541686 Aug 25 '11 at 12:06
@Mehrdad: Also see this : [Handles Comparison: empty classes vs. undefined classes vs. void*](http://stackoverflow.com/questions/4525847/handles-comparison-empty-classes-vs-undefined-classes-vs-void) – Nawaz Aug 25 '11 at 12:06
Ah thanks for the link, that's pretty cool actually! – user541686 Aug 25 '11 at 12:08

score 4 · Answer 2 · answered Aug 25 '11 at 11:52

That is not a difference between class and struct, but between aggregate and non-aggregates. You cannot use the initializer list with a non-aggregate type, but that is unrelated to the class or struct keyword:

class { public: int value; } var = {42};   // compiles
struct { private: int value; } var = {42}; // error

score 3 · Answer 3 · answered Aug 25 '11 at 11:53

3

The difference is between public and private.

Try this instead:

class { public: int value; } var = { 42 };

answered Aug 25 '11 at 11:53

Bo Persson

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score 1 · Answer 4 · answered Aug 25 '11 at 11:52

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It seems that members of a class are private as well as any inheritance is private where as a struct is all public.

Someone else will have to give you more specifics though, sorry.

answered Aug 25 '11 at 11:52

Tony318

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C++ all differences between 'struct' and 'class'?

What are all the differences between structs and classes in C++?

4 Answers4