Why should a pimpl be declared as a struct and not a class?

Question

The canonical form of the pimpl idiom (from Herb Sutter's "Exceptional C++") is as follows:

class X 
{
public:
 /* ... public members ... */
protected:
 /* ... protected members? ... */ 
private:
 /* ... private members? ... */
 struct XImpl;
 XImpl* pimpl_; // opaque pointer to
                // forward-declared class 
};

My question is, why is XImpl declared as a struct instead of a class?

There is little sense is hiding something that is already hidden. The default access of `private` for a `class` vs. `public` for a `struct` seems trivially overkill. — WhozCraig, Sep 18 '14 at 07:21
It makes absolutely no difference whatsoever (unless your compiler is very very buggy). — Marc Glisse, Sep 18 '14 at 07:23
@WhozCraig But that is a forward declaration. In this particular case, it makes absolutely no difference. It doesn't even save typing :-) — juanchopanza, Sep 18 '14 at 07:31
@juanchopanza I completely agree. its the language-difference of the word as a normal decl I was commenting on. you're absolutely right that it truly makes no functional difference at all. — WhozCraig, Sep 18 '14 at 07:33

Angew is no longer proud of SO · Answer 1 · 2014-09-18T08:02:53.637

7

The only difference between struct and class is the default access control of the bases and members (public and private, respectively). You can even declare the same type with one and define it with the other (but note that some compilers can issue warnings on this).

Just use whatever is most natural to you. @WhozCraig correctly points out that since XImpl is already inaccessible outside of the implementation X, making its members private by default seems superfluous. Still, as I've said above, it makes no difference here at all, since it's only the keyword used for the definition which matters.

edited Sep 18 '14 at 08:02

answered Sep 18 '14 at 07:24

Angew is no longer proud of SO

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But it is a forward declaration, so the difference doesn't matter here (except some might get confused seeing `class` in a declaration and `struct` in the definition.) – juanchopanza Sep 18 '14 at 07:31
2

@StilesCrisis Really? Since when? ? C++11 §11(3) [class.access] : "Members of a class defined with the keyword `class` are private by default" – WhozCraig Sep 18 '14 at 07:35
1

Class definitely defaults to private! – Mats Petersson Sep 18 '14 at 07:35
@juanchopanza and some compilers even emit warnings if you define a `class` after forward-declaring it as a `struct` – Arne Mertz Sep 18 '14 at 07:37
@juanchopanza Right, edited the answer to that effect. – Angew is no longer proud of SO Sep 18 '14 at 07:38
@Angew an example for the slight difference (the compiler warning mentioned above): http://coliru.stacked-crooked.com/a/45b934a46a183d27 This is clang specific, it does not appear under gcc. – Arne Mertz Sep 18 '14 at 07:44
2

@ArneMertz IIRR, VS issues this warning as well. But I must say it's one of the few warnings I consider detrimental, and disable it in my projects. – Angew is no longer proud of SO Sep 18 '14 at 08:02

Boris Dalstein · Answer 2 · 2014-09-18T07:28:07.977

2

Well... I don't know why Herb Sutter has decided to use struct, but you can use class instead if you prefer, it is equivalent in this case.

edited Sep 18 '14 at 07:28

answered Sep 18 '14 at 07:22

Boris Dalstein

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score 2 · Answer 3 · answered Sep 18 '14 at 07:24

It doesn't really make a difference since structs and classes are basically the same, but you probably want everything in XImpl to be public because it's only visible to the code that implements X anyway. Using struct instead of class just saves you from having to write public: at the beginning of its definition.

Why should a pimpl be declared as a struct and not a class?

3 Answers3