C++: Order of declaration in public and private

Question

I am unsure about the order of declaration in my class. My compiler says error: "Foo" was not declared in this scope. If I change the order of public and private parts of the class, I end up with the same error. Also, if I want to use the getFoo(), and I am including my header file, the struct Foo type is not visible because it's private. But if I put it in public scope again, then public would have to come before private because otherwise the declaration of myFoo of type Foo can't happen since Foo was not decalred yet. I am confused here... thanks for your help!

/*MyClass.h*/

class MyClass
{


private:

 struct Foo{
  int bar;
 };

Foo myFoo;


public:
 Foo getFoo(){ return myFoo;}

};

Please show some complete code that reproduces the error. There's nothing wrong with what you have shown. See [this demo](http://ideone.com/hZCRtm). — juanchopanza, Jul 18 '14 at 09:58
@juanchopanza: says g++, "foo.cpp:2:1: error: an anonymous struct cannot have function members" — Cheers and hth. - Alf, Jul 18 '14 at 10:00
**-1 for editing the question text after the answers and posting garbage before** — Marco A., Jul 18 '14 at 10:01
@Cheersandhth.-Alf It is not anonymous anymore. It has been given a name, completely changing the nature of the question. http://ideone.com/hZCRtm — juanchopanza, Jul 18 '14 at 10:01
rolled back to original version because answers had already been posted, whose meaning the edit changed — Cheers and hth. - Alf, Jul 18 '14 at 10:06
@Cheersandhth.-Alf Oh no, that invalidates my answer. Never mind, serves me right for answering this kind of question :-) — juanchopanza, Jul 18 '14 at 10:07
Sorry, missing the classname was a typo when copying the code. — tzippy, Jul 18 '14 at 10:10
@Cheersandhth.-Alf I noticed the remaining questions attempt to address the "fixed" code sample, so I rolled forward to revision 2. — juanchopanza, Jul 18 '14 at 10:28

juanchopanza · Answer 1 · 2014-07-18T10:16:34.313

It has nothing to do with public or private. Your inner type has to be defined before it is used:

struct Foo
{
  Bar bar() { return Bar(); } // ERROR
  struct Bar {};
};

struct Foo
{
  struct Bar {};
  Bar bar() { return Bar(); } // OK
};

Note: there has been some confusion concerning the accessibility of a private type. THe type is accessible outside of its class, it just cannot be named. So, this code that accesses the private type is completely legal:

class Foo
{
    struct Bar { 
        void do_stuff() const {}
    };

public:
    Bar bar() { return Bar(); } // OK
};
int main() 
{
    Foo f;
    f.bar().do_stuff(); // use temporary Bar object

    // In C++11, you can even make a named instance
    auto b = f.bar();   // instantiate a Bar object called b
    b.do_stuff();       // use it
}

Is there a logical explanation for allowing such access to private type?.. What about encapsulation? — SomeWittyUsername, Jul 18 '14 at 10:31

score -1 · Answer 2 · answered Jul 18 '14 at 09:52

-1

Your struct Foo should be public otherwise getFoo getter will not work as Foo is accessible only inside your class

answered Jul 18 '14 at 09:52

nib

700
8
15

2

@TonyD But `auto f = x.getFoo();` would work. The `private` is not the issue here. – juanchopanza Jul 18 '14 at 09:56
**-1** there's no problem with the `private`. there is a technical problem with the lack of a name for the class. there is a design problem with the getter. – Cheers and hth. - Alf Jul 18 '14 at 09:57

C++: Order of declaration in public and private

2 Answers2