Question

I have the following code:

class outer
{
    struct inner
    {
        int var1;
        int var2;
        inner() { var1 = 1; var2 = 2; }
    };

    inner inner_instance;

public:
    const inner *get_inner() { return &inner_instance; }
};

int main(int argc, char *argv[])
{
    // 1
    outer outer_instance;
    cout << outer_instance.get_inner()->var1 << endl;

    // 2
    // this cannot be compiled because outer::inner is private
    //const outer::inner *inner_ref = outer_instance.get_inner();
    //cout << inner_ref->var1;

    // 3
    const int *inner_var2 = (int *) outer_instance.get_inner();
    inner_var2++;
    cout << *inner_var2 << endl;

    return 0;
}

I understand why No.2 cannot be compiled. I just do not know what is the design idea of the compiler that allows access to the public fields of a private nested class but not the nested class itself, like No.1. The instance data is still in the memory. If I know the structure of such private nested class, I can still achieve assigning like No.3 does.

Does that mean it is better not to return pointer or reference of a private nested class in a public function? And if I really have to return something about the nested class to the outer world, make it public?

Thanks!

Answer 1

Put the struct inner definition in the public if you want to use it outside outer class.

class outer
{
public:
    struct inner
    {
        int var1;
        int var2;
        inner() { var1 = 1; var2 = 2; }
    };
private:
    inner inner_instance;
public:
    const inner *get_inner() { return &inner_instance; }
};