Re: outer class `this` in local classes without inheritance?

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 27 Jul 2010 08:43:56 CST
On 27 Jul., 06:30, Lorenzo Caminiti <> wrote:

I am new to this groups. I would like to post a C++ programming

You are welcome to do so!

Is there a way to access the outer class `this` from a local class
but without using inheritance?

Let me explain with an example:

    class x { // outer class
        void g() {}

        void f() {
            this->g(); // (1)

            struct y: x { // local class but using inheritance...
                y(x* super): x(*super) {} // (2)
                void h() {
                    this->g(); // (3)

*** Is there a way to do this but without inheriting `y` from `x`? ***

Yes, but that requires that y has a constructor accepting
a (reference or pointer to) x and has a member of pointer or
reference type, e.g. like this:

class x { // outer class
        void g() {}

        void f() {
            this->g(); // (1)

            struct y { // local class, no inheritance
                x* that; // References an object of the outer class
                y(x* super): that(super) {} // (2)
                void h() {
                    that->g(); // (3)

Question: Do you have Java background, where it is possible to
define a "non-static" inner class with implied constructor argument
for an object of the outer class?

At line (3), the local class `y` can access the outer class `x::g()`
simply using `this` because `y` inherits from `x`. However, this
inheritance requires to construct another object `x` when `y` is
constructed as in line (2) adding runtime overhead and requiring `x`
to have an accessible copy constructor.

Yes, and this idiom is not required.

In my application, I need line (3) to essentially look the same as
like line (1) so you can program code inside `y` members as if it were
inside `x` members. A part from that, the local class `y` could look
as it is needed to solve this problem -- with more member variables,
member functions, etc.

But note that there are some restrictions to local classes, among
them it is not feasible to have static data members.

(For example, I tried to redefine `y::operator->()`/`y::operator*()`
to return a pointer/reference to `x` but this does not work because
`this->`/`*this` always retain pointer semantics and do not use the
operators I redefined...)

This should also work, you can simply define:

x* operator->() const { return that; }

as inline function in y. It is not possible to define any member
functions outside of the local class, though.

HTH & Greetings from Bremen,

Daniel Kr?gler

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