Friend Function needs a Helper Function

Question

In my .h file, I have a class like this:

#pragma once
class Widget {
    int private_data;
    friend void foo(Widget& w);
}

As I was implementing foo, it turns out I need a helper function:

static void foohelper(Widget& w) {
    printf("further processing %d", w.private_data);
}

void foo(Widget& w) {
    printf("processing %d", w.private_data);
    foohelper(w);
}

I don't want to put foohelper in the .h file, because it is an implementation detail, but that means there's no way for it to become a friend directly.

In this example, one could get away with just passing the private_data directly, but in a real use case, this doesn't scale well when Widget has more private variables.

I only ever call foohelper from foo, so if c++ supported nested function definitions, I wouldn't have a problem:

void foo(Widget& w) {
    void foohelper(Widget& w) { // compiler: function definition is not allowed here
         printf("further processing %d", w.private_data);
    }
    printf("processing %d", w.private_data);
    foohelper(w);
}

But, c++ doesn't allow this as a solution.

Is it possible for a function to pass friend status to a helper function?

Answer 1

C++ doesn't have inner function, but you can use lambda for that:

void foo(Widget& w) {
    auto foohelper = [](Widget& w) {
         printf("further processing %d", w.private_data);
    };
    printf("processing %d", w.private_data);
    foohelper(w);
}