c++ - Access of nested classes (which behave like friends, but aren't) -

c++ - Access of nested classes (which behave like friends, but aren't) -

without long delay, here code have no clue why does:

#include <iostream> class { private: void print() { std::cout << "a.print() called" << std::endl; }; public: template<typename foo> class b; //note: no friend! public: a(); b<double>* bd; b<int>* bi; }; template<typename foo> class a::b{ a* callback; public: b(a* a):callback(a){}; void print() { callback->print(); }; // why working ??? }; a::a():bd(new b<double>(this)),bi(new b<int>(this)){} int main(int argc, char **argv) { a; // a.print(); // error: ‘void a::print()’ private a.bd->print(); a.bi->print(); a::b<char> c(&a); c.print(); a::b<double> d = *a.bd; d.print(); homecoming 0; }

well, creates ouput:

a.print() called a.print() called a.print() called a.print() called

but why?

background

i started journey downwards rabbit hole when encountered problem through have friends. read friend declaration not forwards declaring (and mentioned answers here , here). while trying set easy illustration (the result of see above), found don't seem need friend @ all.

question

so here bottom line question: why instance of a::b have access a's private function a::print()? (although realize might misunderstand my children are--children opposed base vs. derived)

because nested class fellow member of enclosing class

standard $11.7.1

"a nested class fellow member , such has same access rights other member. members of enclosing class have no special access members of nested class; usual access rules shall obeyed"

and usual access rules specify that:

"a fellow member of class can access names class has access..."

specific examples has been given in standard:

class e { int x; class b { }; class { b b; // ok: e::i can access e::b int y; void f(e* p, int i) { p->x = i; // ok: e::i can access e::x } }; }

c++ class instance parent-child