C++ auto-generated copy constructors with const and non-const refs -

consider following code snippet:

#include <iostream>  struct foo {     foo() = default;     foo(foo &) {         std::cout << "foo(foo &)" << std::endl;     } };  struct bar { foo m; };  int main() {     bar b;     bar b2(b); } 

if code run, foo(foo &) message written. ok, foo doesn't have foo(const foo &) constructor, base(base &) constructor generated base. now, if add following definition of foo:

foo(const foo &) {     std::cout << "foo(const foo &)" << std::endl; } 

the foo(const foo &) message written. ok, foo has copy-constructor const ref parameter, base(const base &) generated compiler. but, if declare foo(const foo &) explicitly deleted:

foo(const foo&) = delete; 

then program won't compile following errors:

prog.cc: in function 'int main()': prog.cc:15:13: error: use of deleted function 'bar::bar(const bar&)'      bar b2(b);              ^ prog.cc:11:8: note: 'bar::bar(const bar&)' implicitly deleted because default definition ill-formed:  struct bar { foo m; };         ^~~ prog.cc:11:8: error: use of deleted function 'foo::foo(const foo&)' prog.cc:8:5: note: declared here      foo(const foo&) = delete;      ^~~ 

in first revision of code, if foo(const foo&) not generated, why don't have error, in third revision? , if it's generated, why isn't called (producing no output message), in second revision?

a copy-constructor implicitly declared , defined if there no user-declared copy-ctor, move-ctor, or move-assignment-operator.

it of form x(const x&) if virtual bases, direct bases , non-static data members t declare copy-ctor accepting const t&.
otherwise of form x(x&).

an implicitly declared copy-ctor defaulted, , defaulted copy-ctor deleted if default-definition (member-wise copy) ill-formed.

you got error because sub-objects copy-ctor accepting const& deleted, making containing classes copy-ctor deleted.