c++ - The case when the copy-constructor implicitly defined as deleted -

the section n3797::12.8/11 [class.copy] says:

an implicitly-declared copy/move constructor inline public member of class. defaulted copy/ move constructor class x defined deleted (8.4.3) if x has:

[...]

— non-static data member of class type m (or array thereof) cannot copied/moved because overload resolution (13.3), applied m’s corresponding constructor, results in ambiguity or function deleted or inaccessible defaulted constructor

the first case ambiguity of corresponding copy/move constructor quite clear. can write following:

#include <iostream> using namespace std;  struct {     a(){ }     a(volatile a&){ }     a(const a&, int = 6){ } };  struct u {     u(){ };     a; };  u u;  u t = u;  int main(){ } 

to reflect that. or function deleted or inaccessible defaulted constructor? what's got function inaccessible default constructor? provide example reflecting that?

simply put:

struct m { m(m const&) =delete; }; struct x { x(x const&) =default; m m; }; // x(x const&) deleted! 

implicitly-declared functions considered "defaulted" ([dcl.fct.def.default] / 5); more familiar pre-c++11 example might like:

struct m { protected: m(m const&); }; struct x { m m; }; // x's implicit copy constructor deleted! 

note if explicitly default function after has been declared, program ill-formed if function implicitly deleted ([dcl.fct.def.default] / 5):

struct m { m(m const&) =delete; }; struct x { x(x const&); m m; };  x::x(x const&) =default; // not allowed.