The case when the copy-constructor implicitly defined as deleted

Question

The section N3797::12.8/11 [class.copy] says:

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/ move constructor for a class X is defined as deleted (8.4.3) if X has:

[...]

— a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (13.3), as applied to M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor

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

#include <iostream>
using namespace std;

struct A
{
    A(){ }
    A(volatile A&){ }
    A(const A&, int a = 6){ }
};

struct U
{
    U(){ };
    A a;
};

U u;

U t = u;

int main(){ }

to reflect that. But what about or a function that is deleted or inaccessible from the defaulted constructor? What's that got with a function inaccessible from the default constructor? Could you provide an example reflecting that?

It says defaulted constructor, not "default constructor" - this is referring to the defaulted copy/move constructor — M.M

user657267 user657267 · Accepted Answer · 2014-11-12T06:05:54

Simply put:

struct M { M(M const&) =delete; };
struct X { X(X const&) =default; M m; }; // X(X const&) is actually deleted!

Implicitly-declared functions are also considered "defaulted" ([dcl.fct.def.default] / 5); a more familiar pre-C++11 example might be something like:

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

Note that if you explicitly default the function after it has been declared, the program is ill-formed if the function would be 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.

The case when the copy-constructor implicitly defined as deleted

2 Answers