BitTickler BitTickler - 1 month ago 8
C++ Question

Warning: Multiple copy constructors defined while deleting them

Environment: VS2015 Update 3, 64 bit (debug|release) compile

In the code below, if I uncomment the

Maybe(X&) = delete
line, I get the warning mentioned in the code below and in the title of the question.

Now, I am aware, that there are certain rules in C++(11?), which might render
the explicit deletion of that constructor obsolete. Only, even after searching
the web for a while, I could not find a definite rule, which would confirm, that
if I only delete
Maybe(const X&) = delete
, the compiler will not auto generate the other copy constructor.

So my question is first and foremost:
Can anyone point me to spot in the C++ specification, which clearly defines the
rules for auto generation of copy constructors? Alternatively, some less official
easy to remember rule of thumb on how to be certain of what will happen would also be welcome.

template <class X>
class Maybe
{
X *m_just;
public:
explicit Maybe(const X& x)
: m_just(new X(x))
{}
Maybe()
: m_just(nullptr)
{}
Maybe(const Maybe<X>&& other)
: m_just(other.m_just)
{
other.m_just = nullptr;
}
Maybe(const Maybe<X>&) = delete;
// If line below is uncommented, this produces the warning:
// warning C4521: 'Maybe<Int32>': multiple copy constructors specified
// Maybe(Maybe<X>&) = delete;

~Maybe()
{
delete m_just;
m_just = nullptr;
}
// ... more members and code which are not related to question
// ...
};


Please do not comment on the whole idea of that class. It is just private tinkering in my private lab... ;)

Answer

The compiler will only generate one of Maybe(Maybe<X>&) and Maybe(const Maybe<X>&).
The conditions for the choice are listed in section 12.8, paragraph 8:

The implicitly-declared copy constructor for a class X will have the form X::X(const X&) if

  • each direct or virtual base class B of X has a copy constructor whose first parameter is of type const B& or const volatile B&, and
  • for all the non-static data members of X that are of a class type M (or array thereof), each such class type has a copy constructor whose first parameter is of type const M& or const volatile M&

Otherwise, the implicitly-declared copy constructor will have the form X::X(X&)

Or more informally, the parameter will be const if and only if everything that needs to be copied can be "const-copied".

Deleting one will not cause the other to be generated.

Since you have no base class and your only member is a pointer, the generated constructor will be of the const variety and you can leave out the line causing an error.

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