Why C++11 implicitly deletes my default constructor?

Question

I wanted to make my class uncopyable using C++11, before I privately inherited my class from boost::noncopyable . In C++11 I implemented following:

class Foo
{
public:
    Foo(const Foo&) = delete;
    Foo& operator=(const Foo&) = delete;
};

With this change compilation of a client code (with VS 2013) gives me following error:

..\main.cpp(9): error C2512: 'Foo' : no appropriate default constructor available

My client code is pretty simple:

int main()
{
    Foo foo;
}

Is there any C++11 rule which implicitly deletes default constructor in my case ?

Answer 1

The default constructor for Foo is not deleted, it is simply not defined. An explicitly defaulted/deleted constructor is a user-declared constructor, and this causes an implicit declaration of a default constructor to be inhibited. Foo has a user-declared constructor, the explicitly deleted copy constructor.

From N3337, §12.1/5 [class.ctor]

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4). ...

---

Note that if the default constructor was indeed deleted, the error message would probably have said as much. For instance, if I change the definition of Foo to

class Foo
{
public:
    int &i;
};

the presence of the data member of reference type implicitly deletes the default constructor. Now, clang and gcc produce the error messages

error: call to implicitly-deleted default constructor of 'Foo'

and

error: use of deleted function 'Foo::Foo()'

Compare these to the error messages produced by your original example

error: no matching constructor for initialization of 'Foo'
error: no matching function for call to 'Foo::Foo()'

---

To fix your example, you'll need to explicitly default the default constructor

Foo() = default;

Answer 2

The implicit default constructor is not deleted, it's just not generated if you have other constructors. This has been the same since very old time.

C++03 [class.ctor]/5:

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X , a default constructor is implicitly declared.

C++11 [class.ctor]/5 (same in C++14):

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X , a constructor having no parameters is implicitly declared as defaulted