Implementing a non-copyable C++ class

Question

In Visual Studio 2019, I am trying, without success, to implement the technique for making a class non-copyable shown for C++11, and later, at the accepted answer at How do I make this C++ object non-copyable?.

My class is,

class Foo {

public:
    Foo();
    Foo(std::string name);
    ~Foo();

    std::string m_name;

    static std::int32_t s_counter;

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

The definition code is,

std::int32_t Foo::s_counter = 0;

Foo::Foo(void)
{
    Foo::s_counter++;
    std::cout << "Foo constructor.  Count = " << Foo::s_counter << std::endl;
}

Foo::Foo(std::string name)
{
    Foo::s_counter++;
    m_name = name;
    std::cout << "Foo " << m_name << " constructor.  Count = " << Foo::s_counter << std::endl;
}

Foo::~Foo()
{
    Foo::s_counter--;
    std::cout << "Foo destructor.  Count = " << Foo::s_counter << std::endl;
}

It is used in,

int main(int argc, char** argv) {

    std::vector<Foo> fooVector;

    {
        Foo myFoo1{ Foo() };
        fooVector.push_back(std::move(myFoo1));

        Foo myFoo2{ Foo("myFoo2") };
        fooVector.push_back(std::move(myFoo2));
    }

    if (Foo::s_counter < 0) {

        std::cout << "Foo object count = " << Foo::s_counter << std::endl;
    }

    std::cin.get();
}

in which I have intentionally scoped the definition of myFoo1 and myFoo2 so as to get object count feedback.

When the copy constructor and assignment constructor are made public, as shown, the compile error is "C2280 'Foo::Foo(const Foo &)': attempting to reference a deleted function". When they are made private, the compile error is "C2248 'Foo::Foo': cannot access private member declared in class 'Foo'".

I believe I am misinterpreting something in the original SO answer, but I cannot see what it is.

Answer 1

Everything's working as expected.

Your code attempts two copies, and both approaches you took to making the class non-copyable succeeded. You can see that, because you got compilation errors.

If you didn't intend to attempt copies in this testcase, it could be these lines:

Foo myFoo1{ Foo() };
Foo myFoo2{ Foo("myFoo2") };

Prior to C++17, those were copy-initialisations. If that's not what you intended, and you're pre-C++17, then you should change them to:

Foo myFoo1{};
Foo myFoo2{"myFoo2"};

… or a good old-fashioned:

Foo myFoo1;
Foo myFoo2("myFoo2");

Meanwhile, your push_backs definitely copy because, though you wrote std::move, your class isn't moveable (has no move constructor) so the std::move actually does nothing.

So:

1. Attempt to copy object
2. Copy is being prevented

All as it should be.