In a boost::noncopyable class, setting the move constructor "=default" does not compile?-CodePudding

Consider the following code. In order for this to compile, I HAVE to provide a body for the move constructor. Having "TestClass(TestClass&& other) = default" does not compile with an error that says "TestClass::TestClass(TestClass&&)' is implicitly deleted because the default definition would be ill-formed:"

Why is this? Why can't it figure out the default?

#include <iostream>
#include <optional>
#include <boost/noncopyable.hpp>

class TestClass : public boost::noncopyable {
public:
    TestClass() {
        std::cout << "Constructed...\n";
    }

    TestClass(TestClass&& other) = default;

    //TestClass(TestClass&& other)  {
    //    std::cout << "Move constructor...\n";
    //}

    TestClass& operator=(TestClass&& other)  {
        std::cout << "Move assignment...\n";
        return *this;
    }
};

TestClass doThing() {
    std::optional<TestClass> tc = std::make_optional<TestClass>(TestClass{});
    return std::move(*tc);
}

int main() {
    const TestClass t = doThing();
}

https://godbolt.org/z/3f5zTT6a9

CodePudding user response：

boost::noncopyable originates from before C 11. So maybe the name isn't making this obvious, but it doesn't only make the class non-copyable, it also makes it non-movable (with move semantics having been added only with C 11). If the base class can't be moved, then a default implementation of the move constructor is not possible.

Also it should be used by private inheritance, not public.

CodePudding user response：

Yeah, I think the misnomer of boost::noncopyable (not nonmovable) confused me. Thanks!