c++destructormove-semanticsnoncopyable

`noncopyable` with custom destructor


I need a noncopyable class which has a declared destructor, and naive approach doesn't work: see https://ideone.com/mU8aoc. What's the problem with the destructor, why moving doesn't work the same way as without it? And of course, how to fix it?

For reference, the complete code (same as by the ideone link above):

class noncopyable {
public:
    noncopyable(noncopyable &&) noexcept;

    noncopyable &operator=(noncopyable &&) noexcept;

protected:
    noncopyable() = default;

    ~noncopyable() = default;

    noncopyable(const noncopyable &) = delete;

    noncopyable &operator=(const noncopyable &) = delete;
};

class C: noncopyable {
public:
    // compiles if this line is uncommented
    // C(C&& c);

    C() {}

    // also compiles if this is commented
    ~C() {}
};

C a() {
    return {};
}

C b() {
    return a();
}

int main() {
    return 0;
}

Solution

  • For your code to work, class C must be moveable. When it has no declared destructor, it gets a compiler-generated implicit move constructor (and move assignment operator). But when it has a declared ("custom" in your parlance) destructor, the move constructor (and move assignment operator) are no longer provided implicitly. This is for your safety: it is assumed that if you need an explicit destructor you will need explicit move functions as well.

    Reference: http://en.cppreference.com/w/cpp/language/move_constructor