Do we have a better way of returning abstract classes in C++?

Question

I want to start throwing some interfaces into my C++ code to make it easier for me to unit test using mocks.

The problem with this is returning abstract classes from a method in C++ is a pain. You can't return by value so you need to return a pointer or a reference.

Given all of the developments in C++ in the last six or seven years, I thought I'd ask if maybe we had a better way to return an abstract base class. An interface without the noise would look something like this, but I'm sure this isn't possible.

IBaseInterface getThing() {return DerivedThing{};}

The way that I remember doing this in the past is to use a pointer (probably a smart pointer now):

std::unique_ptr<IBaseInterface> getThing() {return std::make_unique<DerivedThing>();}

The problem with the pointer is that I'm never actually planning to take advantage of nullptr so the overhead and noise of dealing with a pointer rather than a value gains me no value as a reader.

Is there a better way that I don't know to handle this?

Answer 1

EDIT: provides complete example, including making the polymorphic handle copyable.

#include <iostream>
#include <utility>
#include <memory>

struct IBaseInterface {
    IBaseInterface() = default;
    IBaseInterface(IBaseInterface const&) = default;
    IBaseInterface(IBaseInterface &&) = default;
    IBaseInterface& operator=(IBaseInterface const&) = default;
    IBaseInterface& operator=(IBaseInterface &&) = default;
    virtual ~IBaseInterface() = default;

    virtual std::unique_ptr<IBaseInterface> clone() const = 0;
    virtual void do_thing() = 0;
};

struct handle
{
    handle(std::unique_ptr<IBaseInterface> ptr)
    : _impl(std::move(ptr))
    {}

    handle(handle const& r)
    : _impl(r._impl->clone())
    {}

    handle(handle && r)
    : _impl(std::move(r._impl))
    {}

    handle& operator=(handle const& r)
    {
        auto tmp = r;
        std::swap(_impl, tmp._impl);
        return *this;
    }

    handle& operator=(handle && r)
    {
        _impl = std::move(r._impl);
        return *this;
    }


    // interface here
    void do_thing() { _impl->do_thing(); }

private:
    std::unique_ptr<IBaseInterface> _impl;
};

struct DerivedThing : IBaseInterface
{
    std::unique_ptr<IBaseInterface> clone() const override
    {
        return std::make_unique<DerivedThing>(*this);
    }

    void do_thing() override
    {
        std::cout << "I'm doing something" << std::endl;
    }

};

handle make_thing()
{
    return handle(std::make_unique<DerivedThing>());
};

int main()
{
    auto a = make_thing();
    auto b = a;

    a.do_thing();
    b.do_thing();

    return 0;
}

Now use your handle as if it had (moveable) value semantics