[MS] How can I explicitly specialize a templated C++ constructor, follow-up notes - devamazonaws.blogspot.com

A brief follow-up on How can I explicitly specialize a templated C++ constructor?, which pedantically should probably have been titled something like "How can I explicitly instantiate a templated C++ constructor?" but what's done is done.

Our solution was to use the in_place_type type-holder class to specify what type of object the object manager should contain:

struct ObjectManager
{
    // Concrete should derive from CommonBase
    template<typename Concrete, typename...Args>
    ObjectManager(int reason,
        std::in_place_type_t<Concrete>,
        Args&&...args) :
        m_base(std::make_unique<Concrete>(
                *this, std::forward<Args>(args)...))
    {
        m_base->initialize(reason);
    }

    std::unique_ptr<CommonBase> m_base;
};

We could also apply the "give it a name" principle to the problem and offer a factory method, which is easier to instantiate with an explicit type.

struct ObjectManager
{
    template<typename Concrete, typename...Args>        
    static ObjectManager make(int reason, Args&&...args)
    {                                                   
        return ObjectManager(reason,                    
            std::in_place_type<Concrete>                
            std::forward<Args>(args)...));              
    }                                                   

    ⟦ ... as before ... ⟧
};

// Example usage:
auto manager = ObjectManager::make<Derived>(9, 42);

Note that our solution still uses a type tag parameter (in this case, in_place_type). This is unavoidable because the Object­Manager constructor uses a reference to itself. This is a Catch-22 we ran into when we tried to construct nodes of a hand-made linked list. Consider this alternative:

struct ObjectManager
{
    template<typename Concrete, typename...Args>
    static ObjectManager make(int reason, Args&&...args)
    {
        // Code in italics is wrong
        ObjectManager manager(reason,                  
            std::make_unique<Concrete>(                
                manager, std::forward<Args>(args)...));
        return manager;                                
    }

    template<typename Trait, typename...Args>
    ObjectManager(int reason, std::unique_ptr<CommonBase> base) :
        m_base(std::move(base))
    {
        m_base->initialize(reason);
    }

    std::unique_ptr<CommonBase> m_base;
};

This version creates an Object­Manager that handed out a reference to itself (*this), and then moves that Object­Manager to the return value, thereby changing its address and invalidating the reference.

The above code is eligible for named return value optimization (NRVO), in which case the manager object can be constructed directly in the return value slot. However, NRVO is not a mandatory optimization, so a compiler is permitted to construct the manager separately and then move it¹ to the return value slot when the function exits.

Return value optimization (RVO, formally called copy elision) is guaranteed in the case where you return a freshly-constructed object, which is why our initial version takes the form return ObjectManager(...).

¹ Even though you didn't explicitly write std::move(manager), the compiler is required to move it. In fact, if you write return std::move(manager);, you have inadvertently defeated the std::move optimization!

Post Updated on October 25, 2024 at 03:00PM
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