C++ 中的聚合初始化

Question

I have a base class Event, from which concrete events derive:

struct Event
{
};


struct CollisionEvent : Event
{
    //CollisionEvent(Entity entityA, Entity entityB) : entityA(entityA), entityB(entityB) {}

    Entity entityA;
    Entity entityB;
};

this class is a POD, and I should be able to perform aggregate initializaion. But the following code fails to compile:

called from application:

eventBus->DispatchEvent<CollisionEvent>(mEntities[i], mEntities[j]);



in EventBus.h:

template <typename TEvent, typename... Args>
    void DispatchEvent(Args&&... args)
    {
        auto it = mEventHandlers.find(typeid(TEvent));

        if (it != mEventHandlers.end())
            for (auto &&eventHandler : it->second)
                eventHandler->Invoke(TEvent{ std::forward<Args>(args)... });
    }

saying that it cannot convert initializer list to TEvent. Why?

Answer 1

For aggregate initialization you need to initialize the base class, even if it's empty.

eventBus->DispatchEvent<CollisionEvent>(Event{}, mEntities[i], mEntities[j]);

There were also some changes regarding what is and what isn't an aggregate in recent standards - until C++17 a class having a base class can't be aggregate-initialized, so you can't make it work in C++14. (see https://godbolt.org/z/Pq5PfecKq )