Struct initialization of derived struct with a templated base type

Question

I am trying to use struct initialization on a derived from template struct. The code goes something like this:

template <class Derived>
struct Event{
//the level of access on the ctor has nothing to do with the problem
//protected:
//    Event() = default;
};

struct MoveEvent: Event<MoveEvent>{
    int x, y;
};


int main(){
    //how do I make this work?
  //MoveEvent event = {.x =5, .y = 4};
}

I thought it might have something to do with CTRP, but changing Event<MoveEvent> to Event<int> yields the same problem. Furthermore, I thought it was an issue with POD, but std::is_pod returns true for MoveEvent. So what's the problem here? Why can't I use struct initialization?

@user975989 It doesn't matter, that is a C feature, not a C++ feature — Cory Kramer, Aug 03 '15 at 18:13
Did you try `MoveEvent event = {.5, 4};` already? Also put the verbatim error messages you get in your question please. — πάντα ῥεῖ, Aug 03 '15 at 18:14
Ah. So the problem lies in the inheritance, since a non inheriting `MoveEvent` works. — user975989, Aug 03 '15 at 18:14
@πάνταῥεῖ [that won't work either](http://stackoverflow.com/questions/16983539/why-can-i-not-brace-initialize-a-struct-derived-from-another-struct) — m.s., Aug 03 '15 at 18:16
@m.s. THX for the insight! Learn something new everyday, keeps the Alzheimer's disease away ;-) (there's an age when you don't care so much about your teeth health, or vitamin supply so much, but rather keep your brain going)... — πάντα ῥεῖ, Aug 03 '15 at 18:33

score 6 · Accepted Answer · answered Aug 03 '15 at 18:19

You can only do aggregate initialization on aggregates. An aggregate is, from [dcl.init.aggr]:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

MoveEvent is not an aggregate. Thus, you will have to add a constructor:

template <class Derived>
struct Event {
};

struct MoveEvent: Event<MoveEvent> {
    MoveEvent(int x, int y) : x(x), y(y) { }
    int x, y;
};


int main() {
    MoveEvent event{5, 4}; // NOW this is fine
}

score 3 · Answer 2 · answered Aug 03 '15 at 18:19

You cannot use this syntax for a couple reasons, firstly that "designated initializers" are a C feature, not a C++ feature

MoveEvent event = {.x = 5, .y = 4};

Secondly, you cannot use aggregate initialization for a derived class, because as soon as you introduce inheritance, you no longer have a POD aggregate type.

Your best best is to define a constructor for the derived class

struct MoveEvent: Event<MoveEvent>
{
    MoveEvent(int _x, int _y) : x{_x}, y{_y} {}
    int x, y;
};

Then you can do this

MoveEvent event{5, 4};

Struct initialization of derived struct with a templated base type

2 Answers2

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