Using initializer list for a struct with inheritance

Question

I have a struct that I'd like to initialize using an initializer list

struct Parent{};

struct Child : private Parent {
    int b;
};

int main() {
    Child c{ 1 };
    return 0;
}

Looking at other questions I found this answer that states that it should be possible in c++17.

However when I try the above snippet in VS2017 I get

Error   C2440   'initializing': cannot convert from 'initializer list' to 'child'

Is there any way to utilize this new feature?

Answer 1

With inheritance, each base class subobject of the aggregate is initialized like a member. So to aggregate initialization, Child has two subobjects: Parent , and i . So you need two initializers in your braced-init-list:

Child c{ {}, 1 };

Also, in order for Child to be an aggregate, all subobjects must be public . So you can't have private base classes.

Of course, this assumes that Visual Studio implements the feature correctly. VS2017 15.5 is not C++17 compliant, but 15.7 supports this.

Answer 2

c++17 has introduced an Extension to Aggregate Initialization(P0017R1) which provides for the construction of a derived instance while still explicitly initializing the base class:

struct base { int a1, a2; };
struct derived : base { int b1; };

derived d1{{1, 2}, 3};      // full explicit initialization
derived d1{{}, 1};          // the base is value initialized

Thus using the "Extension to Aggregate Initialization" you'll want to use the code: Child c{ {}, 1 } as mentioned in Nicol Bolas's answer again with his caveat that you'd need to use public not private inheritance to do aggregate initialization at all.

Unfortunately visual-studio-2017 did not support P0017R1 until version 15.7 . So it may be necessary to upgrade your Visual Studio to accomplish this.

---

If that's not possible, and you're able to get by without polymorphisim, you can temporarily define:

struct Child {
    Parent a;
    int b;
};

Which would allow you to use the consistent code: Child c{ {}, 1 } now and whenever you change back to inheritance after upgrading to 15.7.