C++ 17: Aggregate Improvements

C++17 extends how aggregates can be initialised, but first what are “aggregates”?

Aggregates are either arrays, or simple C-like classes that have no user-provided constructors, all members public, no virtual functions. Before C++17 they could not have base classes either.

Before you could do the following to initialise an aggregate:

struct agg
{
    int id;
    string name;
};

agg b{ 1, "name" };

With C++17 you can also do:

struct agg_base
{
    int id;
    string name;
};

agg_base b{ 1, "name" };

struct agg_derived : agg_base
{
    string tag;
};

agg_derived d{ {2, "name2"}, "tag" };

Curly brackets in the last line initialise base members first.

Why is this useful? Deriving a structure from another would disable aggregate initialisation before C++17, so you would have to declare custom constructors - more work 👷‍♂️