combining ctor

main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

struct foo {
	foo(int i): i{i} {}
	int i = 0;
};

struct foos;
struct foos_ref {
	foos_ref(const foos& ref): ptr{&ref} {}
	const foos* ptr;
};
struct foos {
	foos(foos&&) = default;
	foos& operator=(foos&&) = default;
	foos(std::initializer_list<foo> args): m_foos{args} {}
	foos(std::initializer_list<foos_ref> args1, std::initializer_list<foo> args2 = {}):
		foos{args2}  {
		for(const auto& x: args1) {
			m_foos.insert(m_foos.end(), x.ptr->m_foos.begin(), x.ptr->m_foos.end());
		}
	}
	std::vector<foo> m_foos;
};

int main() {
	auto foo1 = foo{1};
	auto foo2 = foo{2};
	auto foo3 = foo{3};
	auto foos1 = foos{foo1, foo2};
	auto foos2 = foos{{foos1}, {foo3}};
	auto foos3 = foos{foos2};
	auto foos4 = foos{foos1, foos2};
}