Simplify some C++ abstractions (#1518)

* Remove namespaces
* Prefer `bool operator==`, not `friend auto operator==`
* Prefer not to use `using`
* Use a `constexpr` function instead of a template for `flipTable`

include/defaultinitvec.hpp

@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: MIT */
+
+#ifndef RGBDS_DEFAULT_INIT_ALLOC_HPP
+#define RGBDS_DEFAULT_INIT_ALLOC_HPP
+
+#include <memory>
+#include <vector>
+
+/*
+ * Allocator adaptor that interposes construct() calls to convert value-initialization
+ * (which is what you get with e.g. `vector::resize`) into default-initialization (which does not
+ * zero out non-class types).
+ * From
+ * https://stackoverflow.com/questions/21028299/is-this-behavior-of-vectorresizesize-type-n-under-c11-and-boost-container/21028912#21028912
+ */
+
+template<typename T, typename A = std::allocator<T>>
+class default_init_allocator : public A {
+	using a_t = std::allocator_traits<A>;
+public:
+	template<typename U>
+	struct rebind {
+		using other = default_init_allocator<U, typename a_t::template rebind_alloc<U>>;
+	};
+
+	using A::A; // Inherit the allocator's constructors
+
+	template<typename U>
+	void construct(U *ptr) noexcept(std::is_nothrow_default_constructible_v<U>) {
+		::new (static_cast<void *>(ptr)) U;
+	}
+	template<typename U, typename... Args>
+	void construct(U *ptr, Args &&...args) {
+		a_t::construct(static_cast<A &>(*this), ptr, std::forward<Args>(args)...);
+	}
+};
+
+template<typename T>
+using DefaultInitVec = std::vector<T, default_init_allocator<T>>;
+
+#endif // RGBDS_DEFAULT_INIT_ALLOC_HPP