c++, enum class as micro database by reflection

> Start
> First Table: url schema ident
> Second Table: url schema alias
> Third Table: relation
> Reflection
> Back: Home

By c++26 reflection, c++ enum class can be used as micro database.

This article takes url schema as example.

First Table: url schema ident

First table: defines url schema ident.

Url schema ident contains http, (socket), websocket, tcp (socket), udp (socket), socks4, socks5, empty, noident, unknown.

Every enum class enumerator is unique. This table is the database keys table.

url_schema::ident

class url_schema
{
public:
	enum class ident
	{
		http,
		websocket,
		tcp,	// tcp socket
		udp,	// udp socket
		socks4,
		socks5,
		empty,
		noident,
		unknown
	};
};

url_schema::ident is used to identify which url schema from the url.

empty: the url has no explicit schema at the first of the url string.

noident: the url starts with :// without schema string, likely a bad-url.

unknown: the url schema is not implemented currently, such as chat://

Second Table: url schema alias

Second table: defines url schema alias. Every ident has too many aliases.

url_schema::alias

class url_schema
{
public:
	enum class alias
	{
		http,
		https,
		websocket,
		websockets,
		tcp,
		tcps,
		udp,
		udps,
		socket,
		sockets,
		sock,
		socks,
		socks4,
		socks5,
		empty,
		noalias,
		unknown
	};
};

url_schema::alias is used to match the first substring of the url.

empty: the url has no explicit schema at the first of the url string.

noalias: the url starts with :// without schema string, likely a bad-url.

unknown: the url schema is not implemented currently, such as chat://

Third Table: relation

Third table: the relation table of the first table and second table. It defines how to search the unique ident by alias. It also defines if the tls is enabled or not.

url_schema::relation

class url_schema
{
public:
	enum class relation
	{
		http_http_no,
		http_https_yes,
		websocket_websocket_no,
		websocket_websockets_yes,
		tcp_tcp_no,
		tcp_tcps_yes,
		udp_udp_no,
		udp_udps_yes,
		tcp_socket_no,
		tcp_sockets_yes,
		tcp_sock_no,
		tcp_socks_yes,
		socks4_socks4_no,
		socks5_socks5_no,
		empty_empty_no,
		noident_noalias_no,
		unknown_unknown_no
	};
};

url_schema::relation is the relation table.

??_yes: tls enabled
??_no: tls not enabled

Reflection

After the three tables are built in c++ code,
the next step is to use c++26 reflection to
search, match, transform and handle the url schema.

Reflection example

#include <meta>
#include <map>
#include <iostream>

namespace url
{
	class url_schema
	{
	public:
		enum class ident
		{
			http,
			websocket,
			tcp,
			udp,
			socks4,
			socks5,
			empty,
			noident,
			unknown
		};
	};
}

int main()
{
	std::map<url::url_schema::ident, std::string> map;
	constexpr auto table = std::define_static_array(
		std::meta::enumerators_of(
			^^url::url_schema::ident
		)
	);
	template for (constexpr auto & ident: auto (table))
	{
		map.emplace(
			[:ident:],
			std::string{
				std::meta::identifier_of(ident)
			}
		);
	}
	for (const auto & [ident, string]: map)
	{
		std::cout
			<< static_cast<int>(ident) << " => " << string
			<< std::endl
		;
	}
}

Output:

0 => http
1 => websocket
2 => tcp
3 => udp
4 => socks4
5 => socks5
6 => empty
7 => noident
8 => unknown

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Fri Jul 10 02:51:26 AM UTC 2026

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