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From: Bonita Montero <Bonita.Montero@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: encapsulating directory operations
Date: Mon, 9 Jun 2025 13:04:45 +0200
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Am 09.06.2025 um 09:59 schrieb Lawrence D'Oliveiro:

> Not necessarily that simple. It might be easier if C++ had
> try/finally (like Python does), but it doesn’t.

The initial thought with C++ exceptions is that all the unrolling
happens with RAII-classe so that you don't need finally. But some-
times you won't write a class for each resource you hold. For this
cases a helper-class like experimental::scope_exit is very comfor-
table. Unfortunately that's still not in the standard so that I
had to write it on my own:

#pragma once
#include <utility>
#include <concepts>
#include "nui.h"

template<std::invocable Fn>
struct defer final
{
	defer( Fn &&fn ) :
		m_enabled( true ),
		m_fn( std::forward<Fn>( fn ) )
	{
	}
	defer( defer const & ) = delete;
	void operator =( defer const & ) = delete;
	~defer()
	{
		if( m_enabled ) [[likely]]
			m_fn();
	}
	void operator ()()
	{
		if( !m_enabled ) [[unlikely]]
			return;
		m_fn();
		m_enabled = false;
	}
	void disable()
	{
		m_enabled = false;
	}
private:
	bool m_enabled;
	NO_UNIQUE_ADDRESS Fn m_fn;
};

template<std::invocable ... Fns>
inline void disable( defer<Fns> &... defs )
{
	(defs.disable(), ...);
}

template<std::invocable Fn, std::invocable FnNext = Fn>
struct xdefer final
{
	xdefer( Fn &&fn, xdefer<FnNext> *next = nullptr ) :
		m_enabled( true ),
		m_next( next ),
		m_fn( std::forward<Fn>( fn ) )
	{
	}
	xdefer( xdefer const & ) = delete;
	void operator =( xdefer const & ) = delete;
	~xdefer()
	{
		bool enabled = m_enabled;
		if( m_next ) [[likely]]
			m_next->m_enabled = enabled;
		if( enabled ) [[likely]]
			m_fn();
	}
	void operator ()()
	{
		if( !m_enabled ) [[unlikely]]
			return;
		m_fn();
		m_enabled = false;
		if( !m_next ) [[unlikely]]
			return;
		m_next->m_enabled = true;
		(*m_next)();
	}
	void disable()
	{
		m_enabled = false;
	}
private:
	template<std::invocable Fn1, std::invocable Fn2>
	friend struct xdefer;
	bool m_enabled;
	xdefer<FnNext> *m_next;
	NO_UNIQUE_ADDRESS Fn m_fn;
};