From Newsgroup: comp.lang.c++

Sorry, should go here.

Am 11.12.2025 um 15:43 schrieb Bonita Montero:

Niche, isn't it ?

#pragma once
#include <atomic>
#include <concepts>

template<typename Callable>
-a -a requires requires( Callable callable ) { { callable() } -> std::same_as<void>; }
-a -a -a -a || requires( Callable callable ) { { callable() } -> std::convertible_to<bool>; }
void xcall_once( std::atomic_int &flag, Callable callable )
{
-a -a using namespace std;
-a -a for( int ref = flag.load( memory_order_acquire ); )
-a -a {
-a -a -a -a if( ref > 0 )
-a -a -a -a -a -a return;
-a -a -a -a if( ref < 0 )
-a -a -a -a {
-a -a -a -a -a -a flag.wait( ref, memory_order_relaxed );
-a -a -a -a -a -a ref = flag.load( memory_order_acquire );
-a -a -a -a -a -a continue;
-a -a -a -a }
-a -a -a -a if( flag.compare_exchange_strong( ref, -1,
memory_order_relaxed, memory_order_relaxed ) )
-a -a -a -a -a -a break;
-a -a }
-a -a bool succ = true;
-a -a if constexpr( requires( Callable callable ) { { callable() } -> same_as<void>; } )
-a -a -a -a callable();
-a -a else
-a -a -a -a succ = (bool)callable();
-a -a flag.store( (int)succ, memory_order_release );
-a -a flag.notify_all();
}

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: comp.lang.c++

This is the final code:

#pragma once
#include <concepts>
#include <atomic>

struct xonce_flag
{
-a -a xonce_flag() noexcept = default;
private:
-a -a friend bool xcall_once( xonce_flag &, std::invocable auto );
-a -a std::atomic<signed char> m_flag = 0;
};

bool xcall_once( xonce_flag &xflag, std::invocable auto callable )
{
-a -a using namespace std;
-a -a atomic<signed char> &flag = xflag.m_flag;
-a -a for( signed char ref = flag.load( memory_order_acquire ); ; )
-a -a -a -a if( ref > 0 )
-a -a -a -a -a -a return true;
-a -a -a -a else if( ref < 0 )
-a -a -a -a {
-a -a -a -a -a -a flag.wait( ref, memory_order_relaxed );
-a -a -a -a -a -a ref = flag.load( memory_order_acquire );
-a -a -a -a -a -a continue;
-a -a -a -a }
-a -a -a -a else if( flag.compare_exchange_strong( ref, -1, memory_order_relaxed, memory_order_acquire ) )
-a -a -a -a -a -a break;
-a -a bool succ = true;
-a -a try
-a -a {
-a -a -a -a if constexpr( requires { (bool)callable(); } )
-a -a -a -a -a -a succ = (bool)callable();
-a -a -a -a else
-a -a -a -a -a -a callable();
-a -a }
-a -a catch( ... )
-a -a {
-a -a -a -a flag.store( 0, memory_order_release );
-a -a -a -a flag.notify_all();
-a -a -a -a throw;
-a -a }
-a -a flag.store( (signed char)succ, memory_order_release );
-a -a flag.notify_all();
-a -a return succ;
}

--- Synchronet 3.21a-Linux NewsLink 1.2