From Newsgroup: comp.lang.c++

template<typename First, typename ... Further>
struct exc_xguard_unpack_last
{
using further = std::tuple<Further *...>;
using type = typename exc_xguard_unpack_last<Further ...>::type;
};

template<typename Last>
struct exc_xguard_unpack_last<Last>
{
using type = Last;
};

template<typename ... PassThrough>
struct exc_xguard
{
template<typename Func, typename Nested>
FORCEINLINE auto operator ()( Func func, Nested nested ) const
{
static_assert( sizeof ...(PassThrough) >= 1 );
try
{
using return_type = std::invoke_result_t<Func>;
return [&]<typename ... Pass>( this auto const &self, std::tuple<Pass
*...> ) L_FORCEINLINE -> return_type
{
try
{
if constexpr( sizeof ...(Pass) == 1 )
return func();
else
return self( typename exc_xguard_unpack_last<Pass ...>::further() );
}
catch( typename exc_xguard_unpack_last<Pass ...>::type & )
{
throw;
}
}( std::tuple<PassThrough *...>() );
}
catch( ... )
{
throw nested();
}
}
};


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From Newsgroup: comp.lang.c++

Am 08.10.2025 um 18:48 schrieb Bonita Montero:

...
template<typename ... PassThrough>
struct exc_xguard
{
-a-a-a-atemplate<typename Func, typename Nested>
-a-a-a-aFORCEINLINE auto operator ()( Func func, Nested nested ) const
-a-a-a-a{
-a-a-a-a-a-a-a static_assert( sizeof ...(PassThrough) >= 1 );
-a-a-a-a-a-a-a try
-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a using return_type = std::invoke_result_t<Func>;
-a-a-a-a-a-a-a-a-a-a-a return [&]<typename ... Pass>( this auto const &self,
std::tuple<Pass *...> ) L_FORCEINLINE -> return_type
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a try
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a if constexpr( sizeof ...(Pass) == 1 )
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a return func();
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a else
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a return self( typename
exc_xguard_unpack_last<Pass ...>::further() );
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a }
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a catch( typename exc_xguard_unpack_last<Pass ...>::type & )
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a throw;
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a }
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a }( std::tuple<PassThrough *...>() );
-a-a-a-a-a-a-a }
-a-a-a-a-a-a-a catch( ... )
-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a throw nested();
-a-a-a-a-a-a-a }
-a-a-a-a}
};

I forget a throw_with_nested:

template<typename ... PassThrough>
struct exc_xguard
{
template<typename Func, typename Nested>
FORCEINLINE auto operator ()( Func func, Nested nested ) const
{
static_assert( sizeof ...(PassThrough) >= 1 );
try
{
using return_type = std::invoke_result_t<Func>;
return [&]<typename ... Pass>( this auto const &self, std::tuple<Pass
*...> ) L_FORCEINLINE -> return_type
{
try
{
if constexpr( sizeof ...(Pass) == 1 )
return func();
else
return self( typename exc_xguard_unpack_last<Pass ...>::further() );
}
catch( typename exc_xguard_unpack_last<Pass ...>::type & )
{
throw;
}
}( std::tuple<PassThrough *...>() );
}
catch( ... )
{
std::throw_with_nested( nested() );
}
}
};
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From Newsgroup: comp.lang.c++

On Wed, 2025-10-08 at 18:48 +0200, Bonita Montero wrote:

template<typename First, typename ... Further>
struct exc_xguard_unpack_last
{
using further = std::tuple<Further *...>;
using type = typename exc_xguard_unpack_last<Further ...>::type;
};

template<typename Last>
struct exc_xguard_unpack_last<Last>
{
using type = Last;
};

template<typename ... PassThrough>
struct exc_xguard
{
template<typename Func, typename Nested>
FORCEINLINE auto operator ()( Func func, Nested nested ) const
{
static_assert( sizeof ...(PassThrough) >= 1 );
try
{
using return_type = std::invoke_result_t<Func>;
return [&]<typename ... Pass>( this auto const &self, std::tuple<Pass
*...> ) L_FORCEINLINE -> return_type
{
try
{
if constexpr( sizeof ...(Pass) == 1 )
return func();
else
return self( typename exc_xguard_unpack_last<Pass ...>::further() );
}
catch( typename exc_xguard_unpack_last<Pass ...>::type & )
{
throw;
}
}( std::tuple<PassThrough *...>() );
}
catch( ... )
{
throw nested();
}
}
};

As usual, the code itself is meaningless (likely so?).
What is the purpose of the codes, and useful in what environment/condition?
C++ is expressive. But more importantly, expressive for what? If the author is not clear of the logic of his idea/concept, what would the 'expressive' mean? That is the general C++ problem: It greatly helps writing obscure codes.
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From Newsgroup: comp.lang.c++

Am 08.10.2025 um 19:45 schrieb wij:

As usual, the code itself is meaningless (likely so?).
What is the purpose of the codes, and useful in what environment/condition?

C++ is expressive. But more importantly, expressive for what? If the author is
not clear of the logic of his idea/concept, what would the 'expressive' mean? That is the general C++ problem: It greatly helps writing obscure codes.

For skilled C++-developers the purpose is easy to guess from the source.

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: comp.lang.c++

On 08/10/2025 17:48, Bonita Montero wrote some code in news:///0c64k6$1nqj1$1@raubtier-asyl.eternal-september.org

Well, I don't really know all the new-fangled language-ending C++ stuff,
but from my last point in C++, supposing the new stuff was by and for
humans, many of whom are trying to capitalise on python and
functional-paradigm programmers.

Is it, perhaps, a very implementation specific routine for guaranteeing
or measuring a supposed C-stack availability or quantity of available recursions, and maybe to run a function right at the end of the
available stack? Does modern C++ throw an exception when at the end of
the stack?

Perhaps to add markers to detect programming errors, or to take
advantage of programming errors, or to decide on what routines to offer,
when to give up a naive recursive process, etc. perhaps the call will
give for "func" an implementation specific assembler routine that
detects proximity to the end of stack and raises a C++ exception.

Seems to be the theme of the comp.* heirarchy at the moment.

--
Tristan Wibberley

The message body is Copyright (C) 2025 Tristan Wibberley except
citations and quotations noted. All Rights Reserved except that you may,
of course, cite it academically giving credit to me, distribute it
verbatim as part of a usenet system or its archives, and use it to
promote my greatness and general superiority without misrepresentation
of my opinions other than my opinion of my greatness and general
superiority which you _may_ misrepresent. You definitely MAY NOT train
any production AI system with it but you may train experimental AI that
will only be used for evaluation of the AI methods it implements.

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From Newsgroup: comp.lang.c++

Am 08.10.2025 um 18:48 schrieb Bonita Montero:

template<typename First, typename ... Further>
struct exc_xguard_unpack_last
{
-a-a-a-ausing further = std::tuple<Further *...>;
-a-a-a-ausing type = typename exc_xguard_unpack_last<Further ...>::type;
};

template<typename Last>
struct exc_xguard_unpack_last<Last>
{
-a-a-a-ausing type = Last;
};

template<typename ... PassThrough>
struct exc_xguard
{
-a-a-a-atemplate<typename Func, typename Nested>
-a-a-a-aFORCEINLINE auto operator ()( Func func, Nested nested ) const
-a-a-a-a{
-a-a-a-a-a-a-a static_assert( sizeof ...(PassThrough) >= 1 );
-a-a-a-a-a-a-a try
-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a using return_type = std::invoke_result_t<Func>;
-a-a-a-a-a-a-a-a-a-a-a return [&]<typename ... Pass>( this auto const &self,
std::tuple<Pass *...> ) L_FORCEINLINE -> return_type
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a try
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a if constexpr( sizeof ...(Pass) == 1 )
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a return func();
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a else
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a return self( typename
exc_xguard_unpack_last<Pass ...>::further() );
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a }
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a catch( typename exc_xguard_unpack_last<Pass ...>::type & )
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a throw;
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a }
-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a }( std::tuple<PassThrough *...>() );
-a-a-a-a-a-a-a }
-a-a-a-a-a-a-a catch( ... )
-a-a-a-a-a-a-a {
-a-a-a-a-a-a-a-a-a-a-a throw nested();
-a-a-a-a-a-a-a }
-a-a-a-a}
};

There's a bug inside that. Do you notice it ?
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