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.

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