passing a parameter pack over a legacy function signature using forward_as_tuple

问题

In my app I would like to pass in a parameter pack over a legacy function signature, and change the values. Here is code that illustrates my question with my attempts as comments:

#include <tuple>
#include <cassert>

void LegacySignature( void* param );

template< typename... ArgsT >
// using ???; // attempt: can 'template alias' or 'using declaration' make the pack's type visible so I can use it inside the LegacyFunction?
void MyFunc( ArgsT&... args )
{
  auto userArgsTuple = std::forward_as_tuple< ArgsT&... >( args... );

  LegacySignature( &userArgsTuple );
}

void LegacySignature( void* param )
{
//  auto userArgsTuple = reinterpret_cast<???>( param ); // attempt: how can I get the parameter pack's type declared so I can use it here?

  // do something with the params like change num to 44 and tf to true;
  //userArgsTuple->num = 44; // desired functionality
  //userArgsTuple->tf = true; // desired functionality
}

int main()
{
  int num { 33 };
  bool tf { false };
  MyFunc( num, tf );
  assert( num == 44 && tf == true );

  return 0;
}

Is there a way to make the parameter pack a declarable lvalue?

回答1:

I'm assuming what you want is a function pointer to your legacy signature.

Here is a C++11 approach.

template<class Sig, class F>
struct magic_callback_t;

template<class R, class...Args, class F>
struct magic_callback_t<R(Args...), F> {
  F f;
  void* pvoid() const { return this; }
  using result_sig = R(*)(void*, Args...);
  result_sig pfunc() const {
    return [](void* pvoid, Args...args)->R{
      auto* self = static_cast<magic_callback_t*>(pvoid);
      return (self->f)(std::forward<Args>(args)...);
    };
  }
};
template<class Sig, class F>
magic_callback_t<Sig, F> magic_callback( F&& f ) {
  return {std::forward<F>(f)};
}

Now we just do this:

auto callback = magic_callback( [&](){
  // use whatever as if we where in the enclosing scope
});

void(*)(void*) legacy_ptr = callback.pfunc();
legacy_ptr( callback.pvoid() );

will call the lambda you passed to magic_callback.

If you want to store stuff as a tuple, you can. Just capture the tuple in the lambda, then use std::get to access it in the body of the lambda. Use mutable if you want it to be mutable.

回答2:

The code below fixes the sample code so that it answers the question as to how to pass a parameter pack over a legacy function signature using forward_as_tuple.

#include <tuple>
#include <cassert>
#include <memory>
#include <functional>

#define ARGSET int, bool

void LegacySignature( long* param ); // ie, LPARAM

template< typename... ArgsT >
struct MyParams
{
  MyParams( ArgsT... args ) : rvalRefs { std::forward_as_tuple( args... ) } {} // The resulting forward_as_tuple tuple has rvalue reference data members 
  std::tuple< ArgsT... > rvalRefs;
};


void LegacySignature( long* legSigParam )
{
  auto userArgsTuple( reinterpret_cast< MyParams< ARGSET >* >( legSigParam ) );

  // do something with the params like change num to 44 and tf to true;
  std::get< 0 >( userArgsTuple->rvalRefs ) = 44; // index types can probably be worked out using enums
  std::get< 1 >( userArgsTuple->rvalRefs ) = true;
}

int main()
{
  int num { 33 };
  bool tf { false };
  MyParams< ARGSET > myParams( num, tf );

  std::unique_ptr< MyParams< ARGSET > > legSigParamPtr = std::make_unique< MyParams< ARGSET > >( myParams );
  LegacySignature( ( long* )legSigParamPtr.get() );
  assert( std::get< 0 >( legSigParamPtr->rvalRefs ) == 44 && std::get< 1 >( legSigParamPtr->rvalRefs ) == true );

  return 0;
}

来源：https://stackoverflow.com/questions/41131574/passing-a-parameter-pack-over-a-legacy-function-signature-using-forward-as-tuple