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?
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;
}