Compare std::function with callback lambda - c++

Question

As continue of thread Compare of std::function with lambda
I have problem with capture this to lambda - and compare after that with "correct callback".
#include <iostream>
#include <functional>
using namespace std;
//////////////////////////////////////////////////////////////////////////
class IFoo
{
public:
virtual void onAppStarted( int, int ) = 0;
};
//////////////////////////////////////////////////////////////////////////
template <typename T>
struct FunctionTypeDeduction;
template<class ClassType, class ReturnType, class... Args>
struct FunctionTypeDeduction< ReturnType( ClassType::* )( Args... ) >
{
using type = ReturnType( *)( Args... );
using functorType = std::function<ReturnType( Args... ) >;
};
//////////////////////////////////////////////////////////////////////////
class SubcribeSystem
{
public:
using tSomeMethodType = FunctionTypeDeduction< decltype( &IFoo::onAppStarted ) >::type;
using tSomeMethodCallback = FunctionTypeDeduction< decltype( &IFoo::onAppStarted ) >::functorType;
public:
void subribeOnSomeMethod( const tSomeMethodCallback& arr )
{
arr( 3, 19 );
}
};
#define SUBCRIBE_FOR_SOMEMETHOD( eventsObj, lambda ) \
do \
{ \
SubcribeSystem::tSomeMethodType const func_ptr = lambda; \
eventsObj.subribeOnSomeMethod( lambda ); \
} \
while(false);
//////////////////////////////////////////////////////////////////////////
class Bar
{
public:
void fn( SubcribeSystem& events )
{
auto onAppStart = []( int width, int height )
{
cout << width << " " << height << endl;
};
auto onAppStart2 = []( bool width, int height )
{
cout << width << " " << height << endl;
};
auto onAppStart3 = [this]( int width, int height )
{
cout << width << " " << height << endl;
someOtherMethod( width );
};
SUBCRIBE_FOR_SOMEMETHOD( events, onAppStart ); // expect all ok
//SUBCRIBE_FOR_SOMEMETHOD( events, onAppStart2 ); // expect error cause bool first param
SUBCRIBE_FOR_SOMEMETHOD( events, onAppStart3 ); // expect all ok, but not !!!
}
void someOtherMethod( int x )
{
cout << "processed callback " << x << endl;
}
};
int main()
{
Bar bar;
SubcribeSystem sub;
bar.fn( sub );
}

Answer 1

In your macro you have
SubcribeSystem::tSomeMethodType const func_ptr = lambda;
Where SubcribeSystem::tSomeMethodType is a
FunctionTypeDeduction< decltype( &IFoo::onAppStarted ) >::type
which is a function pointer. So you are trying to convert the lambda into a function pointer. Unfortunately a lambda that captures, which onAppStart3 does, cannot be converted to a function pointer.
The first two lambdas work as they do not capture so they have an implicit function pointer conversion operator.