I am trying to write a class that "manages" delegates in c++. I already have the delegate class implemented for me. I want this delegate manager class to have two functions:
One would take a pointer to instance of a delegate of a certain type with a given input-argument/return type, and cache it.
The other function would take a member function of the right type in order to bind the cached delegate instance to it.
Currently, I have:
template<typename... Args>
struct FunctionParamsPack { };
This is the container for the types of the parameters this function takes. ie for foo(int i, double d)
that would be int
and double
. I am following the advice from here .
then I have the DelegateInfoPack
class:
template<typename FuncRetType,typename... FuncParams>
struct DelegateInfoPack{
//for look-up by components in the program
typedef typename DelegateClass<FuncRetType, FuncParams...> _Delegate;
//for the delegate manager
typedef typename FuncRetType _FuncRetType;
typedef typename FunctionParamsPack<FuncParams...> _FuncParams;
};
This struct is included by that the components in the program and it typedefs three typenames, two of which are to be used in the DelegateManger class:
template<typename DelegateInfoPack>
class DelegateManager
{
typedef typename DelegateInfoPack::_Delegate _Delegate;
typedef typename DelegateInfoPack::_FuncRetType _FuncRetType;
typedef typename DelegateInfoPack::_FuncParams _FuncParams;
void CacheDelegate(_Delegate* del,...) {}
template<typename UserClass>
void BindDelegate(..., _FuncRetType(UserClass::*fp)( _FuncParams())) {} //Doesn't work!
}
My problem is with the BindDelegate()
function. I am not able to create the correct signature for the member function of the type with a given return type and input parameter types.
basically, I need to know the way to have the right function pointer type with a given return type and argument type so my BindDelegate takes it as an argument.
One approach is to use partial specialization:
template<typename> class DelegateManager;
template<typename FuncRetType,typename... FuncParams>
class DelegateManager<DelegateInfoPack<FuncRetType,FuncParams...>>
{
template<typename UserClass>
void BindDelegate(_FuncRetType(UserClass::*fp)(FuncParams...))
{
}
};
Another approach is to have a class which generates the appropriate function type
template <typename FuncRetType,typename FuncParams>
struct FunctionPointer;
template <typename FuncRetType,typename...ARGS>
struct FunctionPointer<FuncRetType,FunctionParamsPack<ARGS...>> {
typedef FuncRetType (Type)(ARGS...);
};
Then use that in your BindDelegate member function:
template<typename UserClass>
void
BindDelegate(
typename FunctionPointer<_FuncRetType,_FuncParams>::Type UserClass::*fp
)
{ ... }
Or maybe even put this into your DelegateInfoPack class:
template<typename FuncRetType,typename... FuncParams>
struct DelegateInfoPack {
.
.
.
typedef FuncRetType (_FuncType)(FuncParams...);
};
and use that in your DelegateManager
template<typename DelegateInfoPack>
struct DelegateManager
{
.
.
.
typedef typename DelegateInfoPack::_FuncType _FuncType;
template<typename UserClass>
void BindDelegate(_FuncType UserClass::*fp)
{
}
};
As an additional way to solve your task - C++11 introduces new language features that can make your code more flexible with using standard elements
#include <iostream>
#include <functional>
#include <tuple>
#include <iostream>
using std::cout;
using std::endl;
using namespace std::placeholders;
// helpers for tuple unrolling
template<int ...> struct seq {};
template<int N, int ...S> struct gens : gens<N-1, N-1, S...> {};
template<int ...S> struct gens<0, S...>{ typedef seq<S...> type; };
// simple function
double foo_fn(int x, float y, double z)
{
return x + y + z;
}
// structure with memner function to call
struct foo_struct
{
// member function to be used as a delegate
double foo_fn(int x, float y, double z)
{
return x + y + z;
}
// this member function has different signature - but it can be used too
// please note that argument order is changed too
double foo_fn_4(int x, double z, float y, long xx)
{
return x + y + z + xx;
}
};
// delegate class that holds as delegate as its params for future call
template <typename Ret, typename ...Args>
struct delayed_call
{
// tuple can be used as FunctionParamsPack type
typedef std::tuple<Args...> params_type;
// std::function as delegate type
typedef std::function<Ret(Args...)> function_type;
// stored parameters
params_type params;
// stored delegate
function_type func;
// invocation
Ret operator()()
{
return callFunc(typename gens<sizeof...(Args)>::type());
}
// direct invocation
Ret operator()(Args... args)
{
return func(args...);
}
// internal invocation with tuple unrolling
template<int ...S>
double callFunc(seq<S...>)
{
return func(std::get<S>(params) ...);
}
};
int main(void)
{
// arguments
std::tuple<int, float, double> t = std::make_tuple(1, 5, 10);
// var #1 - you can use simple function as delegate
delayed_call<double, int,float, double> saved_foo_fn{t, foo_fn};
foo_struct fs;
// var #2 - you can use member function as delegate
delayed_call<double, int,float, double> saved_foo_fn_struct{t, std::bind(&foo_struct::foo_fn, fs, _1, _2, _3)};
// var #3 - you can use member function with different signature as delegate.
// bind 0 to xx and change argument order
delayed_call<double, int,float, double> saved_foo_fn_struct_4{t, std::bind(&foo_struct::foo_fn_4, fs, _1, _3, _2, 0l)};
// var #4 - you can use lambda function as delegate
delayed_call<double, int,float, double> saved_lambda{t, [](int x, float y, double z)
{
return x + y + z;
}
};
cout << "saved_foo_fn: " << saved_foo_fn() << endl;
cout << "saved_foo_fn_struct: " << saved_foo_fn_struct() << endl;
cout << "saved_foo_fn_struct_4: " << saved_foo_fn_struct_4() << endl;
cout << "saved_lambda: " << saved_lambda() << endl;
cout << "direct call with (1,2,3) to a member: " << saved_foo_fn_struct(1, 2, 3) << endl;
}
Output:
saved_foo_fn: 16
saved_foo_fn_struct: 16
saved_foo_fn_struct_4: 16
saved_lambda: 16
direct call with (1,2,3) to a member: 6
So you are not limited with only member functions, but also can use any callable type with even different signature
If placeholders::_1... look ugly for you - there is a solution
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