c – 具有完全可维护性的多个调度解决方案
作者：互联网
有人能想出一个很好的方法来实现多个调度,如下面的Object :: foo重载？
class A {
public:
virtual void accept (Visitor&) = 0;
};
class B : public A {
virtual void accept (Visitor&) override;
};
class C : public A {
virtual void accept (Visitor&) override;
};
class D : public A {
virtual void accept (Visitor&) override;
};
class Object {
public:
virtual double foo (A*, A*) { std::cout << "Object::foo A,A\n"; return 3.14; }
virtual double foo (B*, B*) { std::cout << "Object::foo B,B\n"; return 3.14; }
virtual double foo (B*, C*) { std::cout << "Object::foo B,C\n"; return 3.14; }
virtual double foo (C*, B*) { std::cout << "Object::foo C,B\n"; return 3.14; }
virtual double foo (C*, C*) { std::cout << "Object::foo C,C\n"; return 3.14; }
virtual char foo (A*, A*, A*) const { std::cout << "Object::foo A,A,A\n"; return '&'; }
virtual char foo (C*, B*, D*) const { std::cout << "Object::foo C,B,D\n"; return '!'; } // Overload of foo with three arguments.
virtual void bar (A*, A*, A*) const { std::cout << "Object::bar A,A,A\n"; }
virtual void bar (B*, B*, B*) const { std::cout << "Object::bar B,B,B\n"; }
virtual void bar (B*, C*, B*) const { std::cout << "Object::bar B,C,B\n"; }
virtual void bar (B*, C*, C*) const { std::cout << "Object::bar B,C,C\n"; }
virtual void bar (B*, C*, D*) const { std::cout << "Object::bar B,C,D\n"; }
virtual void bar (C*, B*, D*) const { std::cout << "Object::bar C,B,D\n"; }
virtual void bar (C*, C*, C*) const { std::cout << "Object::bar C,C,C\n"; }
virtual void bar (D*, B*, C*) const { std::cout << "Object::bar D,B,C\n"; }
double fooMultipleDispatch (A*, A*);
char fooMultipleDispatch (A*, A*, A*);
void barMultipleDispatch (A*, A*, A*);
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
double multipleDispatch (const ObjectFooVisitor<2>& visitor, const Z1<Is...>&, const Z2<Js...>&) {return foo (visitor.getArray<Is>()[Js]...);}
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
char multipleDispatch (const ObjectFooVisitor<3>& visitor, const Z1<Is...>&, const Z2<Js...>&) const {return foo (visitor.getArray<Is>()[Js]...);}
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
void multipleDispatch (const ObjectBarVisitor& visitor, const Z1<Is...>&, const Z2<Js...>&) {bar (visitor.getArray<Is>()[Js]...);}
};
我正在努力执行多次调度的客户端代码如下所示：
int main() {
A* a[] = {new B, new C, new D};
Object* object = new Object;
double d = object->foo (a[0], a[1]); // Object::foo A,A (no multiple dispatch)
d = object->fooMultipleDispatch (a[0], a[1]); // Object::foo B,C
std::cout << "d = " << d << std::endl; // 3.12
const char k = object->fooMultipleDispatch (a[1], a[0], a[2]); // Object::foo C,B,D
std::cout << "k = " << k << std::endl; // !
object->bar (a[1], a[0], a[2]); // Object::bar A,A,A (no multiple dispatch)
object->barMultipleDispatch (a[1], a[0], a[2]); // Object::bar C,B,D
Thing* thing = new Thing;
int num = thing->baz (a[1], a[0], a[2]); // Thing::baz A,A,A (no multiple dispatch)
num = thing->bazMultipleDispatch (a[1], a[0], a[2]); // Thing::baz C,B,D
std::cout << "num = " << num << std::endl; // 5
}
您可以通过我的设计推断出我的解决方案在维护类别中失败了.每次要重载一个带有重载的新函数时,需要编写一个新的相应的Visitor类等(例如在本例中为ObjectFooVisitor,函数Object :: fooMultipleDispatch等).理想的设计不应该需要这种类型的维护工作.
一个示例访问者函数我只需要多个调度Object :: foo,如下所示：
template<>
class ObjectFooVisitor<2> : public Visitor { // For Object::foo overrides with two arguments.
private:
std::tuple<std::array<B*, 2>, std::array<C*, 2>> tupleOfArrays;
std::array<int, 2> tupleIndices;
// ....
};
double Object::fooMultipleDispatch (A* a1, A* a2) {
ObjectFooVisitor<2> visitor;
a1->accept(visitor); // Stores the dynamic type of a1
a2->accept(visitor); // and a2 into ObjectFooVisitor<2>'s array data members.
return MultipleDispatcher<Object, ObjectFooVisitor<2>, double, 2, 0, index_sequence<0>>(this, visitor).execute(); // 2 because there are two arguments in the Object::foo overloads.
}
因此,我所遵循的主要思想是存储动态类型指针数组的元组,然后使用此存储调用适当的重载.但必须有其他设计才能让这项工作更好.
如果你想看到它,这是我的完整解决方案(编译GCC 4.9.2,需要SFINAE支持).随意尝试改进它以使其更易于维护,但我确信需要新的设计.
#include <iostream>
#include <array>
#include <tuple>
#include <type_traits>
class Object; class B; class C; class D;
class Visitor {
public:
virtual void visit (B*) = 0;
virtual void visit (C*) = 0;
virtual void visit (D*) = 0;
};
class A {
public:
virtual void accept (Visitor&) = 0;
};
class B : public A {
virtual void accept (Visitor&) override;
};
class C : public A {
virtual void accept (Visitor&) override;
};
class D : public A {
virtual void accept (Visitor&) override;
};
template <int, int> struct ArrayType; // Extra template parameter N needed here to allow std::array of any size.
template <int N> struct ArrayType<N,0> { using type = std::array<B*, N>; };
template <int N> struct ArrayType<N,1> { using type = std::array<C*, N>; };
template <int N> struct ArrayType<N,2> { using type = std::array<D*, N>; };
template <int> class ObjectFooVisitor;
template<>
class ObjectFooVisitor<2> : public Visitor { // For Object::foo overrides with two arguments.
private:
std::tuple<std::array<B*, 2>, std::array<C*, 2>> tupleOfArrays;
std::array<int, 2> tupleIndices;
int numAccepted = 0;
protected:
virtual void visit (B* b) override {std::get<0>(tupleOfArrays)[numAccepted] = b; tupleIndices[numAccepted++] = 0;}
virtual void visit (C* c) override {std::get<1>(tupleOfArrays)[numAccepted] = c; tupleIndices[numAccepted++] = 1;}
virtual void visit (D*) override {}
public:
ObjectFooVisitor() { std::get<0>(tupleOfArrays) = {{nullptr, nullptr}}; std::get<1>(tupleOfArrays) = {{nullptr, nullptr}}; }
template <int N> const typename ArrayType<2,N>::type& getArray() const {return std::get<N>(tupleOfArrays);}
const std::array<int, 2>& getTupleIndices() const {return tupleIndices;}
};
template<>
class ObjectFooVisitor<3> : public Visitor { // For Object::foo overrides with three arguments.
private:
std::tuple<std::array<B*, 3>, std::array<C*, 3>, std::array<D*, 3>> tupleOfArrays;
std::array<int, 3> tupleIndices;
int numAccepted = 0;
protected:
virtual void visit (B* b) override {std::get<0>(tupleOfArrays)[numAccepted] = b; tupleIndices[numAccepted++] = 0;}
virtual void visit (C* c) override {std::get<1>(tupleOfArrays)[numAccepted] = c; tupleIndices[numAccepted++] = 1;}
virtual void visit (D* d) override {std::get<2>(tupleOfArrays)[numAccepted] = d; tupleIndices[numAccepted++] = 2;}
public:
ObjectFooVisitor() { std::get<0>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; std::get<1>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; std::get<2>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; }
template <int N> const typename ArrayType<3,N>::type& getArray() const {return std::get<N>(tupleOfArrays);}
const std::array<int, 3>& getTupleIndices() const {return tupleIndices;}
};
class ObjectBarVisitor : public Visitor {
private:
std::tuple<std::array<B*, 3>, std::array<C*, 3>, std::array<D*, 3>> tupleOfArrays;
std::array<int, 3> tupleIndices;
int numAccepted = 0;
protected:
virtual void visit (B* b) override {std::get<0>(tupleOfArrays)[numAccepted] = b; tupleIndices[numAccepted++] = 0;}
virtual void visit (C* c) override {std::get<1>(tupleOfArrays)[numAccepted] = c; tupleIndices[numAccepted++] = 1;}
virtual void visit (D* d) override {std::get<2>(tupleOfArrays)[numAccepted] = d; tupleIndices[numAccepted++] = 2;}
public:
ObjectBarVisitor() { std::get<0>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; std::get<1>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; std::get<2>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; }
template <int N> const typename ArrayType<3,N>::type& getArray() const {return std::get<N>(tupleOfArrays);}
const std::array<int, 3>& getTupleIndices() const {return tupleIndices;}
};
class ThingBazVisitor : public Visitor {
private:
std::tuple<std::array<B*, 3>, std::array<C*, 3>, std::array<D*, 3>> tupleOfArrays;
std::array<int, 3> tupleIndices;
int numAccepted = 0;
protected:
virtual void visit (B* b) override {std::get<0>(tupleOfArrays)[numAccepted] = b; tupleIndices[numAccepted++] = 0;}
virtual void visit (C* c) override {std::get<1>(tupleOfArrays)[numAccepted] = c; tupleIndices[numAccepted++] = 1;}
virtual void visit (D* d) override {std::get<2>(tupleOfArrays)[numAccepted] = d; tupleIndices[numAccepted++] = 2;}
public:
ThingBazVisitor() { std::get<0>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; std::get<1>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; std::get<2>(tupleOfArrays) = {{nullptr, nullptr, nullptr}}; }
template <int N> const typename ArrayType<3,N>::type& getArray() const {return std::get<N>(tupleOfArrays);}
const std::array<int, 3>& getTupleIndices() const {return tupleIndices;}
};
void B::accept (Visitor& visitor) {visitor.visit(this);}
void C::accept (Visitor& visitor) {visitor.visit(this);}
void D::accept (Visitor& visitor) {visitor.visit(this);}
class Object {
public:
virtual double foo (A*, A*) { std::cout << "Object::foo A,A\n"; return 3.14; }
virtual double foo (B*, B*) { std::cout << "Object::foo B,B\n"; return 3.14; }
virtual double foo (B*, C*) { std::cout << "Object::foo B,C\n"; return 3.14; }
virtual double foo (C*, B*) { std::cout << "Object::foo C,B\n"; return 3.14; }
virtual double foo (C*, C*) { std::cout << "Object::foo C,C\n"; return 3.14; }
virtual char foo (A*, A*, A*) const { std::cout << "Object::foo A,A,A\n"; return '&'; } // This is needed for the foo overload to be multiple dispatched, even if it is never used, otherwise the other possible foo overloads with three arguments will have no place to go to.
virtual char foo (C*, B*, D*) const { std::cout << "Object::foo C,B,D\n"; return '!'; } // Overload of foo with three arguments. Furthermore, the function itself is const and returns char instead of double. Simply define char fooMultipleDispatch (A*, A*, A*), ObjectFooVisitor<3> (the old ObjectFooVisitor now renamed to ObjectFooVisitor<2>) and overload multipleDispatch with char multipleDispatch (const ObjectFooVisitor<3>& visitor, const Z1<Is...>&, const Z2<Js...>&).
virtual void bar (A*, A*, A*) const { std::cout << "Object::bar A,A,A\n"; }
virtual void bar (B*, B*, B*) const { std::cout << "Object::bar B,B,B\n"; }
virtual void bar (B*, C*, B*) const { std::cout << "Object::bar B,C,B\n"; }
virtual void bar (B*, C*, C*) const { std::cout << "Object::bar B,C,C\n"; }
virtual void bar (B*, C*, D*) const { std::cout << "Object::bar B,C,D\n"; }
virtual void bar (C*, B*, D*) const { std::cout << "Object::bar C,B,D\n"; }
virtual void bar (C*, C*, C*) const { std::cout << "Object::bar C,C,C\n"; }
virtual void bar (D*, B*, C*) const { std::cout << "Object::bar D,B,C\n"; }
double fooMultipleDispatch (A*, A*);
char fooMultipleDispatch (A*, A*, A*);
void barMultipleDispatch (A*, A*, A*);
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
double multipleDispatch (const ObjectFooVisitor<2>& visitor, const Z1<Is...>&, const Z2<Js...>&) {return foo (visitor.getArray<Is>()[Js]...);}
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
char multipleDispatch (const ObjectFooVisitor<3>& visitor, const Z1<Is...>&, const Z2<Js...>&) const {return foo (visitor.getArray<Is>()[Js]...);}
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
void multipleDispatch (const ObjectBarVisitor& visitor, const Z1<Is...>&, const Z2<Js...>&) {bar (visitor.getArray<Is>()[Js]...);}
};
class Thing {
public:
virtual int baz (A*, A*, A*) { std::cout << "Thing::baz A,A,A\n"; return 5; }
virtual int baz (B*, B*, B*) { std::cout << "Thing::baz B,B,B\n"; return 5; }
virtual int baz (B*, C*, B*) { std::cout << "Thing::baz B,C,B\n"; return 5; }
virtual int baz (B*, C*, C*) { std::cout << "Thing::baz B,C,C\n"; return 5; }
virtual int baz (B*, C*, D*) { std::cout << "Thing::baz B,C,D\n"; return 5; }
virtual int baz (C*, B*, D*) { std::cout << "Thing::baz C,B,D\n"; return 5; }
virtual int baz (C*, C*, C*) { std::cout << "Thing::baz C,C,C\n"; return 5; }
virtual int baz (D*, B*, C*) { std::cout << "Thing::baz D,B,C\n"; return 5; }
int bazMultipleDispatch (A*, A*, A*);
template <template <int...> class Z1, template <int...> class Z2, int... Is, int... Js>
int multipleDispatch (const ThingBazVisitor& visitor, const Z1<Is...>&, const Z2<Js...>&) {return baz (visitor.getArray<Is>()[Js]...);} // Since Thing only has baz interested in multiple dispatching, it does not need its own MultipleDispatch inner class like Object does (but if other Thing methods want multiple dispatching, then as in the Object::multipleDispatch overloads).
};
template <typename, typename, typename, int, int, typename, typename = void, int = 0> struct MultipleDispatcher;
template <typename T, typename V, typename R, int Num, int N, template <int...> class Z, int... Is, int I>
struct MultipleDispatcher<T, V, R, Num, N, Z<I, Is...>, typename std::enable_if<N != Num-1>::type> : MultipleDispatcher<T, V, R, Num, N, Z<I+1, Is...>, typename std::enable_if<N != Num-1>::type> {
T* t;
const V& visitor;
MultipleDispatcher (T* o, const V& v) : MultipleDispatcher<T, V, R, Num, N, Z<I+1, Is...>, typename std::enable_if<N != Num-1>::type>(o,v), t(o), visitor(v) {}
R execute();
};
template <typename T, typename V, typename R, int Num, int Last, template <int...> class Z, int... Is, int I>
struct MultipleDispatcher<T, V, R, Num, Last, Z<I, Is...>, typename std::enable_if<Last == Num-1>::type> : MultipleDispatcher<T, V, R, Num, Last, Z<I+1, Is...>, typename std::enable_if<Last == Num-1>::type> {
T* t;
const V& visitor;
MultipleDispatcher (T* o, const V& v) : MultipleDispatcher<T, V, R, Num, Last, Z<I+1, Is...>, typename std::enable_if<Last == Num-1>::type>(o,v), t(o), visitor(v) {}
R execute();
};
template <typename T, typename V, typename R, int Num, int N, template <int...> class Z, int... Is>
struct MultipleDispatcher<T, V, R, Num, N, Z<Num, Is...>, typename std::enable_if<N != Num-1>::type> {
T* t;
const V& visitor;
MultipleDispatcher (T* o, const V& v) : t(o), visitor(v) {}
R execute() {return R();} // End of recursion
};
template <typename T, typename V, typename R, int Num, int Last, template <int...> class Z, int... Is>
struct MultipleDispatcher<T, V, R, Num, Last, Z<Num, Is...>, typename std::enable_if<Last == Num-1>::type> { // This unique specialization is needed to avoid compiling ambiguity.
T* t;
const V& visitor;
MultipleDispatcher (T* o, const V& v) : t(o), visitor(v) {}
R execute() {return R();} // End of recursion
};
template <typename T, typename V, typename R, int Num, int N, template <int...> class Z, int... Is, int I>
R MultipleDispatcher<T, V, R, Num, N, Z<I, Is...>, typename std::enable_if<N != Num-1>::type>::execute() {
if (I == visitor.getTupleIndices()[N])
return MultipleDispatcher<T, V, R, Num, N+1, Z<0, I, Is...>, void>(t, visitor).execute(); // Do we need to specify the std::enable_if part here? Apparently not. We will allow N+1 to be anything, and there is apparently no ambiguity.
else
return MultipleDispatcher<T, V, R, Num, N, Z<I+1, Is...>, typename std::enable_if<N != Num-1>::type>::execute();
}
template <int...> struct index_sequence {};
template <int N, int... Is>
struct make_index_sequence_helper : make_index_sequence_helper<N-1, N-1, Is...> {};
template <int... Is>
struct make_index_sequence_helper<0, Is...> {
using type = index_sequence<Is...>;
};
template <int N>
using make_index_sequence = typename make_index_sequence_helper<N>::type;
template <typename, typename> struct ReverseHelper;
template <template <int...> class Z, typename Pack>
struct ReverseHelper<Z<>, Pack> {
using type = Pack;
};
template <template <int...> class Z, int First, int... Rest, int... Is>
struct ReverseHelper<Z<First, Rest...>, Z<Is...>> : ReverseHelper<Z<Rest...>, Z<First, Is...>> {};
template <typename> struct Reverse;
template <template <int...> class Z, int... Is>
struct Reverse<Z<Is...>> : ReverseHelper<Z<Is...>, Z<>> {};
template <typename T, typename V, typename R, int Num, int Last, template <int...> class Z, int... Is, int I>
R MultipleDispatcher<T, V, R, Num, Last, Z<I, Is...>, typename std::enable_if<Last == Num-1>::type>::execute() {
if (I == visitor.getTupleIndices()[Last])
return t->template multipleDispatch (visitor, typename Reverse<Z<I, Is...>>::type{}, make_index_sequence<Num>{}); // This compiles on GCC 4.9.2 but not on GCC 4.8.1. Template disambiguator needed.
else
return MultipleDispatcher<T, V, R, Num, Last, Z<I+1, Is...>, typename std::enable_if<Last == Num-1>::type>::execute();
}
double Object::fooMultipleDispatch (A* a1, A* a2) {
ObjectFooVisitor<2> visitor;
a1->accept(visitor); // Stores the dynamic type of a1
a2->accept(visitor); // and a2 into ObjectFooVisitor<2>'s array data members.
return MultipleDispatcher<Object, ObjectFooVisitor<2>, double, 2, 0, index_sequence<0>>(this, visitor).execute(); // 2 because there are two arguments in the Object::foo overloads.
}
char Object::fooMultipleDispatch (A* a1, A* a2, A* a3) {
ObjectFooVisitor<3> visitor;
a1->accept(visitor);
a2->accept(visitor);
a3->accept(visitor);
return MultipleDispatcher<Object, ObjectFooVisitor<3>, char, 3, 0, index_sequence<0>>(this, visitor).execute(); // 3 because there are three arguments in this particular Object::foo overload.
}
void Object::barMultipleDispatch (A* a1, A* a2, A* a3) {
ObjectBarVisitor visitor;
a1->accept(visitor);
a2->accept(visitor);
a3->accept(visitor);
MultipleDispatcher<Object, ObjectBarVisitor, void, 3, 0, index_sequence<0>>(this, visitor).execute(); // 3 because there are two arguments in the Object::foo overloads.
}
int Thing::bazMultipleDispatch (A* a1, A* a2, A* a3) {
ThingBazVisitor visitor;
a1->accept(visitor);
a2->accept(visitor);
a3->accept(visitor);
return MultipleDispatcher<Thing, ThingBazVisitor, int, 3, 0, index_sequence<0>>(this, visitor).execute(); // 3 because there are three arguments in the Thing::baz overloads.
}
// Test
int main() {
A* a[] = {new B, new C, new D};
Object* object = new Object;
double d = object->foo (a[0], a[1]); // Object::foo A,A (no multiple dispatch)
d = object->fooMultipleDispatch (a[0], a[1]); // Object::foo B,C
std::cout << "d = " << d << std::endl; // 3.12
const char k = object->fooMultipleDispatch (a[1], a[0], a[2]); // Object::foo C,B,D
std::cout << "k = " << k << std::endl; // !
object->bar (a[1], a[0], a[2]); // Object::bar A,A,A (no multiple dispatch)
object->barMultipleDispatch (a[1], a[0], a[2]); // Object::bar C,B,D
Thing* thing = new Thing;
int num = thing->baz (a[1], a[0], a[2]); // Thing::baz A,A,A (no multiple dispatch)
num = thing->bazMultipleDispatch (a[1], a[0], a[2]); // Thing::baz C,B,D
std::cout << "num = " << num << std::endl; // 5
}
解决方法:
我为多次发送所做的工作(将我的评论转化为答案)：
// Generic IVisitor
// Do: using MyIVisitor = IVisitorTs<Child1, Child2, ...>
template <typename ... Ts> class IVisitorTs;
template <typename T, typename ... Ts>
class IVisitorTs<T, Ts...> : public IVisitorTs<Ts...>
{
public:
using tuple_type = std::tuple<T, Ts...>;
using IVisitorTs<Ts...>::visit;
virtual ~IVisitorTs() = default;
virtual void visit(const T& t) = 0;
};
template <typename T> class IVisitorTs<T>
{
public:
using tuple_type = std::tuple<T>;
virtual ~IVisitorTs() = default;
virtual void visit(const T& t) = 0;
};
namespace detail {
// retrieve the index of T in Ts...
template <typename T, typename ... Ts> struct get_index;
template <typename T, typename ... Ts>
struct get_index<T, T, Ts...> : std::integral_constant<std::size_t, 0> {};
template <typename T, typename Tail, typename ... Ts>
struct get_index<T, Tail, Ts...> :
std::integral_constant < std::size_t, 1 + get_index<T, Ts...>::value > {};
// retrieve the index of T in Tuple<Ts...>
template <typename T, typename Tuple> struct get_index_in_tuple;
template <typename T, template <typename...> class C, typename ... Ts>
struct get_index_in_tuple<T, C<Ts...>> : get_index<T, Ts...> {};
// get element of a multiarray
template <std::size_t I>
struct multi_array_getter
{
template <typename T, std::size_t N>
static constexpr auto get(const T& a, const std::array<std::size_t, N>& index)
-> decltype(multi_array_getter<I - 1>::get(a[index[N - I]], index))
{
return multi_array_getter<I - 1>::get(a[index[N - I]], index);
}
};
template <>
struct multi_array_getter<0>
{
template <typename T, std::size_t N>
static constexpr auto get(const T& a, const std::array<std::size_t, N>& index)
-> decltype(a)
{
return a;
}
};
// Provide an implementation of visitor
// by forwarding to C implementation (which may be non virtual)
template <typename IVisitor, typename C, typename...Ts> struct IVisitorImpl;
template <typename IVisitor, typename C, typename T, typename...Ts>
struct IVisitorImpl<IVisitor, C, T, Ts...> : IVisitorImpl<IVisitor, C, Ts...>
{
virtual void visit(const T& t) override { C::visit(t); }
};
template <typename IVisitor, typename C, typename T>
struct IVisitorImpl<IVisitor, C, T> : IVisitor, C
{
virtual void visit(const T& t) override { C::visit(t); }
};
// helper to expand child type to IVisitorImpl
template <typename IVisitor, typename C>
struct IVisitorImplType;
template <typename ... Ts, typename C>
struct IVisitorImplType<IVisitorTs<Ts...>, C>
{
using type = IVisitorImpl<IVisitorTs<Ts...>, C, Ts...>;
};
// Create an multi array of pointer of function
// (with all combinaisons of overload).
template <typename Ret, typename F, typename Arg>
class GetAllOverload
{
private:
template <typename...Ts>
struct Functor
{
// function which will be in array.
static Ret call(F&f, const Arg& arg)
{
return call_helper(f, arg, make_index_sequence<sizeof...(Ts)>());
}
private:
// The final dispatched function
template <std::size_t ... Is>
static Ret call_helper(F&f, const Arg& arg, index_sequence<Is...>)
{
using RetTuple = std::tuple<Ts&...>;
// static cast is suffisant if arg is the abstract type
// when given arg is concrete type, reinterpret_cast is required.
// TODO: build a smaller table with only possible value to avoid that
return f(reinterpret_cast<typename std::tuple_element<Is, RetTuple>::type>(std::get<Is>(arg))...);
}
};
// helper class to create the multi array of function pointer
template <std::size_t N, typename Tuple, typename...Ts>
struct Builder;
template <typename...Ts, typename...Ts2>
struct Builder<1, std::tuple<Ts...>, Ts2...>
{
using RetType = std::array<Ret (*)(F&, const Arg&), sizeof...(Ts)>;
static constexpr RetType build()
{
return RetType{ &Functor<Ts2..., Ts>::call... };
}
};
template <std::size_t N, typename ...Ts, typename...Ts2>
struct Builder<N, std::tuple<Ts...>, Ts2...>
{
template <typename T>
using RecType = Builder<N - 1, std::tuple<Ts...>, Ts2..., T>;
using T0 = typename std::tuple_element<0, std::tuple<Ts...>>::type;
using RetType = std::array<decltype(RecType<T0>::build()), sizeof...(Ts)>;
static constexpr RetType build() {
return RetType{ RecType<Ts>::build()... };
}
};
public:
template <std::size_t N, typename VisitorTuple>
static constexpr auto get()
-> decltype(Builder<N, VisitorTuple>::build())
{
return Builder<N, VisitorTuple>::build();
}
};
template <typename Ret, typename IVisitor, typename F, std::size_t N>
class dispatcher
{
private:
std::array<std::size_t, N> index;
struct visitorCallImpl
{
template <typename T>
void visit(const T&) const
{
*index = get_index_in_tuple<T, IVisitor>::value;
}
void setIndexPtr(std::size_t& index) { this->index = &index; }
private:
std::size_t* index = nullptr;
};
template <std::size_t I, typename Tuple>
void set_index(const Tuple&t)
{
using VisitorType = typename IVisitorImplType<IVisitor, visitorCallImpl>::type;
VisitorType visitor;
visitor.setIndexPtr(index[I]);
std::get<I>(t).accept(visitor);
}
public:
template <typename Tuple, std::size_t ... Is>
Ret operator () (F&& f, const Tuple&t, index_sequence<Is...>)
{
const int dummy[] = {(set_index<Is>(t), 0)...};
static_cast<void>(dummy); // silent the warning unused varaible
constexpr auto a = GetAllOverload<Ret, F&&, Tuple>::
template get<sizeof...(Is), typename IVisitor::tuple_type>();
auto func = multi_array_getter<N>::get(a, index);
return (*func)(f, t);
}
};
} // namespace detail
template <typename Ret, typename Visitor, typename F, typename ... Ts>
Ret dispatch(F&& f, Ts&...args)
{
constexpr std::size_t size = sizeof...(Ts);
detail::dispatcher<Ret, Visitor, F&&, size> d;
return d(std::forward<F>(f), std::tie(args...), make_index_sequence<size>());
}
用法示例
struct A;
struct B;
struct C;
struct D;
using IAVisitor = IVisitorTs<A, B, C, D>;
struct A {
virtual ~A() = default;
virtual void accept(IAVisitor& v) const { v.visit(*this); }
};
struct B : A {
virtual void accept(IAVisitor& v) const override { v.visit(*this); }
};
struct C : A {
virtual void accept(IAVisitor& v) const override { v.visit(*this); }
};
struct D : A {
virtual void accept(IAVisitor& v) const override { v.visit(*this); }
};
class Object {
public:
virtual double foo (A*, A*) { std::cout << "Object::foo A,A\n"; return 3.14; }
virtual double foo (B*, B*) { std::cout << "Object::foo B,B\n"; return 3.14; }
virtual double foo (B*, C*) { std::cout << "Object::foo B,C\n"; return 3.14; }
virtual double foo (C*, B*) { std::cout << "Object::foo C,B\n"; return 3.14; }
virtual double foo (C*, C*) { std::cout << "Object::foo C,C\n"; return 3.14; }
virtual char foo (A*, A*, A*) const { std::cout << "Object::foo A,A,A\n"; return '&'; }
virtual char foo (C*, B*, D*) const { std::cout << "Object::foo C,B,D\n"; return '!'; } // Overload of foo with three arguments.
virtual void bar (A*, A*, A*) const { std::cout << "Object::bar A,A,A\n"; }
virtual void bar (B*, B*, B*) const { std::cout << "Object::bar B,B,B\n"; }
virtual void bar (B*, C*, B*) const { std::cout << "Object::bar B,C,B\n"; }
virtual void bar (B*, C*, C*) const { std::cout << "Object::bar B,C,C\n"; }
virtual void bar (B*, C*, D*) const { std::cout << "Object::bar B,C,D\n"; }
virtual void bar (C*, B*, D*) const { std::cout << "Object::bar C,B,D\n"; }
virtual void bar (C*, C*, C*) const { std::cout << "Object::bar C,C,C\n"; }
virtual void bar (D*, B*, C*) const { std::cout << "Object::bar D,B,C\n"; }
double fooMultipleDispatch (A*, A*);
char fooMultipleDispatch (A*, A*, A*);
};
class FooDispatcher
{
public:
explicit FooDispatcher(Object& object) : object(object) {}
template <typename T1, typename T2>
double operator() (T1& a1, T2& a2) const
{
return object.foo(&a1, &a2);
}
template <typename T1, typename T2, typename T3>
char operator() (T1& a1, T2& a2, T3& a3) const
{
return object.foo(&a1, &a2, &a3);
}
private:
Object& object;
};
double Object::fooMultipleDispatch (A* a1, A* a2)
{
return dispatch<double, IAVisitor>(FooDispatcher(*this), *a1, *a2);
}
char Object::fooMultipleDispatch (A* a1, A* a2, A* a3)
{
return dispatch<char, IAVisitor>(FooDispatcher(*this), *a1, *a2, *a3);
}
int main() {
A a_a;
B a_b;
C a_c;
D a_d;
A* a[] = {&a_b, &a_c, &a_d, &a_a};
Object object;
double d = object.foo (a[0], a[1]); // Object::foo A,A (no multiple dispatch)
d = object.fooMultipleDispatch (a[0], a[1]); // Object::foo B,C
std::cout << "d = " << d << std::endl; // 3.14
object.fooMultipleDispatch (a[0], a[3]); // B,A -> so best match is Object::foo A,A
const char k = object.fooMultipleDispatch (a[1], a[0], a[2]); // Object::foo C,B,D
std::cout << "k = " << k << std::endl; // !
}
标签：multiple-dispatch,c,virtual 来源： https://codeday.me/bug/20190929/1831016.html