这有点假设,因为我不太担心性能 - 只是想知道哪个选项实际上是最快/最有效的,或者是否没有任何区别。
假设我有以下代码用于支持重载的访问者模板:
#define IMPLEMENT_VISITOR_WITH_SUPERCLASS(superclass) \
typedef superclass visitor_super_t; \
virtual void visit(Visitor& v) { v.visit(*this); }
//-----------------------------------------------------------------------------
// Implementation detail:
// Selective dispatcher for the visitor - required to handle overloading.
//
template <typename T>
struct VisitorDispatch {
static void dispatch(Visitor* v, T* t) { v->visit(*t); }
};
// Specalization for cases where dispatch is not defined
template <> struct VisitorDispatch<void> {
static void dispatch(Visitor* v, void* t) { throw std::bad_cast(""); }
};
//-----------------------------------------------------------------------------
// Derive visitors from this and 'Visitor'.
template <typename T>
class VTarget
{
public:
// Don't really need a virtual dtor.
virtual void dispatch(T& t) = 0;
};
//-----------------------------------------------------------------------------
class Visitor
{
public:
virtual ~Visitor() = 0;
template <typename T>
void visit(T& t) {
typedef VTarget<T> target_t;
target_t* tgt = dynamic_cast<target_t*>(this);
if (tgt) {
tgt->dispatch(t);
}
else {
// Navigate up inhertiance hierarchy.
// requires 'super' to be defined in all classes in hierarchy
// applicable to this visitor.
typedef typename T::visitor_super_t super;
super* s = static_cast<super*>(&t);
VisitorDispatch<super>::dispatch(this, s);
}
}
};
//-----------------------------------------------------------------------------
inline Visitor::~Visitor() {}
然后将其用于创建通用访问者:
class CommonBase {
IMPLEMENT_VISITOR_WITH_SUPERCLASS(void)
virtual ~CommonBase() = 0;
};
class A : public CommonBase {
IMPLEMENT_VISITOR_WITH_SUPERCLASS(CommonBase)
};
class B : public CommonBase {
IMPLEMENT_VISITOR_WITH_SUPERCLASS(CommonBase)
};
class MyVisitor
: public Visitor
, public VTarget<CommonBase>
, public VTarget<A>
, public VTarget<B>
{
public:
virtual void dispatch(CommonBase& obj);
virtual void dispatch(A& obj);
virtual void dispatch(B& obj);
};
使用访问者最终会导致dynamic_cast<>
's from Visitor
to VTarget<T>
,这是一个交叉转换。
可以实现的另一种方式是创建Visitor
一个虚拟基础VTarget<T>
-MyVisitor
然后不再需要直接从访问者继承。dynamic_cast<>
Visitor::visit 代码中的Visitor
.
执行强制转换时,一种方法比另一种方法快吗?或者您是否只因拥有虚拟基地而受到规模损失?