问题
I have been looking for a way to get around the slowness of the dynamic cast type checking. Before you start saying I should redesign everything, let me inform you that the design was decided on 5 years ago. I can't fix all 400,000 lines of code that came after (I wish I could), but I can make some changes. I have run this little test on type identification:
#include <iostream>
#include <typeinfo>
#include <stdint.h>
#include <ctime>
using namespace std;
#define ADD_TYPE_ID \
static intptr_t type() { return reinterpret_cast<intptr_t>(&type); }\
virtual intptr_t getType() { return type(); }
struct Base
{
ADD_TYPE_ID;
};
template <typename T>
struct Derived : public Base
{
ADD_TYPE_ID;
};
int main()
{
Base* b = new Derived<int>();
cout << "Correct Type: " << (b->getType() == Derived<int>::type()) << endl; // true
cout << "Template Type: " << (b->getType() == Derived<float>::type()) << endl; // false
cout << "Base Type: " << (b->getType() == Base::type()) << endl; // false
clock_t begin = clock();
{
for (size_t i = 0; i < 100000000; i++)
{
if (b->getType() == Derived<int>::type())
Derived <int>* d = static_cast<Derived<int>*> (b);
}
}
clock_t end = clock();
double elapsed = double(end - begin) / CLOCKS_PER_SEC;
cout << "Type elapsed: " << elapsed << endl;
begin = clock();
{
for (size_t i = 0; i < 100000000; i++)
{
Derived<int>* d = dynamic_cast<Derived<int>*>(b);
if (d);
}
}
end = clock();
elapsed = double(end - begin) / CLOCKS_PER_SEC;
cout << "Type elapsed: " << elapsed << endl;
begin = clock();
{
for (size_t i = 0; i < 100000000; i++)
{
Derived<int>* d = dynamic_cast<Derived<int>*>(b);
if ( typeid(d) == typeid(Derived<int>*) )
static_cast<Derived<int>*> (b);
}
}
end = clock();
elapsed = double(end - begin) / CLOCKS_PER_SEC;
cout << "Type elapsed: " << elapsed << endl;
return 0;
}
It seems that using the class id (first times solution above) would be the fastest way to do type-checking at runtime. Will this cause any problems with threading? Is there a better way to check for types at runtime (with not much re-factoring)?
Edit: Might I also add that this needs to work with the TI compilers, which currently only support up to '03
回答1:
First off, note that there's a big difference between dynamic_cast and RTTI: The cast tells you whether you can treat a base object as some further derived, but not necessarily most-derived object. RTTI tells you the precise most-derived type. Naturally the former is more powerful and more expensive.
So then, there are two natural ways you can select on types if you have a polymorphic hierarchy. They're different; use the one that actually applies.
void method1(Base * p)
{
if (Derived * q = dynamic_cast<Derived *>(p))
{
// use q
}
}
void method2(Base * p)
{
if (typeid(*p) == typeid(Derived))
{
auto * q = static_cast<Derived *>(p);
// use q
}
}
Note also that method 2 is not generally available if the base class is a virtual base. Neither method applies if your classes are not polymorphic.
In a quick test I found method 2 to be significantly faster than your manual ID-based solution, which in turn is faster than the dynamic cast solution (method 1).
回答2:
How about comparing the classes' virtual function tables?
Quick and dirty proof of concept:
void* instance_vtbl(void* c)
{
return *(void**)c;
}
template<typename C>
void* class_vtbl()
{
static C c;
return instance_vtbl(&c);
}
// ...
begin = clock();
{
for (size_t i = 0; i < 100000000; i++)
{
if (instance_vtbl(b) == class_vtbl<Derived<int>>())
Derived <int>* d = static_cast<Derived<int>*> (b);
}
}
end = clock();
elapsed = double(end - begin) / CLOCKS_PER_SEC;
cout << "Type elapsed: " << elapsed << endl;
With Visual C++'s /Ox switch, this appears 3x faster than the type/getType trick.
回答3:
Given this type of code
class A {
};
class B : public A {
}
A * a;
B * b = dynamic_cast<B*> (a);
if( b != 0 ) // do something B specific
The polymorphic (right?) way to fix it is something like this
class A {
public:
virtual void specific() { /* do nothing */ }
};
class B : public A {
public:
virtual void specific() { /* do something B specific */ }
}
A * a;
if( a != 0 ) a->specific();
回答4:
When MSVC 2005 first came out, dynamic_cast<> for 64-bit code was much slower than for 32-bit code. We wanted a quick and easy fix. This is what our code looks like. It probably violates all kinds of good design rules, but the conversion to remove dynamic_cast<> can be automated with a script.
class dbbMsgEph {
public:
virtual dbbResultEph * CastResultEph() { return 0; }
virtual const dbbResultEph * CastResultEph() const { return 0; }
};
class dbbResultEph : public dbbMsgEph {
public:
virtual dbbResultEph * CastResultEph() { return this; }
virtual const dbbResultEph * CastResultEph() const { return this; }
static dbbResultEph * Cast( dbbMsgEph * );
static const dbbResultEph * Cast( const dbbMsgEph * );
};
dbbResultEph *
dbbResultEph::Cast( dbbMsgEph * arg )
{
if( arg == 0 ) return 0;
return arg->CastResultEph();
}
const dbbResultEph *
dbbResultEph::Cast( const dbbMsgEph * arg )
{
if( arg == 0 ) return 0;
return arg->CastResultEph();
}
When we used to have
dbbMsgEph * pMsg;
dbbResultEph * pResult = dynamic_cast<dbbResultEph *> (pMsg);
we changed it to
dbbResultEph * pResult = dbbResultEph::Cast (pMsg);
using a simple sed(1) script. And virtual function calls are pretty efficient.
回答5:
//in release module(VS2008) this is true：
cout << "Base Type: " << (b->getType() == Base::type()) << endl;
I guess it's because the optimization.So I change the implementation of Derived::type()
template <typename T>
struct Derived : public Base
{
static intptr_t type()
{
cout << "different type()" << endl;
return reinterpret_cast<intptr_t>(&type);
}
virtual intptr_t getType() { return type(); }
};
Then it's different.So how to deal with it if use this method???
来源：https://stackoverflow.com/questions/25495733/circumventing-rtti-on-legacy-code