[英]C++ equivalent of java's instanceof
实现与 java 的instanceof
等效的 C++ 的首选方法是什么?
尝试使用:
if(NewType* v = dynamic_cast<NewType*>(old)) {
// old was safely casted to NewType
v->doSomething();
}
这需要您的编译器启用 rtti 支持。
编辑:我对这个答案有一些很好的评论!
每次需要使用 dynamic_cast(或 instanceof)时,最好问问自己这是否是必要的。 这通常是设计不佳的标志。
典型的解决方法是将您正在检查的类的特殊行为放入基类的虚函数中,或者可能引入诸如访问者之类的东西,您可以在不更改接口的情况下为子类引入特定行为(除了添加访问者接受接口课程)。
正如所指出的,dynamic_cast 不是免费的。 处理大多数(但不是所有情况)的简单且一致执行的 hack 基本上是添加一个枚举,代表您的类可以拥有的所有可能类型,并检查您是否得到了正确的类型。
if(old->getType() == BOX) {
Box* box = static_cast<Box*>(old);
// Do something box specific
}
这不是一个好的 oo 设计,但它可以是一种解决方法,其成本或多或少只是一个虚函数调用。 无论 RTTI 是否启用,它也能正常工作。
请注意,此方法不支持多级继承,因此如果您不小心,可能会以如下所示的代码结束:
// Here we have a SpecialBox class that inherits Box, since it has its own type
// we must check for both BOX or SPECIAL_BOX
if(old->getType() == BOX || old->getType() == SPECIAL_BOX) {
Box* box = static_cast<Box*>(old);
// Do something box specific
}
根据你想要做什么,你可以这样做:
template<typename Base, typename T>
inline bool instanceof(const T*) {
return std::is_base_of<Base, T>::value;
}
用:
if (instanceof<BaseClass>(ptr)) { ... }
但是,这纯粹是对编译器已知的类型进行操作。
编辑:
此代码应该适用于多态指针:
template<typename Base, typename T>
inline bool instanceof(const T *ptr) {
return dynamic_cast<const Base*>(ptr) != nullptr;
}
示例: http : //cpp.sh/6qir
我认为这个问题今天仍然有意义。 使用 C++11 标准,您现在可以在不使用dynamic_cast
的instanceof
下实现instanceof
函数,如下所示:
if (dynamic_cast<B*>(aPtr) != nullptr) {
// aPtr is instance of B
} else {
// aPtr is NOT instance of B
}
但是您仍然依赖于RTTI
支持。 所以这是我根据一些宏和元编程魔法解决这个问题的方法。 唯一的缺点是恕我直言,这种方法不适用于多重继承工作。
宏实例
#include <set>
#include <tuple>
#include <typeindex>
#define _EMPTY_BASE_TYPE_DECL() using BaseTypes = std::tuple<>;
#define _BASE_TYPE_DECL(Class, BaseClass) \
using BaseTypes = decltype(std::tuple_cat(std::tuple<BaseClass>(), Class::BaseTypes()));
#define _INSTANCE_OF_DECL_BODY(Class) \
static const std::set<std::type_index> baseTypeContainer; \
virtual bool instanceOfHelper(const std::type_index &_tidx) { \
if (std::type_index(typeid(ThisType)) == _tidx) return true; \
if (std::tuple_size<BaseTypes>::value == 0) return false; \
return baseTypeContainer.find(_tidx) != baseTypeContainer.end(); \
} \
template <typename... T> \
static std::set<std::type_index> getTypeIndexes(std::tuple<T...>) { \
return std::set<std::type_index>{std::type_index(typeid(T))...}; \
}
#define INSTANCE_OF_SUB_DECL(Class, BaseClass) \
protected: \
using ThisType = Class; \
_BASE_TYPE_DECL(Class, BaseClass) \
_INSTANCE_OF_DECL_BODY(Class)
#define INSTANCE_OF_BASE_DECL(Class) \
protected: \
using ThisType = Class; \
_EMPTY_BASE_TYPE_DECL() \
_INSTANCE_OF_DECL_BODY(Class) \
public: \
template <typename Of> \
typename std::enable_if<std::is_base_of<Class, Of>::value, bool>::type instanceOf() { \
return instanceOfHelper(std::type_index(typeid(Of))); \
}
#define INSTANCE_OF_IMPL(Class) \
const std::set<std::type_index> Class::baseTypeContainer = Class::getTypeIndexes(Class::BaseTypes());
然后你可以使用这些东西(谨慎)如下:
DemoClassHierarchy.hpp*
#include "InstanceOfMacros.h"
struct A {
virtual ~A() {}
INSTANCE_OF_BASE_DECL(A)
};
INSTANCE_OF_IMPL(A)
struct B : public A {
virtual ~B() {}
INSTANCE_OF_SUB_DECL(B, A)
};
INSTANCE_OF_IMPL(B)
struct C : public A {
virtual ~C() {}
INSTANCE_OF_SUB_DECL(C, A)
};
INSTANCE_OF_IMPL(C)
struct D : public C {
virtual ~D() {}
INSTANCE_OF_SUB_DECL(D, C)
};
INSTANCE_OF_IMPL(D)
以下代码提供了一个小演示来验证基本的正确行为。
InstanceOfDemo.cpp
#include <iostream>
#include <memory>
#include "DemoClassHierarchy.hpp"
int main() {
A *a2aPtr = new A;
A *a2bPtr = new B;
std::shared_ptr<A> a2cPtr(new C);
C *c2dPtr = new D;
std::unique_ptr<A> a2dPtr(new D);
std::cout << "a2aPtr->instanceOf<A>(): expected=1, value=" << a2aPtr->instanceOf<A>() << std::endl;
std::cout << "a2aPtr->instanceOf<B>(): expected=0, value=" << a2aPtr->instanceOf<B>() << std::endl;
std::cout << "a2aPtr->instanceOf<C>(): expected=0, value=" << a2aPtr->instanceOf<C>() << std::endl;
std::cout << "a2aPtr->instanceOf<D>(): expected=0, value=" << a2aPtr->instanceOf<D>() << std::endl;
std::cout << std::endl;
std::cout << "a2bPtr->instanceOf<A>(): expected=1, value=" << a2bPtr->instanceOf<A>() << std::endl;
std::cout << "a2bPtr->instanceOf<B>(): expected=1, value=" << a2bPtr->instanceOf<B>() << std::endl;
std::cout << "a2bPtr->instanceOf<C>(): expected=0, value=" << a2bPtr->instanceOf<C>() << std::endl;
std::cout << "a2bPtr->instanceOf<D>(): expected=0, value=" << a2bPtr->instanceOf<D>() << std::endl;
std::cout << std::endl;
std::cout << "a2cPtr->instanceOf<A>(): expected=1, value=" << a2cPtr->instanceOf<A>() << std::endl;
std::cout << "a2cPtr->instanceOf<B>(): expected=0, value=" << a2cPtr->instanceOf<B>() << std::endl;
std::cout << "a2cPtr->instanceOf<C>(): expected=1, value=" << a2cPtr->instanceOf<C>() << std::endl;
std::cout << "a2cPtr->instanceOf<D>(): expected=0, value=" << a2cPtr->instanceOf<D>() << std::endl;
std::cout << std::endl;
std::cout << "c2dPtr->instanceOf<A>(): expected=1, value=" << c2dPtr->instanceOf<A>() << std::endl;
std::cout << "c2dPtr->instanceOf<B>(): expected=0, value=" << c2dPtr->instanceOf<B>() << std::endl;
std::cout << "c2dPtr->instanceOf<C>(): expected=1, value=" << c2dPtr->instanceOf<C>() << std::endl;
std::cout << "c2dPtr->instanceOf<D>(): expected=1, value=" << c2dPtr->instanceOf<D>() << std::endl;
std::cout << std::endl;
std::cout << "a2dPtr->instanceOf<A>(): expected=1, value=" << a2dPtr->instanceOf<A>() << std::endl;
std::cout << "a2dPtr->instanceOf<B>(): expected=0, value=" << a2dPtr->instanceOf<B>() << std::endl;
std::cout << "a2dPtr->instanceOf<C>(): expected=1, value=" << a2dPtr->instanceOf<C>() << std::endl;
std::cout << "a2dPtr->instanceOf<D>(): expected=1, value=" << a2dPtr->instanceOf<D>() << std::endl;
delete a2aPtr;
delete a2bPtr;
delete c2dPtr;
return 0;
}
输出:
a2aPtr->instanceOf<A>(): expected=1, value=1
a2aPtr->instanceOf<B>(): expected=0, value=0
a2aPtr->instanceOf<C>(): expected=0, value=0
a2aPtr->instanceOf<D>(): expected=0, value=0
a2bPtr->instanceOf<A>(): expected=1, value=1
a2bPtr->instanceOf<B>(): expected=1, value=1
a2bPtr->instanceOf<C>(): expected=0, value=0
a2bPtr->instanceOf<D>(): expected=0, value=0
a2cPtr->instanceOf<A>(): expected=1, value=1
a2cPtr->instanceOf<B>(): expected=0, value=0
a2cPtr->instanceOf<C>(): expected=1, value=1
a2cPtr->instanceOf<D>(): expected=0, value=0
c2dPtr->instanceOf<A>(): expected=1, value=1
c2dPtr->instanceOf<B>(): expected=0, value=0
c2dPtr->instanceOf<C>(): expected=1, value=1
c2dPtr->instanceOf<D>(): expected=1, value=1
a2dPtr->instanceOf<A>(): expected=1, value=1
a2dPtr->instanceOf<B>(): expected=0, value=0
a2dPtr->instanceOf<C>(): expected=1, value=1
a2dPtr->instanceOf<D>(): expected=1, value=1
现在出现的最有趣的问题是,这个邪恶的东西是否比使用dynamic_cast
更有效。 因此,我编写了一个非常基本的性能测量应用程序。
InstanceOfPerformance.cpp
#include <chrono>
#include <iostream>
#include <string>
#include "DemoClassHierarchy.hpp"
template <typename Base, typename Derived, typename Duration>
Duration instanceOfMeasurement(unsigned _loopCycles) {
auto start = std::chrono::high_resolution_clock::now();
volatile bool isInstanceOf = false;
for (unsigned i = 0; i < _loopCycles; ++i) {
Base *ptr = new Derived;
isInstanceOf = ptr->template instanceOf<Derived>();
delete ptr;
}
auto end = std::chrono::high_resolution_clock::now();
return std::chrono::duration_cast<Duration>(end - start);
}
template <typename Base, typename Derived, typename Duration>
Duration dynamicCastMeasurement(unsigned _loopCycles) {
auto start = std::chrono::high_resolution_clock::now();
volatile bool isInstanceOf = false;
for (unsigned i = 0; i < _loopCycles; ++i) {
Base *ptr = new Derived;
isInstanceOf = dynamic_cast<Derived *>(ptr) != nullptr;
delete ptr;
}
auto end = std::chrono::high_resolution_clock::now();
return std::chrono::duration_cast<Duration>(end - start);
}
int main() {
unsigned testCycles = 10000000;
std::string unit = " us";
using DType = std::chrono::microseconds;
std::cout << "InstanceOf performance(A->D) : " << instanceOfMeasurement<A, D, DType>(testCycles).count() << unit
<< std::endl;
std::cout << "InstanceOf performance(A->C) : " << instanceOfMeasurement<A, C, DType>(testCycles).count() << unit
<< std::endl;
std::cout << "InstanceOf performance(A->B) : " << instanceOfMeasurement<A, B, DType>(testCycles).count() << unit
<< std::endl;
std::cout << "InstanceOf performance(A->A) : " << instanceOfMeasurement<A, A, DType>(testCycles).count() << unit
<< "\n"
<< std::endl;
std::cout << "DynamicCast performance(A->D) : " << dynamicCastMeasurement<A, D, DType>(testCycles).count() << unit
<< std::endl;
std::cout << "DynamicCast performance(A->C) : " << dynamicCastMeasurement<A, C, DType>(testCycles).count() << unit
<< std::endl;
std::cout << "DynamicCast performance(A->B) : " << dynamicCastMeasurement<A, B, DType>(testCycles).count() << unit
<< std::endl;
std::cout << "DynamicCast performance(A->A) : " << dynamicCastMeasurement<A, A, DType>(testCycles).count() << unit
<< "\n"
<< std::endl;
return 0;
}
结果各不相同,主要取决于编译器优化的程度。 使用g++ -std=c++11 -O0 -o instanceof-performance InstanceOfPerformance.cpp
编译性能测量程序,我本地机器上的输出是:
InstanceOf performance(A->D) : 699638 us
InstanceOf performance(A->C) : 642157 us
InstanceOf performance(A->B) : 671399 us
InstanceOf performance(A->A) : 626193 us
DynamicCast performance(A->D) : 754937 us
DynamicCast performance(A->C) : 706766 us
DynamicCast performance(A->B) : 751353 us
DynamicCast performance(A->A) : 676853 us
嗯,这个结果非常发人深省,因为时间证明新方法与dynamic_cast
方法相比并没有快多少。 它甚至不太有效用于在此进行测试,如果一个指针的特殊测试用例A
是实例A
。 但是通过使用编译器优化来调整我们的二进制文件,潮流会发生转变。 相应的编译器命令是g++ -std=c++11 -O3 -o instanceof-performance InstanceOfPerformance.cpp
。 在我本地机器上的结果是惊人的:
InstanceOf performance(A->D) : 3035 us
InstanceOf performance(A->C) : 5030 us
InstanceOf performance(A->B) : 5250 us
InstanceOf performance(A->A) : 3021 us
DynamicCast performance(A->D) : 666903 us
DynamicCast performance(A->C) : 698567 us
DynamicCast performance(A->B) : 727368 us
DynamicCast performance(A->A) : 3098 us
如果您不依赖多重继承,不反对优秀的旧 C 宏、RTTI 和模板元编程,并且不会懒于向类层次结构的类添加一些小指令,那么这种方法可以稍微提升您的应用程序关于它的性能,如果你经常检查一个指针的实例。 但请谨慎使用。 不保证此方法的正确性。
注意:所有演示都是在 2012 年中期 MacBook Pro 上的 macOS Sierra 下使用clang (Apple LLVM version 9.0.0 (clang-900.0.39.2))
编译的。
编辑:我还使用gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
在 Linux 机器上测试了性能。 在这个平台上,性能优势不像在带有 clang 的 macO 上那么显着。
输出(没有编译器优化):
InstanceOf performance(A->D) : 390768 us
InstanceOf performance(A->C) : 333994 us
InstanceOf performance(A->B) : 334596 us
InstanceOf performance(A->A) : 300959 us
DynamicCast performance(A->D) : 331942 us
DynamicCast performance(A->C) : 303715 us
DynamicCast performance(A->B) : 400262 us
DynamicCast performance(A->A) : 324942 us
输出(经过编译器优化):
InstanceOf performance(A->D) : 209501 us
InstanceOf performance(A->C) : 208727 us
InstanceOf performance(A->B) : 207815 us
InstanceOf performance(A->A) : 197953 us
DynamicCast performance(A->D) : 259417 us
DynamicCast performance(A->C) : 256203 us
DynamicCast performance(A->B) : 261202 us
DynamicCast performance(A->A) : 193535 us
众所周知, dynamic_cast
效率低下。 它向上遍历继承层次结构,如果您有多个继承级别,并且需要检查对象是否是其类型层次结构中任何一种类型的实例,那么它是唯一的解决方案。
但是,如果仅检查对象是否正是您指定的类型的更有限形式的instanceof
就足以满足您的需求,那么下面的函数会更有效率:
template<typename T, typename K>
inline bool isType(const K &k) {
return typeid(T).hash_code() == typeid(k).hash_code();
}
以下是如何调用上述函数的示例:
DerivedA k;
Base *p = &k;
cout << boolalpha << isType<DerivedA>(*p) << endl; // true
cout << boolalpha << isType<DerivedB>(*p) << endl; // false
您将指定模板类型A
(作为您要检查的类型),并传入您要测试的对象作为参数(将从模板类型K
推断出)。
#include <iostream.h>
#include<typeinfo.h>
template<class T>
void fun(T a)
{
if(typeid(T) == typeid(int))
{
//Do something
cout<<"int";
}
else if(typeid(T) == typeid(float))
{
//Do Something else
cout<<"float";
}
}
void main()
{
fun(23);
fun(90.67f);
}
这对我来说非常适合使用Code :: Blocks IDE和GCC编译器
#include<iostream>
#include<typeinfo>
#include<iomanip>
#define SIZE 20
using namespace std;
class Publication
{
protected:
char title[SIZE];
int price;
public:
Publication()
{
cout<<endl<<" Enter title of media : ";
cin>>title;
cout<<endl<<" Enter price of media : ";
cin>>price;
}
virtual void show()=0;
};
class Book : public Publication
{
int pages;
public:
Book()
{
cout<<endl<<" Enter number of pages : ";
cin>>pages;
}
void show()
{
cout<<endl<<setw(12)<<left<<" Book Title"<<": "<<title;
cout<<endl<<setw(12)<<left<<" Price"<<": "<<price;
cout<<endl<<setw(12)<<left<<" Pages"<<": "<<pages;
cout<<endl<<" ----------------------------------------";
}
};
class Tape : public Publication
{
int duration;
public:
Tape()
{
cout<<endl<<" Enter duration in minute : ";
cin>>duration;
}
void show()
{
cout<<endl<<setw(10)<<left<<" Tape Title"<<": "<<title;
cout<<endl<<setw(10)<<left<<" Price"<<": "<<price;
cout<<endl<<setw(10)<<left<<" Duration"<<": "<<duration<<" minutes";
cout<<endl<<" ----------------------------------------";
}
};
int main()
{
int n, i, type;
cout<<endl<<" Enter number of media : ";
cin>>n;
Publication **p = new Publication*[n];
cout<<endl<<" Enter "<<n<<" media details : ";
for(i=0;i<n;i++)
{
cout<<endl<<" Select Media Type [ 1 - Book / 2 - Tape ] ";
cin>>type;
if ( type == 1 )
{
p[i] = new Book();
}
else
if ( type == 2 )
{
p[i] = new Tape();
}
else
{
i--;
cout<<endl<<" Invalid type. You have to Re-enter choice";
}
}
for(i=0;i<n;i++)
{
if ( typeid(Book) == typeid(*p[i]) )
{
p[i]->show();
}
}
return 0;
}
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