实现C ++等效于instanceof的首选方法是什么？
答案 0 :(得分：190)
尝试使用：
if(NewType* v = dynamic_cast<NewType*>(old)) {
// old was safely casted to NewType
v->doSomething();
}
这要求您的编译器启用rtti支持。
编辑： 我对这个答案有一些很好的评论！
每当你需要使用dynamic_cast（或instanceof）时，你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要检查的类的特殊行为放入基类的虚函数中，或者引入类似visitor的内容，您可以在其中引入子类的特定行为而不更改接口（除了当然要添加访客接受界面。）
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）的简单且始终如一的执行hack基本上是添加一个枚举，表示您的类可以拥有的所有可能类型，并检查您是否拥有正确的类型。
if(old->getType() == BOX) {
Box* box = static_cast<Box*>(old);
// Do something box specific
}
这不是很好的oo设计，但它可以是一种解决方法，其成本或多或少只是虚拟函数调用。无论是否启用RTTI，它都可以工作。
请注意，此方法不支持多级继承，因此如果您不小心，可能会看起来像这样的代码：
// 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
}
答案 1 :(得分：32)
根据您的目的，您可以这样做：
template<typename Base, typename T>
inline bool instanceof(const T*) {
return std::is_base_of<Base, T>::value;
}
使用：
if (instanceof<BaseClass>(ptr)) { ... }
但是，这纯粹依赖于编译器已知的类型。
修改
此代码适用于多态指针：
template<typename Base, typename T>
inline bool instanceof(const T *ptr) {
return dynamic_cast<const Base*>(ptr) != nullptr;
}
答案 2 :(得分：2)
我认为这个问题今天仍然有用。使用C ++ 11标准，您现在可以在不使用instanceof的情况下实现dynamic_cast功能：
if (dynamic_cast<B*>(aPtr) != nullptr) {
// aPtr is instance of B
} else {
// aPtr is NOT instance of B
}
但您仍然依赖RTTI支持。所以这是我的解决方案，取决于一些宏和Metaprogramming魔术。 imho唯一的缺点是这种方法 多继承。
<强> InstanceOfMacros.h
#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());
然后您可以按如下方式使用此内容（谨慎）：
<强> 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型演示，用于验证基本的正确行为。
<强> 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;
}
<强>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，如果这些邪恶的东西比使用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。
<强> 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，主要基于编译器优化的程度。使用g++ -std=c++11 -O0 -o instanceof-performance InstanceOfPerformance.cpp在本地计算机上输出编译性能测量程序：
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，这个结果非常清醒，因为时间表明新方法与dynamic_cast方法相比并不快。对于测试A的指针是A的实例的特殊测试用例来说效率更低。 但是潮流通过使用编译器otpimization调整我们的二进制来转变。相应的编译器命令是g++ -std=c++11 -O3 -o instanceof-performance InstanceOfPerformance.cpp。在我的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
如果你不依赖于多重继承，不能反对好的旧C宏，RTTI和模板元编程，并且不太懒，不能在类层次结构的类中添加一些小指令，那么这种方法可以提升你的应用程序如果你经常最终检查一个指针的实例，那么就它的性能而言。 但请谨慎使用。这种方法的正确性无法保证。
注意：所有演示都是在MacBook Pro Mid 2012上的macOS Sierra下使用clang (Apple LLVM version 9.0.0 (clang-900.0.39.2))编译的。
修改
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，性能优势并不像具有铿锵声的macOs那么重要。
输出（无编译器优化）：
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（使用编译器优化）：
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
答案 3 :(得分：0)
dynamic_cast效率低下。它遍历继承层次结构，如果您有多个继承级别，它是唯一的解决方案，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。 / p>
但是如果instanceof更有限的形式只检查对象是否与您指定的类型完全相符，那么下面的函数会更有效：
template<typename T, typename K>
inline bool isType(const K &k) {
return typeid(T).hash_code() == typeid(k).hash_code();
}
以下是您如何调用上述功能的示例：
DerivedA k;
Base *p = &k;
cout << boolalpha << isType<DerivedA>(*p) << endl; // true
cout << boolalpha << isType<DerivedB>(*p) << endl; // false
您指定模板类型A（作为您要检查的类型），并将要测试的对象作为参数传入（从中推断出模板类型K ）。
答案 4 :(得分：-4)
#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);
}
答案 5 :(得分：-10)
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;
}