C ++相当于instanceof

Question

C ++相当于instanceof

实现C ++等价的首选方法是instanceof什么？

Answer 1

尝试使用：if(NewType* v = dynamic_cast<NewType*>(old)) {&nbsp; &nbsp;// old was safely casted to NewType&nbsp; &nbsp;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好评！每次你需要使用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，或者引入类似访问者的内容，您可以在不改变界面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) {&nbsp; &nbsp;Box* box = static_cast<Box*>(old);&nbsp; &nbsp;// Do something box specific}这不是好设计，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，可能会以如下代码结束：// Here we have a SpecialBox class that inherits Box, since it has its own type// we must check for both BOX or SPECIAL_BOXif(old->getType() == BOX || old->getType() == SPECIAL_BOX) {&nbsp; &nbsp;Box* box = static_cast<Box*>(old);&nbsp; &nbsp;// Do something box specific}

Answer 2

没有dynamic_cast的实现实例我认为这个问题今天仍然有用。使用C ++ 11标准，您现在可以实现一个instanceof函数而不使用dynamic_cast这样的：if (dynamic_cast<B*>(aPtr) != nullptr) {&nbsp; // aPtr is instance of B} else {&nbsp; // aPtr is NOT instance of B}但你依然依赖RTTI支持。所以这是我的解决方案，取决于一些宏和Metaprogramming魔术。唯一的缺点是恕我直言，这种做法并没有对工作的多重继承。InstanceOfMacros.h#include <set>#include <tuple>#include <typeindex>#define _EMPTY_BASE_TYPE_DECL() using BaseTypes = std::tuple<>;#define _BASE_TYPE_DECL(Class, BaseClass) \&nbsp; using BaseTypes = decltype(std::tuple_cat(std::tuple<BaseClass>(), Class::BaseTypes()));#define _INSTANCE_OF_DECL_BODY(Class)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; static const std::set<std::type_index> baseTypeContainer;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; virtual bool instanceOfHelper(const std::type_index &_tidx) {&nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; &nbsp; if (std::type_index(typeid(ThisType)) == _tidx) return true;&nbsp; &nbsp; &nbsp; \&nbsp; &nbsp; if (std::tuple_size<BaseTypes>::value == 0) return false;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; &nbsp; return baseTypeContainer.find(_tidx) != baseTypeContainer.end();&nbsp; \&nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; template <typename... T>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp; static std::set<std::type_index> getTypeIndexes(std::tuple<T...>) { \&nbsp; &nbsp; return std::set<std::type_index>{std::type_index(typeid(T))...};&nbsp; \&nbsp; }#define INSTANCE_OF_SUB_DECL(Class, BaseClass) \&nbsp;protected:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp; using ThisType = Class;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp; _BASE_TYPE_DECL(Class, BaseClass)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp; _INSTANCE_OF_DECL_BODY(Class)#define INSTANCE_OF_BASE_DECL(Class)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp;protected:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; using ThisType = Class;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; _EMPTY_BASE_TYPE_DECL()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; _INSTANCE_OF_DECL_BODY(Class)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp;public:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp; template <typename Of>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; \&nbsp; typename std::enable_if<std::is_base_of<Class, Of>::value, bool>::type instanceOf() { \&nbsp; &nbsp; return instanceOfHelper(std::type_index(typeid(Of)));&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;\&nbsp; }#define INSTANCE_OF_IMPL(Class) \&nbsp; const std::set<std::type_index> Class::baseTypeContainer = Class::getTypeIndexes(Class::BaseTypes());演示然后你可以使用这些东西（谨慎），如下所示：DemoClassHierarchy.hpp *#include "InstanceOfMacros.h"struct A {&nbsp; virtual ~A() {}&nbsp; INSTANCE_OF_BASE_DECL(A)};INSTANCE_OF_IMPL(A)struct B : public A {&nbsp; virtual ~B() {}&nbsp; INSTANCE_OF_SUB_DECL(B, A)};INSTANCE_OF_IMPL(B)struct C : public A {&nbsp; virtual ~C() {}&nbsp; INSTANCE_OF_SUB_DECL(C, A)};INSTANCE_OF_IMPL(C)struct D : public C {&nbsp; virtual ~D() {}&nbsp; 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。InstanceOfDemo.cpp#include <iostream>#include <memory>#include "DemoClassHierarchy.hpp"int main() {&nbsp; A *a2aPtr = new A;&nbsp; A *a2bPtr = new B;&nbsp; std::shared_ptr<A> a2cPtr(new C);&nbsp; C *c2dPtr = new D;&nbsp; std::unique_ptr<A> a2dPtr(new D);&nbsp; std::cout << "a2aPtr->instanceOf<A>(): expected=1, value=" << a2aPtr->instanceOf<A>() << std::endl;&nbsp; std::cout << "a2aPtr->instanceOf<B>(): expected=0, value=" << a2aPtr->instanceOf<B>() << std::endl;&nbsp; std::cout << "a2aPtr->instanceOf<C>(): expected=0, value=" << a2aPtr->instanceOf<C>() << std::endl;&nbsp; std::cout << "a2aPtr->instanceOf<D>(): expected=0, value=" << a2aPtr->instanceOf<D>() << std::endl;&nbsp; std::cout << std::endl;&nbsp; std::cout << "a2bPtr->instanceOf<A>(): expected=1, value=" << a2bPtr->instanceOf<A>() << std::endl;&nbsp; std::cout << "a2bPtr->instanceOf<B>(): expected=1, value=" << a2bPtr->instanceOf<B>() << std::endl;&nbsp; std::cout << "a2bPtr->instanceOf<C>(): expected=0, value=" << a2bPtr->instanceOf<C>() << std::endl;&nbsp; std::cout << "a2bPtr->instanceOf<D>(): expected=0, value=" << a2bPtr->instanceOf<D>() << std::endl;&nbsp; std::cout << std::endl;&nbsp; std::cout << "a2cPtr->instanceOf<A>(): expected=1, value=" << a2cPtr->instanceOf<A>() << std::endl;&nbsp; std::cout << "a2cPtr->instanceOf<B>(): expected=0, value=" << a2cPtr->instanceOf<B>() << std::endl;&nbsp; std::cout << "a2cPtr->instanceOf<C>(): expected=1, value=" << a2cPtr->instanceOf<C>() << std::endl;&nbsp; std::cout << "a2cPtr->instanceOf<D>(): expected=0, value=" << a2cPtr->instanceOf<D>() << std::endl;&nbsp; std::cout << std::endl;&nbsp; std::cout << "c2dPtr->instanceOf<A>(): expected=1, value=" << c2dPtr->instanceOf<A>() << std::endl;&nbsp; std::cout << "c2dPtr->instanceOf<B>(): expected=0, value=" << c2dPtr->instanceOf<B>() << std::endl;&nbsp; std::cout << "c2dPtr->instanceOf<C>(): expected=1, value=" << c2dPtr->instanceOf<C>() << std::endl;&nbsp; std::cout << "c2dPtr->instanceOf<D>(): expected=1, value=" << c2dPtr->instanceOf<D>() << std::endl;&nbsp; std::cout << std::endl;&nbsp; std::cout << "a2dPtr->instanceOf<A>(): expected=1, value=" << a2dPtr->instanceOf<A>() << std::endl;&nbsp; std::cout << "a2dPtr->instanceOf<B>(): expected=0, value=" << a2dPtr->instanceOf<B>() << std::endl;&nbsp; std::cout << "a2dPtr->instanceOf<C>(): expected=1, value=" << a2dPtr->instanceOf<C>() << std::endl;&nbsp; std::cout << "a2dPtr->instanceOf<D>(): expected=1, value=" << a2dPtr->instanceOf<D>() << std::endl;&nbsp; delete a2aPtr;&nbsp; delete a2bPtr;&nbsp; delete c2dPtr;&nbsp; return 0;}输出：a2aPtr->instanceOf<A>(): expected=1, value=1a2aPtr->instanceOf<B>(): expected=0, value=0a2aPtr->instanceOf<C>(): expected=0, value=0a2aPtr->instanceOf<D>(): expected=0, value=0a2bPtr->instanceOf<A>(): expected=1, value=1a2bPtr->instanceOf<B>(): expected=1, value=1a2bPtr->instanceOf<C>(): expected=0, value=0a2bPtr->instanceOf<D>(): expected=0, value=0a2cPtr->instanceOf<A>(): expected=1, value=1a2cPtr->instanceOf<B>(): expected=0, value=0a2cPtr->instanceOf<C>(): expected=1, value=1a2cPtr->instanceOf<D>(): expected=0, value=0c2dPtr->instanceOf<A>(): expected=1, value=1c2dPtr->instanceOf<B>(): expected=0, value=0c2dPtr->instanceOf<C>(): expected=1, value=1c2dPtr->instanceOf<D>(): expected=1, value=1a2dPtr->instanceOf<A>(): expected=1, value=1a2dPtr->instanceOf<B>(): expected=0, value=0a2dPtr->instanceOf<C>(): expected=1, value=1a2dPtr->instanceOf<D>(): expected=1, value=1性能现在出现的最有趣的问题是，如果这种邪恶的东西比使用它更有效dynamic_cast。因此，我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) {&nbsp; auto start = std::chrono::high_resolution_clock::now();&nbsp; volatile bool isInstanceOf = false;&nbsp; for (unsigned i = 0; i < _loopCycles; ++i) {&nbsp; &nbsp; Base *ptr = new Derived;&nbsp; &nbsp; isInstanceOf = ptr->template instanceOf<Derived>();&nbsp; &nbsp; delete ptr;&nbsp; }&nbsp; auto end = std::chrono::high_resolution_clock::now();&nbsp; return std::chrono::duration_cast<Duration>(end - start);}template <typename Base, typename Derived, typename Duration>Duration dynamicCastMeasurement(unsigned _loopCycles) {&nbsp; auto start = std::chrono::high_resolution_clock::now();&nbsp; volatile bool isInstanceOf = false;&nbsp; for (unsigned i = 0; i < _loopCycles; ++i) {&nbsp; &nbsp; Base *ptr = new Derived;&nbsp; &nbsp; isInstanceOf = dynamic_cast<Derived *>(ptr) != nullptr;&nbsp; &nbsp; delete ptr;&nbsp; }&nbsp; auto end = std::chrono::high_resolution_clock::now();&nbsp; return std::chrono::duration_cast<Duration>(end - start);}int main() {&nbsp; unsigned testCycles = 10000000;&nbsp; std::string unit = " us";&nbsp; using DType = std::chrono::microseconds;&nbsp; std::cout << "InstanceOf performance(A->D)&nbsp; : " << instanceOfMeasurement<A, D, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "InstanceOf performance(A->C)&nbsp; : " << instanceOfMeasurement<A, C, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "InstanceOf performance(A->B)&nbsp; : " << instanceOfMeasurement<A, B, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "InstanceOf performance(A->A)&nbsp; : " << instanceOfMeasurement<A, A, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << "\n"&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "DynamicCast performance(A->D) : " << dynamicCastMeasurement<A, D, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "DynamicCast performance(A->C) : " << dynamicCastMeasurement<A, C, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "DynamicCast performance(A->B) : " << dynamicCastMeasurement<A, B, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; std::cout << "DynamicCast performance(A->A) : " << dynamicCastMeasurement<A, A, DType>(testCycles).count() << unit&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << "\n"&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; << std::endl;&nbsp; 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本地计算机上的输出编译性能测量程序是：InstanceOf performance(A->D)&nbsp; : 699638 usInstanceOf performance(A->C)&nbsp; : 642157 usInstanceOf performance(A->B)&nbsp; : 671399 usInstanceOf performance(A->A)&nbsp; : 626193 usDynamicCast performance(A->D) : 754937 usDynamicCast performance(A->C) : 706766 usDynamicCast performance(A->B) : 751353 usDynamicCast 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。对于测试指针A是否是实例的特殊测试用例来说效率更低A。但是，通过使用编译器otpimization调整我们的二进制来转变趋势。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)&nbsp; : 3035 usInstanceOf performance(A->C)&nbsp; : 5030 usInstanceOf performance(A->B)&nbsp; : 5250 usInstanceOf performance(A->A)&nbsp; : 3021 usDynamicCast performance(A->D) : 666903 usDynamicCast performance(A->C) : 698567 usDynamicCast performance(A->B) : 727368 usDynamicCast performance(A->A) : 3098 us如果你不依赖于多重继承，不是好旧的C宏，RTTI和模板元编程的对手，并且不太懒于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一点点关于它的性能，如果你经常最终检查指针的实例。但要谨慎使用它。这种方法的正确性无法保证。注意：所有演示都是clang (Apple LLVM version 9.0.0 (clang-900.0.39.2))在MacBook Pro Mid 2012上使用macOS Sierra 编译的。编辑： 我还测试了Linux机器上的性能gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609。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)&nbsp; : 390768 usInstanceOf performance(A->C)&nbsp; : 333994 usInstanceOf performance(A->B)&nbsp; : 334596 usInstanceOf performance(A->A)&nbsp; : 300959 usDynamicCast performance(A->D) : 331942 usDynamicCast performance(A->C) : 303715 usDynamicCast performance(A->B) : 400262 usDynamicCast performance(A->A) : 324942 us输出（带编译器优化）：InstanceOf performance(A->D)&nbsp; : 209501 usInstanceOf performance(A->C)&nbsp; : 208727 usInstanceOf performance(A->B)&nbsp; : 207815 usInstanceOf performance(A->A)&nbsp; : 197953 usDynamicCast performance(A->D) : 259417 usDynamicCast performance(A->C) : 256203 usDynamicCast performance(A->B) : 261202 usDynamicCast performance(A->A) : 193535 us