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C ++相當(dāng)于instanceof

C++

紅顏莎娜 2019-08-12 17:07:22

C ++相當(dāng)于instanceof實(shí)現(xiàn)C ++等價(jià)的首選方法是instanceof什么？

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3 回答

Cats萌萌

TA貢獻(xiàn)1805條經(jīng)驗(yàn) 獲得超9個(gè)贊

嘗試使用：

if(NewType* v = dynamic_cast<NewType*>(old)) {

// old was safely casted to NewType

v->doSomething();

}

這要求您的編譯器啟用rtti支持。

編輯：我對(duì)這個(gè)答案有一些好評(píng)！

每次你需要使用dynamic_cast（或instanceof）時(shí)，你最好問問自己這是否是必要的。這通常是設(shè)計(jì)不佳的標(biāo)志。

典型的解決方法是將要檢查的類的特殊行為放入基類的虛函數(shù)中，或者引入類似訪問者的內(nèi)容，您可以在不改變界面的情況下為子類引入特定行為（除了添加訪問者接受接口）課程）。

正如所指出的，dynamic_cast不是免費(fèi)的。處理大多數(shù)（但不是所有情況）的簡(jiǎn)單且始終如一的執(zhí)行hack基本上是添加一個(gè)枚舉，表示您的類可以擁有的所有可能類型，并檢查您是否擁有正確的類型。

if(old->getType() == BOX) {

Box* box = static_cast<Box*>(old);

// Do something box specific

}

這不是好設(shè)計(jì)，但它可以是一種解決方法，其成本或多或少只是虛擬函數(shù)調(diào)用。無(wú)論是否啟用RTTI，它都可以工作。

請(qǐng)注意，此方法不支持多級(jí)繼承，因此如果您不小心，可能會(huì)以如下代碼結(jié)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

}

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搖曳的薔薇

TA貢獻(xiàn)1793條經(jīng)驗(yàn) 獲得超6個(gè)贊

沒有dynamic_cast的實(shí)現(xiàn)實(shí)例

我認(rèn)為這個(gè)問題今天仍然有用。使用C ++ 11標(biāo)準(zhǔn)，您現(xiàn)在可以實(shí)現(xiàn)一個(gè)instanceof函數(shù)而不使用dynamic_cast這樣的：

if (dynamic_cast<B*>(aPtr) != nullptr) {

// aPtr is instance of B

} else {

// aPtr is NOT instance of B

}

但你依然依賴RTTI支持。所以這是我的解決方案，取決于一些宏和Metaprogramming魔術(shù)。唯一的缺點(diǎn)是恕我直言，這種做法并沒有對(duì)工作的多重繼承。

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());

演示

然后你可以使用這些東西（謹(jǐn)慎），如下所示：

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)

以下代碼提供了一個(gè)小型演示，用于驗(yàn)證基本的正確行為。

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(): expected=0, value=" << a2aPtr->instanceOf() << 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(): expected=1, value=" << a2bPtr->instanceOf() << 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(): expected=0, value=" << a2cPtr->instanceOf() << 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(): expected=0, value=" << c2dPtr->instanceOf() << 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(): expected=0, value=" << a2dPtr->instanceOf() << 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;

}

輸出：

a2aPtr->instanceOf<A>(): expected=1, value=1

a2aPtr->instanceOf(): 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(): 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(): 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(): 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(): expected=0, value=0

a2dPtr->instanceOf<C>(): expected=1, value=1

a2dPtr->instanceOf<D>(): expected=1, value=1

性能

現(xiàn)在出現(xiàn)的最有趣的問題是，如果這種邪惡的東西比使用它更有效dynamic_cast。因此，我寫了一個(gè)非?；镜男阅軠y(cè)量應(yīng)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;

}

結(jié)果各不相同，主要基于編譯器優(yōu)化的程度。使用g++ -std=c++11 -O0 -o instanceof-performance InstanceOfPerformance.cpp本地計(jì)算機(jī)上的輸出編譯性能測(cè)量程序是：

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，這個(gè)結(jié)果非常清醒，因?yàn)闀r(shí)間表明新方法與dynamic_cast方法相比并不快。對(duì)于測(cè)試指針A是否是實(shí)例的特殊測(cè)試用例來(lái)說(shuō)效率更低A。但是，通過(guò)使用編譯器otpimization調(diào)整我們的二進(jìn)制來(lái)轉(zhuǎn)變趨勢(shì)。相應(yīng)的編譯器命令是g++ -std=c++11 -O3 -o instanceof-performance InstanceOfPerformance.cpp。在我的本地機(jī)器上的結(jié)果是驚人的：

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

如果你不依賴于多重繼承，不是好舊的C宏，RTTI和模板元編程的對(duì)手，并且不太懶于在類層次結(jié)構(gòu)的類中添加一些小指令，那么這種方法可以提升你的應(yīng)用程序一點(diǎn)點(diǎn)關(guān)于它的性能，如果你經(jīng)常最終檢查指針的實(shí)例。但要謹(jǐn)慎使用它。這種方法的正確性無(wú)法保證。

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 編譯的。

編輯：我還測(cè)試了Linux機(jī)器上的性能gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609。在這個(gè)平臺(tái)上，性能優(yōu)勢(shì)并不像具有鏗鏘聲的macOs那么重要。

輸出（無(wú)編譯器優(yōu)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

輸出（帶編譯器優(yōu)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

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C ++相當(dāng)于instanceof

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