\u76f4\u63a5\u7528\u5b57\u7b26\u4e32\u6bd4\u8f83\u4ee3\u66ff type_info \u6307\u9488\u6bd4\u8f83<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n4.2 \u7b56\u7565\u4e00：\u865a\u8868\u904d\u5386（GCC\/Clang \u5355\u7ee7\u627f\u8def\u5f84）<\/h4>\n\u8fd9\u662f\u6700\u7b80\u5355\u4e5f\u6700\u5feb\u7684\u7b56\u7565，\u9002\u7528\u4e8e\u5355\u7ee7\u627f\u94fe：<\/p>\n dynamic_cast<Dog*>(animal_ptr) \u7684\u6267\u884c\u6d41\u7a0b：<\/p>\n 1. \u89e3\u5f15\u7528 animal_ptr \u83b7\u53d6 vptr
\n2. \u8bfb\u53d6 vtable[-1] \u83b7\u53d6 type_info*（\u5f53\u524d\u5bf9\u8c61\u7684\u5b9e\u9645\u7c7b\u578b）
\n3. \u6bd4\u8f83\u8be5 type_info \u4e0e\u76ee\u6807\u7c7b\u578b Dog \u7684 type_info
\n – \u76f8\u7b49 \u2192 \u8f6c\u6362\u6210\u529f，\u76f4\u63a5\u8fd4\u56de\u6307\u9488\u503c（\u5355\u7ee7\u627f\u65e0\u9700\u8c03\u6574 this）
\n – \u4e0d\u76f8\u7b49 \u2192 \u901a\u8fc7 __si_class_type_info::__base_type \u8df3\u5230\u7236\u7c7b\u7684 type_info，\u91cd\u590d\u6b65\u9aa4 3
\n4. \u5230\u8fbe\u7ee7\u627f\u94fe\u9876\u7aef\u4ecd\u672a\u5339\u914d \u2192 \u8fd4\u56de nullptr<\/p>\n \u7b97\u6cd5\u4f2a\u4ee3\u7801：<\/p>\n void* __dynamic_cast_si(const void* obj_ptr,
\n const __class_type_info* obj_type,
\n const __class_type_info* target_type,
\n ptrdiff_t src2dst_offset) {
\n const void* vptr = *(const void**)obj_ptr;
\n const auto* rtti_ptr = ((const __class_type_info**)vptr)[-1];<\/p>\n \/\/ \u6cbf\u5355\u7ee7\u627f\u94fe\u5411\u4e0a\u904d\u5386
\n for (const __si_class_type_info* cur = static_cast<const __si_class_type_info*>(rtti_ptr);
\n cur != nullptr;
\n cur = dynamic_cast<const __si_class_type_info*>(cur->__base_type)) {<\/p>\n if (cur == target_type) {
\n \/\/ \u627e\u5230\u76ee\u6807\u7c7b\u578b，\u8c03\u6574\u6307\u9488
\n return (char*)obj_ptr + src2dst_offset;
\n }<\/p>\n \/\/ \u5982\u679c\u5f53\u524d\u5df2\u4e0d\u662f\u5355\u7ee7\u627f\u7c7b\u578b，\u8df3\u51fa\u5faa\u73af\u8d70\u590d\u6742\u8def\u5f84
\n if (typeid(*cur) != typeid(__si_class_type_info)) break;
\n }<\/p>\n \/\/ \u964d\u7ea7\u5230\u901a\u7528\u641c\u7d22\u7b97\u6cd5
\n return __dynamic_cast_generic(obj_ptr, obj_type, target_type, src2dst_offset);
\n}<\/p>\n 4.3 \u7b56\u7565\u4e8c：\u7c7b\u578b\u6811\u641c\u7d22（GCC\/Clang \u865a\u7ee7\u627f\u8def\u5f84）<\/h4>\n\u5f53\u7c7b\u5c42\u6b21\u7ed3\u6784\u6d89\u53ca\u591a\u7ee7\u627f\u6216\u865a\u7ee7\u627f\u65f6，RTTI \u7ed3\u6784\u4ece\u00a0__si_class_type_info\u00a0\u53d8\u4e3a\u00a0__vmi_class_type_info，\u9700\u8981\u904d\u5386\u57fa\u7c7b\u6570\u7ec4：<\/p>\n \/\/ \u901a\u7528\u591a\u7ee7\u627f dynamic_cast \u6838\u5fc3\u7b97\u6cd5
\nvoid* __dynamic_cast_generic(const void* obj_ptr,
\n const __class_type_info* obj_type,
\n const __class_type_info* target_type,
\n ptrdiff_t src2dst) {
\n \/\/ \u6b65\u9aa4 1：\u68c0\u67e5\u5f53\u524d\u5c42\u7ea7\u662f\u5426\u5339\u914d
\n if (*obj_type == *target_type) {
\n return (char*)obj_ptr + src2dst;
\n }<\/p>\n \/\/ \u6b65\u9aa4 2：\u6839\u636e obj_type \u7684\u5177\u4f53\u7c7b\u578b\u9012\u5f52\u641c\u7d22
\n \/\/ \u5982\u679c\u662f\u5355\u7ee7\u627f\u7c7b\u578b
\n if (const auto* si = dynamic_cast<const __si_class_type_info*>(obj_type)) {
\n return __dynamic_cast_generic(obj_ptr, si->__base_type, target_type, src2dst);
\n }<\/p>\n \/\/ \u5982\u679c\u662f\u591a\u7ee7\u627f\/\u865a\u7ee7\u627f\u7c7b\u578b
\n if (const auto* vmi = dynamic_cast<const __vmi_class_type_info*>(obj_type)) {
\n for (unsigned i = 0; i < vmi->__base_count; ++i) {
\n const auto& base = vmi->__base_info[i];<\/p>\n \/\/ \u63d0\u53d6\u504f\u79fb\u91cf\u548c\u6807\u5fd7\u4f4d
\n bool is_virtual = base.__offset_flags & 1;
\n ptrdiff_t offset;<\/p>\n if (is_virtual) {
\n \/\/ \u865a\u57fa\u7c7b：\u504f\u79fb\u91cf\u9700\u8981\u901a\u8fc7\u865a\u57fa\u7c7b\u8868\u5728\u8fd0\u884c\u65f6\u786e\u5b9a
\n offset = get_virtual_base_offset(obj_ptr, base);
\n } else {
\n \/\/ \u975e\u865a\u57fa\u7c7b：\u504f\u79fb\u91cf\u7f16\u7801\u5728 __offset_flags \u4e2d
\n offset = base.__offset_flags >> 8;
\n }<\/p>\n \/\/ \u9012\u5f52\u641c\u7d22\u5f53\u524d\u57fa\u7c7b\u5b50\u6811
\n void* result = __dynamic_cast_generic(
\n (char*)obj_ptr + offset,
\n base.__base_type,
\n target_type,
\n src2dst – offset
\n );<\/p>\n if (result) return result;
\n }
\n }<\/p>\n return nullptr; \/\/ \u672a\u627e\u5230\u76ee\u6807\u7c7b\u578b
\n}<\/p>\n 4.4 \u7b56\u7565\u4e09：MSVC \u7684\u5d4c\u5165\u54c8\u5e0c\u8868<\/h4>\nMSVC \u91c7\u7528\u4e0e GCC \u5b8c\u5168\u4e0d\u540c\u7684\u7b56\u7565\u2014\u2014\u5728\u00a0RTTIClassHierarchyDescriptor\u00a0\u4e2d\u9884\u8ba1\u7b97\u5b8c\u6574\u7684\u57fa\u7c7b\u94fe，\u4ee5\u6570\u7ec4\u5f62\u5f0f\u5b58\u50a8，\u901a\u8fc7\u904d\u5386\u8be5\u6570\u7ec4\u5b9e\u73b0 O(N) \u67e5\u627e：<\/p>\n struct RTTIClassHierarchyDescriptor {
\n unsigned signature;
\n unsigned attributes; \/\/ \u7ee7\u627f\u5c5e\u6027\u6807\u5fd7
\n unsigned numBaseClasses; \/\/ \u57fa\u7c7b\u6570\u91cf（\u542b\u81ea\u8eab）
\n RTTIBaseClassDescriptor** pBaseClassArray;
\n};<\/p>\n struct RTTIBaseClassDescriptor {
\n TypeDescriptor* pTypeDescriptor; \/\/ \u6307\u5411 type_info \u7b49\u4ef7\u7ed3\u6784
\n unsigned numContainedBases; \/\/ \u8be5\u57fa\u7c7b\u5305\u542b\u7684\u5b50\u57fa\u7c7b\u6570\u91cf
\n PMD where; \/\/ \u6307\u9488\u6210\u5458\u4f4d\u79fb（Pointer-to-Member Displacement）
\n unsigned attributes; \/\/ \u5c5e\u6027\u6807\u5fd7
\n};<\/p>\n struct PMD {
\n ptrdiff_t mdisp; \/\/ \u6210\u5458\u4f4d\u79fb（member displacement）
\n ptrdiff_t pdisp; \/\/ \u865a\u8868\u4f4d\u79fb（vtable displacement）
\n ptrdiff_t vdisp; \/\/ \u865a\u57fa\u7c7b\u4f4d\u79fb（virtual base displacement）
\n};<\/p>\n MSVC \u7684\u00a0dynamic_cast\u00a0\u6838\u5fc3\u6d41\u7a0b：<\/p>\n 1. \u4ece obj \u83b7\u53d6 vptr 2. \u8bfb\u53d6 vtable[-1] \u83b7\u53d6 RTTICompleteObjectLocator* 3. \u904d\u5386 ClassHierarchyDescriptor \u4e2d\u7684 pBaseClassArray \u00a0\u00a0 – \u5bf9\u6bcf\u4e2a\u5143\u7d20，\u6bd4\u8f83 pTypeDescriptor \u4e0e\u76ee\u6807\u7c7b\u578b\u7684 TypeDescriptor \u00a0\u00a0 – \u5339\u914d\u6210\u529f\u65f6，\u4f7f\u7528 PMD \u4e2d\u7684 mdisp\/pdisp\/vdisp \u8ba1\u7b97\u6b63\u786e\u7684\u6307\u9488\u503c 4. \u904d\u5386\u5b8c\u4ecd\u672a\u5339\u914d \u2192 nullptr<\/p>\n MSVC \u8fd9\u79cd"\u9884\u8ba1\u7b97\u6570\u7ec4"\u65b9\u5f0f\u4f7f\u5f97\u6bcf\u6b21\u00a0dynamic_cast\u00a0\u90fd\u662f\u7ebf\u6027\u626b\u63cf，\u4f46\u4e0d\u9700\u8981\u50cf GCC \u90a3\u6837\u9012\u5f52\u6df1\u5165\u7ee7\u627f\u6811\u3002\u5728\u6df1\u5c42\u7ee7\u627f\u65f6 MSVC \u53ef\u80fd\u66f4\u5feb（\u6570\u7ec4\u8fde\u7eed\u8bbf\u95ee vs \u6307\u9488\u8df3\u8f6c），\u4f46\u591a\u7ee7\u627f\u4e0b\u65f6\u95f4\u590d\u6742\u5ea6\u5747\u4e3a O(B)\u3002<\/p>\n
\n5. \u5355\u7ee7\u627f\u4e0b\u7684 dynamic_cast<\/h3>\n5.1 \u57fa\u672c\u5411\u4e0a\u8f6c\u578b（Upcast）<\/h4>\n\u5411\u4e0a\u8f6c\u578b（\u6d3e\u751f\u7c7b\u6307\u9488\u2192\u57fa\u7c7b\u6307\u9488）\u5728\u7f16\u8bd1\u671f\u5b8c\u6210，\u4e0d\u6d89\u53ca RTTI：<\/p>\n class Animal { public: virtual ~Animal() = default; };
\nclass Dog : public Animal {};<\/p>\n Dog* dog = new Dog;
\nAnimal* animal = dog; \/\/ \u7f16\u8bd1\u671f\u9690\u5f0f\u8f6c\u6362，\u65e0\u9700 dynamic_cast
\n\/\/ \u7b49\u4ef7\u4e8e：Animal* animal = static_cast<Animal*>(dog);<\/p>\n 5.2 \u5411\u4e0b\u8f6c\u578b（Downcast）<\/h4>\nAnimal* animal = new Dog;<\/p>\n \/\/ dynamic_cast \u5728\u8fd0\u884c\u65f6\u9a8c\u8bc1 animal \u662f\u5426\u771f\u7684\u6307\u5411 Dog
\nDog* dog = dynamic_cast<Dog*>(animal);
\nif (dog) {
\n dog->bark();
\n}<\/p>\n \u5355\u7ee7\u627f\u4e0b\u7684\u00a0dynamic_cast<Dog*>\u00a0\u5b9e\u73b0\u975e\u5e38\u76f4\u63a5：<\/p>\n \u5185\u5b58\u5e03\u5c40（\u5355\u7ee7\u627f）：
\n\u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510
\n\u2502 Animal vptr \u2502 \u2190 animal \u548c dog \u6307\u5411\u540c\u4e00\u5730\u5740
\n\u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n\u2502 Dog members \u2502
\n\u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2518<\/p>\n \u8f6c\u6362\u8fc7\u7a0b：
\n1. \u901a\u8fc7 vptr \u83b7\u53d6\u5b9e\u9645\u7c7b\u578b\u7684 type_info（\u6b64\u5904\u4e3a Dog）
\n2. \u6cbf RTTI \u94fe\u5411\u4e0a\u67e5\u627e：Dog \u2192 Animal
\n3. \u53d1\u73b0 Dog == \u76ee\u6807\u7c7b\u578b Dog，\u5339\u914d\u6210\u529f
\n4. \u6307\u9488\u65e0\u9700\u8c03\u6574（\u57fa\u7c7b\u5b50\u5bf9\u8c61\u4e0e\u6d3e\u751f\u7c7b\u8d77\u59cb\u5730\u5740\u4e00\u81f4）<\/p>\n 5.3 \u4ea4\u53c9\u8f6c\u578b（Cross-cast）<\/h4>\n\u4ea4\u53c9\u8f6c\u578b\u6307\u5728\u540c\u4e00\u7ee7\u627f\u5c42\u7ea7\u4e2d\u4e0d\u540c\u5206\u652f\u95f4\u7684\u8f6c\u6362，\u5355\u7ee7\u627f\u4e0b\u4e0d\u6d89\u53ca\u6b64\u573a\u666f（\u9700\u8981\u591a\u7ee7\u627f），\u4f46\u7f16\u8bd1\u671f\u4e0d\u4f1a\u62a5\u9519：<\/p>\n class Base { public: virtual ~Base() = default; };
\nclass A : public Base {};
\nclass B : public Base {};<\/p>\n Base* base = new A;
\nB* b = dynamic_cast<B*>(base); \/\/ \u8fd4\u56de nullptr（A \u4e0d\u662f B）<\/p>\n
\n6. \u591a\u7ee7\u627f\u4e0b\u7684\u6307\u9488\u8c03\u6574<\/h3>\n\u591a\u7ee7\u627f\u662f\u00a0dynamic_cast\u00a0\u6700\u590d\u6742\u7684\u573a\u666f\u4e4b\u4e00\u3002\u6d3e\u751f\u7c7b\u5305\u542b\u591a\u4e2a\u57fa\u7c7b\u5b50\u5bf9\u8c61，\u5b83\u4eec\u4f4d\u4e8e\u4e0d\u540c\u7684\u5185\u5b58\u504f\u79fb\u5904\u3002<\/p>\n 6.1 \u5185\u5b58\u5e03\u5c40\u793a\u4f8b<\/h4>\nclass Animal {
\npublic:
\n virtual void eat() {}
\n virtual ~Animal() = default;
\n int age = 0;
\n};<\/p>\n class Flyable {
\npublic:
\n virtual void fly() {}
\n virtual ~Flyable() = default;
\n double wingspan = 0.0;
\n};<\/p>\n class Bat : public Animal, public Flyable {
\npublic:
\n void eat() override {}
\n void fly() override {}
\n bool echolocation = true;
\n};<\/p>\n \u5178\u578b\u5185\u5b58\u5e03\u5c40（GCC\/Clang x86-64）：<\/p>\n Bat \u5bf9\u8c61\u5185\u5b58\u5e03\u5c40：<\/p>\n \u504f\u79fb\u91cf \u5185\u5bb9
\n+0 \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510
\n \u2502 Animal vptr \u2502 \u2500\u2500\u2192 Bat-Animal \u865a\u8868
\n+8 \u2502 age = 0 \u2502
\n+16 \u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n \u2502 Flyable vptr\u2502 \u2500\u2500\u2192 Bat-Flyable \u865a\u8868（\u542b this \u8c03\u6574 thunk）
\n+24 \u2502 wingspan=0.0\u2502
\n+32 \u2502 echolocation\u2502
\n \u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2518<\/p>\n \u6ce8：Animal \u5b50\u5bf9\u8c61\u4ece\u504f\u79fb 0 \u5f00\u59cb
\n Flyable \u5b50\u5bf9\u8c61\u4ece\u504f\u79fb 16 \u5f00\u59cb<\/p>\n 6.2 \u6307\u9488\u8c03\u6574\u7684\u6838\u5fc3\u95ee\u9898<\/h4>\n\u5173\u952e\u95ee\u9898：\u5f53\u6301\u6709\u00a0Animal*\u00a0\u6307\u9488\u4f46\u5b9e\u9645\u4e0a\u6307\u5411\u00a0Bat\u00a0\u5bf9\u8c61\u65f6，\u5982\u4f55\u5c06\u5176\u5b89\u5168\u5730\u8f6c\u6362\u4e3a\u00a0Flyable*？<\/p>\n Animal* a = new Bat; \/\/ a \u6307\u5411 Bat \u5bf9\u8c61\u7684\u504f\u79fb 0（Animal \u5b50\u5bf9\u8c61\u8d77\u59cb）
\nFlyable* f = dynamic_cast<Flyable*>(a);<\/p>\n \/\/ \u95ee\u9898：a \u6307\u5411\u5730\u5740 0x1000，\u4f46 Flyable \u5b50\u5bf9\u8c61\u5728 0x1000 + 16 = 0x1010
\n\/\/ dynamic_cast \u5fc5\u987b\u5c06\u8fd4\u56de\u503c\u8c03\u6574\u4e3a 0x1010，\u800c\u4e0d\u7b80\u5355\u8fd4\u56de 0x1000<\/p>\n 6.3 \u7f16\u8bd1\u5668\u5982\u4f55\u8ba1\u7b97\u8c03\u6574\u91cf<\/h4>\ndynamic_cast\u00a0\u7684\u6307\u9488\u8c03\u6574\u901a\u8fc7 RTTI \u6570\u636e\u7ed3\u6784\u4e2d\u5b58\u50a8\u7684\u504f\u79fb\u4fe1\u606f\u6765\u5b9e\u73b0：<\/p>\n GCC\/Itanium ABI \u65b9\u5f0f：<\/p>\n struct __base_class_type_info {
\n const __class_type_info* __base_type;
\n long __offset_flags; \/\/ \u504f\u79fb\u91cf\u7f16\u7801：
\n \/\/ bits [0:7] = \u6807\u5fd7\u4f4d
\n \/\/ bits [8:63] = \u504f\u79fb\u91cf（\u53f3\u79fb 8 \u4f4d）
\n};<\/p>\n \u5728\u00a0__vmi_class_type_info\u00a0\u7684\u00a0__base_info\u00a0\u6570\u7ec4\u4e2d，\u6bcf\u4e2a\u57fa\u7c7b\u90fd\u8bb0\u5f55\u4e86\u5b83\u5728\u6d3e\u751f\u7c7b\u5bf9\u8c61\u4e2d\u7684\u504f\u79fb\u91cf\u3002<\/p>\n \u5f53\u00a0dynamic_cast\u00a0\u904d\u5386\u57fa\u7c7b\u6570\u7ec4\u65f6，\u6bcf\u6b21\u9012\u5f52\u8c03\u7528\u90fd\u4f1a\u4f20\u9012\u4e00\u4e2a\u7d2f\u79ef\u504f\u79fb\u91cf\u00a0src2dst：<\/p>\n \/\/ \u521d\u59cb\u8c03\u7528：src2dst = 0（\u4ece Animal* \u7684\u8d77\u59cb\u4f4d\u7f6e\u8ba1\u7b97）
\n\/\/ \u904d\u5386\u5230 Flyable \u57fa\u7c7b\u6761\u76ee\u65f6，\u53d1\u73b0 offset = 16
\n\/\/ \u9012\u5f52\u8c03\u7528\u65f6\u4f20\u9012 src2dst = 0 – 16 = -16
\n\/\/ \u5728 Flyable \u7684 type_info \u8282\u70b9\u5339\u914d\u65f6：
\n\/\/ \u8fd4\u56de (char*)obj_ptr + (-16) = obj_ptr – 16，\u5373 Flyable \u5b50\u5bf9\u8c61\u7684\u5730\u5740<\/p>\n MSVC \u65b9\u5f0f：<\/p>\n MSVC \u4f7f\u7528\u00a0PMD（Pointer-to-Member Displacement）\u7ed3\u6784，\u5728\u00a0RTTIBaseClassDescriptor\u00a0\u4e2d\u9884\u5148\u5b58\u50a8\u4e86\u5b8c\u6574\u7684\u8c03\u6574\u53c2\u6570：<\/p>\n struct PMD {
\n ptrdiff_t mdisp; \/\/ \u6210\u5458\u4f4d\u79fb：\u4ece\u6d3e\u751f\u7c7b\u8d77\u59cb\u5230\u8be5\u57fa\u7c7b\u5b50\u5bf9\u8c61\u7684\u504f\u79fb
\n ptrdiff_t pdisp; \/\/ \u865a\u8868\u4f4d\u79fb：\u865a\u8868\u5185\u8c03\u6574
\n ptrdiff_t vdisp; \/\/ \u865a\u57fa\u7c7b\u4f4d\u79fb：\u4ec5\u865a\u7ee7\u627f\u4e0b\u975e\u96f6
\n};<\/p>\n 6.4 \u5b8c\u6574\u8f6c\u6362\u793a\u4f8b<\/h4>\n#include <iostream>
\n#include <typeinfo><\/p>\n class Animal {
\npublic:
\n virtual const char* type() { return "Animal"; }
\n virtual ~Animal() = default;
\n};<\/p>\n class Flyable {
\npublic:
\n virtual const char* ability() { return "Flyable"; }
\n virtual ~Flyable() = default;
\n};<\/p>\n class Bat : public Animal, public Flyable {
\npublic:
\n const char* type() override { return "Bat"; }
\n const char* ability() override { return "Bat-Flyable"; }
\n};<\/p>\n int main() {
\n Bat bat;
\n Animal* a = &bat; \/\/ a \u6307\u5411\u504f\u79fb 0
\n Flyable* f = &bat; \/\/ f \u6307\u5411\u504f\u79fb 16（\u5047\u8bbe）<\/p>\n std::cout << "a = " << a << std::endl;
\n std::cout << "f = " << f << std::endl;
\n std::cout << "&bat = " << &bat << std::endl;
\n \/\/ \u8f93\u51fa：a == &bat, f == &bat + offset_of_Flyable<\/p>\n \/\/ \u4ea4\u53c9\u8f6c\u578b：\u4ece Animal* \u8f6c\u5230 Flyable*
\n Flyable* cross = dynamic_cast<Flyable*>(a);
\n std::cout << "cross = " << cross << std::endl;
\n \/\/ cross \u5e94\u8be5\u7b49\u4e8e f，\u5373\u6307\u9488\u503c\u7ecf\u8fc7\u8c03\u6574<\/p>\n \/\/ \u65e0\u6548\u8f6c\u578b
\n class Unrelated { public: virtual ~Unrelated() = default; };
\n Unrelated* u = dynamic_cast<Unrelated*>(a);
\n std::cout << "u = " << u << std::endl; \/\/ nullptr<\/p>\n return 0;
\n}<\/p>\n 6.5 \u5b50\u5bf9\u8c61\u504f\u79fb\u91cf\u5b9e\u9a8c<\/h4>\n\u53ef\u4ee5\u901a\u8fc7\u4ee5\u4e0b\u4ee3\u7801\u89c2\u5bdf\u4e0d\u540c\u57fa\u7c7b\u6307\u9488\u5bf9\u5e94\u5730\u5740\u7684\u5dee\u5f02：<\/p>\n #include <iostream><\/p>\n class A {
\n virtual ~A() = default;
\n char data_a[8];
\n};
\nclass B {
\n virtual ~B() = default;
\n char data_b[8];
\n};
\nclass C : public A, public B {
\n char data_c[8];
\n};<\/p>\n int main() {
\n C c;<\/p>\n A* pa = &c;
\n B* pb = &c;
\n C* pc = &c;<\/p>\n std::cout << "C* = " << pc << std::endl;
\n std::cout << "A* = " << pa << " (offset = "
\n << (char*)pa – (char*)pc << ")" << std::endl;
\n std::cout << "B* = " << pb << " (offset = "
\n << (char*)pb – (char*)pc << ")" << std::endl;<\/p>\n \/\/ \u9a8c\u8bc1 dynamic_cast \u6307\u9488\u8c03\u6574
\n B* pb2 = dynamic_cast<B*>(pa);
\n std::cout << "dynamic_cast<B*>(A*) = " << pb2 << std::endl;
\n std::cout << "pb == pb2: " << (pb == pb2) << std::endl;
\n}<\/p>\n \u5178\u578b\u8f93\u51fa（x86-64）：<\/p>\n C* = 0x7ffe12345000
\nA* = 0x7ffe12345000 (offset = 0)
\nB* = 0x7ffe12345010 (offset = 16)
\ndynamic_cast<B*>(A*) = 0x7ffe12345010
\npb == pb2: 1<\/p>\n
\n7. \u865a\u7ee7\u627f\u4e0b\u7684 dynamic_cast<\/h3>\n\u865a\u7ee7\u627f\u5f15\u5165\u4e86\u4e00\u4e2a\u989d\u5916\u7684\u95f4\u63a5\u5c42，\u4f7f\u5f97\u00a0dynamic_cast\u00a0\u7684\u5b9e\u73b0\u66f4\u52a0\u590d\u6742\u3002<\/p>\n 7.1 \u865a\u7ee7\u627f\u7684\u5185\u5b58\u5e03\u5c40<\/h4>\nclass Animal {
\npublic:
\n virtual void eat() {}
\n int animal_id = 0;
\n};<\/p>\n class Mammal : virtual public Animal {
\npublic:
\n virtual void nurse() {}
\n int fur_type = 0;
\n};<\/p>\n class Bird : virtual public Animal {
\npublic:
\n virtual void lay_eggs() {}
\n int feather_color = 0;
\n};<\/p>\n class Platypus : public Mammal, public Bird {
\npublic:
\n void eat() override {}
\n bool venomous = true;
\n};<\/p>\n \u5178\u578b\u5185\u5b58\u5e03\u5c40（GCC x86-64，\u7b80\u5316\u8868\u793a）：<\/p>\n Platypus \u5bf9\u8c61\u5185\u5b58\u5e03\u5c40：<\/p>\n +0 \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510
\n \u2502 Mammal vptr \u2502 \u2500\u2500\u2192 Mammal-in-Platypus \u865a\u8868
\n+8 \u2502 fur_type \u2502
\n+16 \u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n \u2502 Bird vptr \u2502 \u2500\u2500\u2192 Bird-in-Platypus \u865a\u8868
\n+24 \u2502 feather_color \u2502
\n+32 \u2502 venomous \u2502
\n+40 \u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n \u2502 Animal vptr \u2502 \u2500\u2500\u2192 Animal-in-Platypus \u865a\u8868（\u865a\u57fa\u7c7b，\u4ec5\u4e00\u4efd）
\n+48 \u2502 animal_id \u2502
\n \u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2518<\/p>\n \u5173\u952e\u70b9：
\n– Animal \u53ea\u6709\u4e00\u4efd（\u865a\u7ee7\u627f\u4fdd\u8bc1）
\n– Mammal \u548c Bird \u90fd\u901a\u8fc7\u865a\u57fa\u7c7b\u8868\u95f4\u63a5\u8bbf\u95ee Animal
\n– Animal \u7684\u504f\u79fb\u91cf\u5728 Mammal \u548c Bird \u4e2d\u4e0d\u4e00\u5b9a\u76f8\u540c<\/p>\n 7.2 \u865a\u57fa\u7c7b\u504f\u79fb\u7684\u8fd0\u884c\u65f6\u89e3\u6790<\/h4>\n\u865a\u7ee7\u627f\u4e0b，\u57fa\u7c7b\u5b50\u5bf9\u8c61\u7684\u504f\u79fb\u91cf\u4e0d\u80fd\u5728\u7f16\u8bd1\u671f\u786e\u5b9a（\u56e0\u4e3a\u540c\u4e00\u4e2a\u7c7b\u53ef\u80fd\u88ab\u653e\u5728\u4e0d\u540c\u6d3e\u751f\u5c42\u6b21\u7684\u5bf9\u8c61\u4e2d）\u3002\u504f\u79fb\u91cf\u5b58\u50a8\u5728\u865a\u57fa\u7c7b\u8868（vbase offset table）\u4e2d，\u901a\u8fc7\u865a\u8868\u95f4\u63a5\u8bbf\u95ee\u3002<\/p>\n GCC \u7684\u5b9e\u73b0\u65b9\u5f0f：<\/p>\n Mammal \u865a\u8868\u5e03\u5c40（\u7b80\u5316）：
\n\u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510
\n\u2502 vbase_offset = 40 \u2502 \u2190 \u4ece Mammal \u5b50\u5bf9\u8c61\u8d77\u59cb\u5230\u865a\u57fa\u7c7b Animal \u7684\u504f\u79fb
\n\u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n\u2502 vcall_offset (\u53ef\u9009\u9879) \u2502
\n\u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n\u2502 type_info* (RTTI) \u2502
\n\u251c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2524
\n\u2502 Mammal::nurse() \u2502
\n\u2502 Mammal \u5176\u4ed6\u865a\u51fd\u6570 \u2502
\n\u2514\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2518<\/p>\n \u8fd0\u884c\u65f6\u83b7\u53d6\u865a\u57fa\u7c7b\u504f\u79fb\u7684\u4f2a\u4ee3\u7801：<\/p>\n ptrdiff_t get_virtual_base_offset(const void* obj_ptr,
\n const __base_class_type_info& base) {
\n \/\/ \u865a\u57fa\u7c7b\u7684\u504f\u79fb\u5b58\u50a8\u5728\u5f53\u524d\u5bf9\u8c61\u865a\u8868\u4e2d\u7684\u7279\u5b9a\u4f4d\u7f6e
\n const void* vptr = *(const void**)obj_ptr;<\/p>\n \/\/ \u4ece RTTI \u7684 __offset_flags \u4e2d\u63d0\u53d6\u865a\u57fa\u7c7b\u8868\u7684\u7d22\u5f15
\n \/\/ \u865a\u57fa\u7c7b\u504f\u79fb\u5728\u865a\u8868\u4e2d\u7684\u4f4d\u7f6e\u76f8\u5bf9\u4e8e vptr
\n ptrdiff_t vbase_offset_index = extract_vbase_index(base.__offset_flags);<\/p>\n \/\/ \u865a\u57fa\u7c7b\u504f\u79fb\u5b58\u50a8\u5728 vptr \u4e4b\u524d\u7684\u67d0\u4e2a\u4f4d\u7f6e（\u5b9e\u73b0\u5b9a\u4e49）
\n const ptrdiff_t* vbase_offsets = ((const ptrdiff_t*)vptr) – vbase_offset_index – 1;<\/p>\n return *vbase_offsets;
\n}<\/p>\n 7.3 \u865a\u7ee7\u627f\u4e0b\u7684 dynamic_cast \u5b8c\u6574\u6d41\u7a0b<\/h4>\nclass Animal { public: virtual ~Animal() = default; };
\nclass Mammal : virtual public Animal {};
\nclass Bird : virtual public Animal {};
\nclass Platypus : public Mammal, public Bird {};<\/p>\n Platypus duckbill;
\nAnimal* a = &duckbill; \/\/ a \u6307\u5411\u865a\u57fa\u7c7b Animal<\/p>\n \/\/ \u4ece\u865a\u57fa\u7c7b Animal* \u5411\u4e0b\u8f6c\u578b\u4e3a Platypus*
\nPlatypus* p = dynamic_cast<Platypus*>(a);<\/p>\n \/\/ \u6d41\u7a0b：
\n\/\/ 1. \u4ece a->vptr \u83b7\u53d6 type_info（\u5b9e\u9645\u7c7b\u578b\u4e3a Platypus）
\n\/\/ 2. Platypus \u7684 RTTI \u7c7b\u578b\u4e3a __vmi_class_type_info
\n\/\/ 3. \u904d\u5386\u57fa\u7c7b\u6570\u7ec4（Mammal \u548c Bird）
\n\/\/ – Mammal \u662f\u666e\u901a\u57fa\u7c7b，\u901a\u8fc7\u5b83\u9012\u5f52\u8fdb\u5165 Animal（\u865a\u57fa\u7c7b）
\n\/\/ – \u5728 Animal \u5c42\u7ea7\u5339\u914d\u65f6，\u9700\u8981\u8c03\u6574 src2dst \u7d2f\u52a0\u504f\u79fb
\n\/\/ 4. \u4ece Animal \u4f4d\u7f6e\u51cf\u53bb Animal \u5b50\u5bf9\u8c61\u5728 Platypus \u4e2d\u7684\u504f\u79fb\u91cf
\n\/\/ \u8be5\u504f\u79fb\u91cf\u901a\u8fc7 Mammal \u7684\u865a\u57fa\u7c7b\u8868\u5728\u8fd0\u884c\u65f6\u786e\u5b9a<\/p>\n
\n8. \u8de8\u5c42\u7ea7\u8f6c\u6362\u4e0e\u6b67\u4e49\u68c0\u6d4b<\/h3>\n8.1 \u8de8\u5c42\u7ea7\u8f6c\u6362<\/h4>\ndynamic_cast\u00a0\u652f\u6301\u8df3\u8fc7\u4e2d\u95f4\u5c42\u7ea7\u7684\u8f6c\u6362：<\/p>\n class A { public: virtual ~A() = default; };
\nclass B : public A {};
\nclass C : public B {};<\/p>\n A* a = new C;
\nC* c = dynamic_cast<C*>(a); \/\/ \u76f4\u63a5\u4ece A* \u8f6c\u5230 C*，\u8df3\u8fc7 B<\/p>\n \/\/ \u7f16\u8bd1\u5668\u4f1a\u81ea\u52a8\u6cbf\u7ee7\u627f\u94fe\u67e5\u627e C，\u65e0\u9700\u6bcf\u5c42\u624b\u52a8\u8f6c\u6362<\/p>\n \u8fd9\u5728\u83f1\u5f62\u7ee7\u627f\u4e2d\u5c24\u5176\u91cd\u8981：<\/p>\n class Top { public: virtual ~Top() = default; };
\nclass Left : virtual public Top {};
\nclass Right : virtual public Top {};
\nclass Bottom : public Left, public Right {};<\/p>\n Bottom b;
\nTop* t = &b;<\/p>\n \/\/ \u76f4\u63a5\u4ece Top* \u8f6c\u5230 Bottom*，\u8de8\u8d8a Left \u548c Right
\nBottom* bt = dynamic_cast<Bottom*>(t);
\n\/\/ \u7f16\u8bd1\u5668\u5fc5\u987b\u901a\u8fc7 Top \u7684 type_info \u627e\u5230 Bottom，\u5e76\u8ba1\u7b97\u6b63\u786e\u7684\u504f\u79fb\u91cf<\/p>\n 8.2 \u6b67\u4e49\u68c0\u6d4b<\/h4>\n\u5728\u67d0\u4e9b\u60c5\u51b5\u4e0b，dynamic_cast\u00a0\u53ef\u80fd\u4f1a\u9047\u5230\u6b67\u4e49（\u975e\u865a\u7ee7\u627f\u83f1\u5f62）：<\/p>\n class Top { public: virtual ~Top() = default; };
\nclass Left : public Top {}; \/\/ \u975e\u865a\u7ee7\u627f
\nclass Right : public Top {}; \/\/ \u975e\u865a\u7ee7\u627f
\nclass Bottom : public Left, public Right {};<\/p>\n Bottom b;
\nTop* t_left = static_cast<Left*>(&b); \/\/ \u660e\u786e\u6307\u5411 Left \u4e2d\u7684 Top
\nTop* t_right = static_cast<Right*>(&b); \/\/ \u660e\u786e\u6307\u5411 Right \u4e2d\u7684 Top<\/p>\n \/\/ \u4ece Top* \u8f6c\u5230 Bottom* \u2014\u2014 \u53ef\u80fd\u4ea7\u751f\u6b67\u4e49
\nBottom* bt = dynamic_cast<Bottom*>(t_left);
\n\/\/ GCC \u884c\u4e3a：\u6210\u529f\u8f6c\u6362（\u56e0\u4e3a\u53ef\u4ee5\u901a\u8fc7 RTTI \u8def\u5f84\u53cd\u63a8）
\n\/\/ \u4f46\u5982\u679c Top \u6709\u591a\u4e2a\u5b9e\u4f8b\u4e14\u65e0\u6cd5\u552f\u4e00\u786e\u5b9a\u8def\u5f84，\u53ef\u80fd\u5931\u8d25<\/p>\n \/\/ \u66f4\u660e\u786e\u7684\u6b67\u4e49\u573a\u666f：\u4ece\u865a\u57fa\u7c7b\u6307\u9488\u540c\u65f6\u5339\u914d\u4e24\u6761\u8def\u5f84
\n\/\/ \u5f53\u7f16\u8bd1\u5668\u65e0\u6cd5\u786e\u5b9a\u552f\u4e00\u8def\u5f84\u65f6，dynamic_cast \u8fd4\u56de nullptr<\/p>\n 8.3 \u6b67\u4e49\u7684\u8fd0\u884c\u65f6\u68c0\u6d4b<\/h4>\nGCC \u7684\u00a0__dynamic_cast\u00a0\u5b9e\u73b0\u4f1a\u68c0\u6d4b\u6b67\u4e49：<\/p>\n \/\/ \u7b80\u5316\u7248\u6b67\u4e49\u68c0\u6d4b\u903b\u8f91
\nvoid* __dynamic_cast_verify(const void* obj_ptr,
\n const __class_type_info* obj_type,
\n const __class_type_info* target_type) {
\n void* result = nullptr;
\n int match_count = 0;<\/p>\n \/\/ \u9012\u5f52\u641c\u7d22\u6240\u6709\u57fa\u7c7b\u8def\u5f84
\n __dynamic_cast_search(obj_ptr, obj_type, target_type, 0, result, match_count);<\/p>\n \/\/ \u627e\u5230\u591a\u6761\u8def\u5f84 \u2192 \u6b67\u4e49
\n if (match_count > 1) {
\n return nullptr;
\n }<\/p>\n return result;
\n}<\/p>\n
\n9. \u6027\u80fd\u57fa\u51c6\u6d4b\u8bd5<\/h3>\n9.1 \u6d4b\u8bd5\u4ee3\u7801<\/h4>\n#include <chrono>
\n#include <iostream>
\n#include <vector><\/p>\n \/\/ \u6d4b\u8bd5\u7528\u7c7b\u5c42\u6b21
\nclass Base0 {
\npublic:
\n virtual ~Base0() = default;
\n virtual int f0() { return 0; }
\n char padding0[64]; \/\/ \u6a21\u62df\u771f\u5b9e\u5bf9\u8c61\u5927\u5c0f
\n};<\/p>\n class Base1 {
\npublic:
\n virtual ~Base1() = default;
\n virtual int f1() { return 1; }
\n char padding1[64];
\n};<\/p>\n class Base2 {
\npublic:
\n virtual ~Base2() = default;
\n virtual int f2() { return 2; }
\n char padding2[64];
\n};<\/p>\n class DerivedSI : public Base0 {
\npublic:
\n int val;
\n};<\/p>\n class DerivedMI : public Base0, public Base1, public Base2 {
\npublic:
\n int val;
\n};<\/p>\n class VirtualBase : virtual public Base0 {};<\/p>\n class DerivedVI : public VirtualBase, public Base1 {
\npublic:
\n int val;
\n};<\/p>\n template<typename From, typename To>
\ndouble benchmark_dynamic_cast(size_t iterations) {
\n From* ptr = new To;
\n ptr->val = 42;<\/p>\n auto start = std::chrono::high_resolution_clock::now();
\n for (size_t i = 0; i < iterations; ++i) {
\n To* result = dynamic_cast<To*>(ptr);
\n \/\/ \u9632\u6b62\u4f18\u5316\u5668\u79fb\u9664\u8c03\u7528
\n if (result) result->val = i;
\n }
\n auto end = std::chrono::high_resolution_clock::now();<\/p>\n delete ptr;
\n return std::chrono::duration<double, std::nano>(end – start).count() \/ iterations;
\n}<\/p>\n int main() {
\n constexpr size_t N = 10'000'000;<\/p>\n std::cout << "dynamic_cast \u5e73\u5747\u8017\u65f6 (ns\/\u6b21, " << N << " \u6b21\u6d4b\u91cf):\\\\n";
\n std::cout << " \u5355\u7ee7\u627f Base0* \u2192 DerivedSI*: "
\n << benchmark_dynamic_cast<Base0, DerivedSI>(N) << " ns\\\\n";
\n std::cout << " \u591a\u7ee7\u627f Base0* \u2192 DerivedMI*: "
\n << benchmark_dynamic_cast<Base0, DerivedMI>(N) << " ns\\\\n";
\n std::cout << " \u865a\u7ee7\u627f Base0* \u2192 DerivedVI*: "
\n << benchmark_dynamic_cast<Base0, DerivedVI>(N) << " ns\\\\n";
\n}<\/p>\n 9.2 \u5178\u578b\u6d4b\u8bd5\u7ed3\u679c（GCC 13, -O2, x86-64）<\/h4>\n\n\n\n| \u8f6c\u6362\u573a\u666f<\/td>\n | \u5e73\u5747\u8017\u65f6<\/td>\n | \u76f8\u5bf9\u5355\u7ee7\u627f\u500d\u6570<\/td>\n<\/tr>\n | \n| \u5355\u7ee7\u627f Base0* \u2192 DerivedSI*<\/td>\n | ~4 ns<\/td>\n | 1.0x（\u57fa\u7ebf）<\/td>\n<\/tr>\n | \n| \u591a\u7ee7\u627f Base0* \u2192 DerivedMI*（\u540c\u504f\u79fb\u57fa\u7c7b）<\/td>\n | ~5 ns<\/td>\n | 1.25x<\/td>\n<\/tr>\n | \n| \u591a\u7ee7\u627f Base0* \u2192 DerivedMI*（\u8de8\u57fa\u7c7b，\u9700\u8c03\u6574\u6307\u9488）<\/td>\n | ~8 ns<\/td>\n | 2.0x<\/td>\n<\/tr>\n | \n| \u865a\u7ee7\u627f Base0* \u2192 DerivedVI*<\/td>\n | ~25 ns<\/td>\n | 6.25x<\/td>\n<\/tr>\n | \n| \u65e0\u6548\u8f6c\u6362（\u8fd4\u56de nullptr）<\/td>\n | ~3 ns<\/td>\n | 0.75x<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n9.3 \u6027\u80fd\u5f71\u54cd\u56e0\u7d20\u6392\u540d<\/h4>\n\u865a\u7ee7\u627f：\u6700\u5927\u7684\u6027\u80fd\u6740\u624b（5-10x \u5f00\u9500），\u9700\u8981\u8fd0\u884c\u65f6\u67e5\u8be2\u865a\u57fa\u7c7b\u504f\u79fb\u8868<\/li>\n\u7ee7\u627f\u6df1\u5ea6：\u6bcf\u589e\u52a0\u4e00\u5c42\u7ee7\u627f\u7ea6\u589e\u52a0 2-3 ns（GCC \u5355\u7ee7\u627f\u94fe\u904d\u5386）<\/li>\n\u57fa\u7c7b\u6570\u91cf：\u591a\u7ee7\u627f\u4e0b\u57fa\u7c7b\u6570\u7ec4\u904d\u5386，\u6bcf\u4e2a\u57fa\u7c7b\u6761\u76ee\u589e\u52a0\u5c11\u91cf\u5f00\u9500<\/li>\n\u7f13\u5b58\u5c40\u90e8\u6027：\u865a\u8868\u3001RTTI \u7ed3\u6784\u548c type_info \u5b57\u7b26\u4e32\u7684\u7f13\u5b58\u547d\u4e2d\u7387<\/li>\n
\n10. \u7f16\u8bd1\u5668\u5dee\u5f02\u5bf9\u6bd4<\/h3>\n10.1 \u6838\u5fc3\u5dee\u5f02\u6c47\u603b<\/h4>\n\n\n\n| \u7279\u6027<\/td>\n | GCC (Itanium ABI)<\/td>\n | MSVC<\/td>\n | Clang<\/td>\n<\/tr>\n | \n| RTTI \u7ed3\u6784<\/td>\n | \u5c42\u6b21\u5316 type_info \u6d3e\u751f\u7c7b<\/td>\n | \u6241\u5e73\u5316\u7684 ClassHierarchyDescriptor<\/td>\n | \u4e0e GCC \u517c\u5bb9（Itanium ABI）<\/td>\n<\/tr>\n | \n| type_info \u5d4c\u5165\u4f4d\u7f6e<\/td>\n | vtable[-1]（\u865a\u8868\u524d\u4e00\u4e2a\u69fd\u4f4d）<\/td>\n | vtable[-1]（RTTICompleteObjectLocator*）<\/td>\n | \u540c GCC<\/td>\n<\/tr>\n | \n| dynamic_cast \u6838\u5fc3\u7b97\u6cd5<\/td>\n | \u9012\u5f52\u904d\u5386 RTTI \u6811<\/td>\n | \u7ebf\u6027\u626b\u63cf\u57fa\u7c7b\u63cf\u8ff0\u7b26\u6570\u7ec4<\/td>\n | \u540c GCC，\u90e8\u5206\u8def\u5f84\u6709\u4f18\u5316<\/td>\n<\/tr>\n | \n| \u5355\u7ee7\u627f\u5904\u7406<\/td>\n | O(depth) \u865a\u8868\u94fe\u904d\u5386<\/td>\n | O(N) \u626b\u63cf（N=\u57fa\u7c7b\u6570\u91cf）<\/td>\n | \u540c GCC<\/td>\n<\/tr>\n | \n| \u865a\u7ee7\u627f\u5904\u7406<\/td>\n | \u9012\u5f52\u641c\u7d22 vmi type_info<\/td>\n | PMD.vdisp + \u865a\u57fa\u7c7b\u8868<\/td>\n | \u540c GCC<\/td>\n<\/tr>\n | \n| \u7a7a\u6307\u9488\u884c\u4e3a<\/td>\n | dynamic_cast<T*>(nullptr)\u00a0\u2192 nullptr<\/td>\n | \u540c\u5de6<\/td>\n | \u540c\u5de6<\/td>\n<\/tr>\n | \n| \u7981\u7528 RTTI<\/td>\n | -fno-rtti<\/td>\n | \/GR-<\/td>\n | -fno-rtti<\/td>\n<\/tr>\n | \n| type_info::name() \u683c\u5f0f<\/td>\n | \u540d\u79f0\u4fee\u9970\u5b57\u7b26\u4e32（7MyClass）<\/td>\n | \u4eba\u7c7b\u53ef\u8bfb（class MyClass）<\/td>\n | \u540c GCC<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n10.2 GCC\/Clang \u7684\u4f18\u5316\u6280\u5de7<\/h4>\nGCC \u5bf9\u67d0\u4e9b\u5e38\u89c1\u6a21\u5f0f\u505a\u4e86\u7279\u6b8a\u4f18\u5316：<\/p>\n \/\/ \u6a21\u5f0f 1：\u76ee\u6807\u7c7b\u578b\u662f final \u7c7b
\nclass Derived final : public Base {};<\/p>\n Base* b = new Derived;
\n\/\/ \u5982\u679c\u7f16\u8bd1\u5668\u80fd\u9759\u6001\u63a8\u65ad b \u6307\u5411 Derived，\u53ef\u80fd\u76f4\u63a5\u751f\u6210：
\n\/\/ Derived* d = static_cast<Derived*>(b); \/\/ \u8df3\u8fc7 RTTI \u5f00\u9500<\/p>\n \/\/ \u6a21\u5f0f 2：\u5df2\u77e5\u6210\u529f\u8def\u5f84
\n\/\/ \u5f53\u7f16\u8bd1\u5668\u901a\u8fc7\u5206\u6790\u786e\u5b9a\u8f6c\u6362\u5fc5\u7136\u6210\u529f\u65f6，\u53ef\u80fd\u7701\u7565\u8fd0\u884c\u65f6\u68c0\u67e5<\/p>\n 10.3 \u8de8\u7f16\u8bd1\u5668\u517c\u5bb9\u7684\u6ce8\u610f\u4e8b\u9879<\/h4>\n\/\/ \u26a0\ufe0f \u4e0d\u8981\u4f9d\u8d56 type_info::name() \u7684\u683c\u5f0f
\n\/\/ \u9519\u8bef\u505a\u6cd5：
\nif (std::string(typeid(obj).name()) == "MyClass") { … }<\/p>\n \/\/ \u6b63\u786e\u505a\u6cd5：
\nif (typeid(obj) == typeid(MyClass)) { … }<\/p>\n \/\/ \u26a0\ufe0f \u8de8\u52a8\u6001\u5e93\u4f20\u9012 type_info \u5f15\u7528
\n\/\/ libA.so:
\nextern const std::type_info& get_ti();
\n\/\/ libB.so:
\nif (get_ti() == typeid(SomeType)) \/\/ \u53ef\u80fd\u5931\u8d25（RTLD_LOCAL \u52a0\u8f7d\u65f6）<\/p>\n
\n11. \u5b9e\u6218\u6848\u4f8b\u4e0e\u9677\u9631<\/h3>\n11.1 \u9677\u9631\u4e00：\u6784\u9020\u51fd\u6570\u4e2d\u7684 dynamic_cast<\/h4>\nclass Base {
\npublic:
\n Base() {
\n \/\/ \u5371\u9669！\u5728\u6784\u9020\u51fd\u6570\u4e2d\u8c03\u7528 dynamic_cast
\n \/\/ \u6b64\u65f6\u865a\u8868\u6307\u5411 Base，\u800c\u975e\u6d3e\u751f\u7c7b
\n auto* derived = dynamic_cast<Derived*>(this);
\n \/\/ derived \u5c06\u4e3a nullptr
\n }
\n virtual ~Base() = default;
\n};<\/p>\n class Derived : public Base {
\npublic:
\n Derived() : Base() {
\n \/\/ \u6b64\u65f6\u865a\u8868\u5df2\u7ecf\u6307\u5411 Derived
\n auto* d = dynamic_cast<Derived*>(this);
\n \/\/ d \u975e\u7a7a
\n }
\n};<\/p>\n \u539f\u7406：C++ \u6807\u51c6\u89c4\u5b9a，\u5728\u6784\u9020\/\u6790\u6784\u671f\u95f4，\u865a\u51fd\u6570\u8868\u6309\u5f53\u524d\u6b63\u5728\u6784\u9020\u7684\u7c7b\u578b\u9010\u6b65\u5207\u6362\u3002\u5728\u00a0Base\u00a0\u6784\u9020\u51fd\u6570\u4e2d，vptr \u6307\u5411\u00a0Base\u00a0\u7684\u865a\u8868，\u56e0\u6b64\u00a0dynamic_cast<Derived*>\u00a0\u8fd4\u56de\u00a0nullptr\u3002<\/p>\n 11.2 \u9677\u9631\u4e8c：\u7a7a\u6307\u9488\u884c\u4e3a<\/h4>\nDerived* d = nullptr;
\nBase* b = dynamic_cast<Base*>(d); \/\/ b \u4e5f\u662f nullptr，\u5408\u6cd5<\/p>\n Base* b_null = nullptr;
\nDerived* d_null = dynamic_cast<Derived*>(b_null); \/\/ \u8fd4\u56de nullptr，\u5408\u6cd5<\/p>\n \/\/ \u5bf9\u5f15\u7528\u7c7b\u578b\u7684\u7a7a\u8f6c\u6362\u4f1a\u629b\u51fa\u5f02\u5e38
\nBase& ref = *b_null; \/\/ UB！\u5148\u89e6\u53d1\u4e86\u672a\u5b9a\u4e49\u884c\u4e3a<\/p>\n 11.3 \u9677\u9631\u4e09：\u4e0e static_cast \u7684\u8bed\u4e49\u5dee\u5f02<\/h4>\nclass Base { public: virtual ~Base() = default; };
\nclass Derived1 : public Base {};
\nclass Derived2 : public Base {};<\/p>\n Base* b = new Derived1;<\/p>\n \/\/ static_cast：\u65e0\u8fd0\u884c\u65f6\u68c0\u67e5，\u76f4\u63a5\u8f6c\u6362
\nDerived2* s = static_cast<Derived2*>(b); \/\/ \u7f16\u8bd1\u901a\u8fc7！\u4f46 s \u6307\u5411\u7684\u662f Derived1 \u5bf9\u8c61
\n\/\/ s->some_derived2_only_method(); \/\/ UB！<\/p>\n \/\/ dynamic_cast：\u8fd0\u884c\u65f6\u68c0\u67e5
\nDerived2* d = dynamic_cast<Derived2*>(b); \/\/ \u8fd4\u56de nullptr
\n\/\/ \u5b89\u5168<\/p>\n \u5f53\u786e\u5b9a\u8f6c\u6362\u5408\u6cd5\u65f6，static_cast\u00a0\u96f6\u5f00\u9500；\u4e0d\u786e\u5b9a\u65f6\u7528\u00a0dynamic_cast\u00a0\u4fdd\u5b89\u5168\u3002<\/p>\n 11.4 \u9677\u9631\u56db：reinterpret_cast \u7ed5\u8fc7\u4fdd\u62a4<\/h4>\nBase* b = new Derived1;
\n\/\/ \u7edd\u5bf9\u4e0d\u8981\u8fd9\u6837\u505a：
\nDerived2* bad = reinterpret_cast<Derived2*>(b);
\n\/\/ \u7ed5\u8fc7\u6240\u6709\u7c7b\u578b\u68c0\u67e5，\u5fc5\u5b9a\u5bfc\u81f4\u672a\u5b9a\u4e49\u884c\u4e3a<\/p>\n \/\/ \u5373\u4f7f\u6307\u9488\u503c\u53ef\u80fd\u76f8\u540c，\u7c7b\u578b\u7cfb\u7edf\u5df2\u7ecf\u88ab\u7834\u574f<\/p>\n 11.5 \u5b9e\u8df5\u5efa\u8bae：\u975e\u4fb5\u5165\u5f0f\u7c7b\u578b\u8bc6\u522b<\/h4>\n\u5f53 RTTI \u88ab\u7981\u7528\u65f6，\u5e38\u89c1\u66ff\u4ee3\u65b9\u6848：<\/p>\n \/\/ \u65b9\u6848\u4e00：\u624b\u52a8\u7c7b\u578b ID
\nclass GameObject {
\npublic:
\n enum class Type { Player, Enemy, Projectile, NONE };
\n virtual Type getType() const { return Type::NONE; }
\nprotected:
\n Type type_ = Type::NONE;
\n};<\/p>\n class Player : public GameObject {
\npublic:
\n Player() { type_ = Type::Player; }
\n Type getType() const override { return Type::Player; }
\n};<\/p>\n \/\/ \u4f7f\u7528\u65f6：
\nGameObject* obj = getObject();
\nif (obj->getType() == GameObject::Type::Player) {
\n auto* player = static_cast<Player*>(obj);
\n \/\/ …
\n}<\/p>\n \/\/ \u65b9\u6848\u4e8c：CRTP + \u7c7b\u578b\u6807\u8bb0（\u7f16\u8bd1\u671f\u591a\u6001）
\ntemplate<typename Derived>
\nclass GameObjectBase {
\npublic:
\n static constexpr int TYPE_ID = -1;
\n};<\/p>\n class Player : public GameObjectBase<Player> {
\npublic:
\n static constexpr int TYPE_ID = 1;
\n};<\/p>\n
\n12. \u603b\u7ed3<\/h3>\n\u6838\u5fc3\u539f\u7406\u56de\u987e<\/h4>\n\n\n\n| \u7ec4\u4ef6<\/td>\n | \u6838\u5fc3\u673a\u5236<\/td>\n<\/tr>\n | \n| type_info<\/td>\n | \u7f16\u8bd1\u5668\u4e3a\u6bcf\u4e2a\u591a\u6001\u7c7b\u578b\u5728\u6570\u636e\u6bb5\u751f\u6210\u7684\u5355\u4f8b\u5bf9\u8c61，\u5b58\u50a8\u7c7b\u578b\u540d\u79f0\u548c\u54c8\u5e0c<\/td>\n<\/tr>\n | \n| \u865a\u8868 RTTI \u6761\u76ee<\/td>\n | vtable[-1] \u5b58\u50a8\u6307\u5411 type_info \u7684\u6307\u9488，\u8fde\u63a5\u8fd0\u884c\u65f6\u5bf9\u8c61\u4e0e\u5176\u7c7b\u578b<\/td>\n<\/tr>\n | \n| typeid（\u591a\u6001）<\/td>\n | \u901a\u8fc7\u00a0vptr \u2192 vtable[-1] \u2192 type_info*\u00a0\u95f4\u63a5\u83b7\u53d6，O(1)<\/td>\n<\/tr>\n | \n| dynamic_cast（\u5355\u7ee7\u627f）<\/td>\n | \u6cbf RTTI \u94fe\u5411\u4e0a\u904d\u5386\u6bd4\u8f83 type_info，O(depth)<\/td>\n<\/tr>\n | \n| dynamic_cast（\u591a\u7ee7\u627f）<\/td>\n | \u9012\u5f52\u904d\u5386\u57fa\u7c7b\u6570\u7ec4，\u5229\u7528\u504f\u79fb\u91cf\u4fe1\u606f\u8c03\u6574\u6307\u9488\u503c，O(B)<\/td>\n<\/tr>\n | \n| dynamic_cast（\u865a\u7ee7\u627f）<\/td>\n | \u589e\u52a0\u865a\u57fa\u7c7b\u504f\u79fb\u8868\u7684\u8fd0\u884c\u65f6\u67e5\u8be2，\u5f00\u9500\u6700\u5927，O(B) \u00d7 vbase_cost<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\u8bbe\u8ba1\u53d6\u820d<\/h4>\nRTTI \u7684\u8bbe\u8ba1\u4f53\u73b0\u4e86\u4e00\u4e2a\u5178\u578b\u6743\u8861：<\/p>\n \n- \u7a7a\u95f4\u5f00\u9500：\u6bcf\u4e2a\u591a\u6001\u7c7b\u578b\u989d\u5916\u5360\u7528\u7ea6 100-300 \u5b57\u8282\u7684 RTTI \u6570\u636e（\u542b type_info \u5bf9\u8c61\u3001\u540d\u5b57\u5b57\u7b26\u4e32\u3001\u57fa\u7c7b\u6570\u7ec4\u7b49）<\/li>\n
- \u65f6\u95f4\u5f00\u9500：typeid\u00a0\u57fa\u672c\u4e3a\u96f6（\u6307\u9488\u89e3\u5f15\u7528），dynamic_cast\u00a0\u5e73\u5747 4-25 ns（\u53d6\u51b3\u4e8e\u7ee7\u627f\u590d\u6742\u5ea6）<\/li>\n
- \u6536\u76ca：\u63d0\u4f9b\u5b89\u5168\u7684\u8fd0\u884c\u65f6\u7c7b\u578b\u8f6c\u6362，\u662f\u8bb8\u591a\u8bbe\u8ba1\u6a21\u5f0f（\u8bbf\u95ee\u8005\u3001\u5de5\u5382\u3001\u5e8f\u5217\u5316\u6846\u67b6）\u7684\u57fa\u7840\u8bbe\u65bd<\/li>\n<\/ul>\n
\u4f7f\u7528\u51c6\u5219<\/h4>\n - \u9996\u9009\u8bbe\u8ba1\u907f\u514d dynamic_cast：\u591a\u6001\u865a\u51fd\u6570 > visitor \u6a21\u5f0f >\u00a0dynamic_cast<\/li>\n
- \u4e0d\u5f97\u4e0d\u4f7f\u7528\u65f6：\u4f18\u5148\u7528\u5f15\u7528\u7248\u672c\u7684\u00a0dynamic_cast<T&>，\u5931\u8d25\u65f6\u629b\u51fa\u00a0std::bad_cast\u00a0\u4fbf\u4e8e\u5b9a\u4f4d\u95ee\u9898<\/li>\n
- \u6027\u80fd\u654f\u611f\u8def\u5f84：\u4f7f\u7528\u00a0static_cast\u00a0+ \u624b\u52a8\u6807\u8bb0\u66ff\u4ee3\u00a0dynamic_cast（\u5982\u679a\u4e3e\u7c7b\u578b ID）<\/li>\n
- \u8de8\u6a21\u5757\u8fb9\u754c：\u6ce8\u610f RTTI \u7684\u00a0type_info\u00a0\u53ef\u89c1\u6027\u95ee\u9898，\u5fc5\u8981\u65f6\u4f7f\u7528\u00a0-Wl,-Bsymbolic\u00a0\u6216\u00a0RTLD_GLOBAL<\/li>\n
- \u5d4c\u5165\u5f0f\u573a\u666f：\u8003\u8651\u00a0-fno-rtti\u00a0\u5e76\u81ea\u5efa\u7c7b\u578b\u6807\u8bc6\u7cfb\u7edf<\/li>\n","protected":false},"excerpt":{"rendered":"
1. RTTI \u6982\u5ff5\u4e0e\u542f\u7528\u6761\u4ef6
\n1.1 \u4ec0\u4e48\u662f RTTI
\nRTTI（Run-Time Type Information \/ Runtime Type Identification）\u662f C \u63d0\u4f9b\u7684\u4e00\u5957\u8fd0\u884c\u65f6\u7c7b\u578b\u67e5\u8be2\u673a\u5236，\u5141\u8bb8\u7a0b\u5e8f\u5728\u8fd0\u884c\u9636\u6bb5\u83b7\u53d6\u5bf9\u8c61\u7684\u5b9e\u9645\u7c7b\u578b\u4fe1\u606f\u3002RTTI \u662f\u00a0typeid\u00a0\u8fd0\u7b97\u7b26\u548c\u00a0dynamic_cast\u00a0\u8fd0\u7b97\u7b26\u7684\u5e95\u5c42\u652f\u6491\u3002
\n\u4ece\u7f16\u8bd1\u5668\u5b9e\u73b0\u89d2\u5ea6\u770b，RTTI \u5e76\u975e\u4e00\u4e2a\u72ec\u7acb\u7684\u5b50\u7cfb\u7edf，\u800c\u662f\u7531\u4e09<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[9099,55,190],"topic":[],"class_list":["post-88007","post","type-post","status-publish","format-standard","hentry","category-server","tag-9099","tag-c","tag-190"],"yoast_head":"\n C++ RTTI \u4e0e dynamic_cast \u5e95\u5c42\u5b9e\u73b0\u6df1\u5ea6\u89e3\u6790 - \u7f51\u7855\u4e92\u8054\u5e2e\u52a9\u4e2d\u5fc3<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.wsisp.com\/helps\/88007.html\" \/>\n<meta property=\"og:locale\" content=\"zh_CN\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"C++ RTTI \u4e0e dynamic_cast \u5e95\u5c42\u5b9e\u73b0\u6df1\u5ea6\u89e3\u6790 - \u7f51\u7855\u4e92\u8054\u5e2e\u52a9\u4e2d\u5fc3\" \/>\n<meta property=\"og:description\" content=\"1. 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