[C++] 深入探索 C++ 多态 ① - 虚函数调用链路

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/* g++ -std='c++11' test.cpp -o t && ./t */
#include <iostream>

class Model {
   public:
    virtual void face() {
        std::cout << "model's face!\n";
    }
};

class Gril : public Model {
   public:
    void face() override {
        std::cout << "girl's face!\n";
    }
};

class Man : public Model {
   public:
    void face() override {
        std::cout << "man's face!\n";
    }
};

class Boy : public Model {
   public:
    void face() override {
        std::cout << "boy's face!\n";
    }
};

void takePhoto(Model& m) {
    m.face();
}

int main() {
    Model model;
    Gril girl;
    Man man;
    Boy boy;

    takePhoto(model);
    takePhoto(girl);
    takePhoto(man);
    takePhoto(boy);
    return 0;
}

// 输出：
// model's face!
// girl's face!
// man's face!
// boy's face!

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/* /usr/include/c++/4.8.2/typeinfo */
// The type_info class describes type information generated by an implementation.
class type_info {
 protected:
    const char* __name;
};

/* /usr/include/c++/4.8.2/cxxabi.h */
// Type information for a class.
class __class_type_info : public std::type_info {
 public:
    ...
};

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/* /usr/include/c++/4.8.2/cxxabi.h */
// Type information for a class with a single non-virtual base.
class __si_class_type_info : public __class_type_info {
   public:
    const __class_type_info *__base_type;
    ...
};

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/* /usr/include/c++/4.8.2/cxxabi.h */
// Helper class for __vmi_class_type.
class __base_class_type_info {
   public:
    const __class_type_info *__base_type;  // Base class type.
#ifdef _GLIBCXX_LLP64
    long long __offset_flags;  // Offset and info.
#else
    long __offset_flags;  // Offset and info.
#endif
    ...
};

// Type information for a class with multiple and/or virtual bases.
class __vmi_class_type_info : public __class_type_info {
   public:
    unsigned int __flags;       // Details about the class hierarchy.
    unsigned int __base_count;  // Number of direct bases.

    // The array of bases uses the trailing array struct hack so this
    // class is not constructable with a normal constructor. It is
    // internally generated by the compiler.
    __base_class_type_info __base_info[1];  // Array of bases.
    ...
};

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/* /usr/src/debug/gcc-4.8.5-20150702/libstdc++-v3/libsupc++/tinfo.h */
// Initial part of a vtable, this structure is used with offsetof, so we don't
// have to keep alignments consistent manually.
struct vtable_prefix {
    // Offset to most derived object.
    ptrdiff_t whole_object;

    // Additional padding if necessary.
#ifdef _GLIBCXX_VTABLE_PADDING
    ptrdiff_t padding1;
#endif

    // Pointer to most derived type_info.
    const __class_type_info *whole_type;

    // Additional padding if necessary.
#ifdef _GLIBCXX_VTABLE_PADDING
    ptrdiff_t padding2;
#endif

    // What a class's vptr points to.
    const void *origin;
};

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// g++ -g -O0 -std=c++11 -fdump-class-hierarchy test_virtual.cpp -o t
#include <iostream>

class A {
   public:
    int m_a = 0;
    virtual void vfuncA1() {}
    virtual void vfuncA2() {}
};

int main(int argc, char** argv) {    
    A* a = new A;
    a->vfuncA2();
    return 0;
}