VC++ 2005 编译器 Data Member 的布局

《深度探索C++对象模型》P.92 L.18：

C++ Standard 要求，在同一个 access section（也就是 private、public、protected 等区段）中，member 的排列只需符合“较晚出现的 member 在 class object 中有较高的地址”这一条件即可。也就是说，各个 member 并不一定得连续排列。（注：最后这句与本文内容无关）

而书上某个地方说 C++ 标准对各个 access section 的 member 并没有要求，为了弄清楚 VC++ 编译器中的布局，写了下面的测试代码。

// VC++ 2005 #include <stdio.h> class Base1 {public: Base1(int a, int b, int c) : _i_base1_public(a) , _i_base1_protected(b) , _i_base1_private(c) { } static void show_layout() { printf("&Base1::_i_base1_public : 0x%p/n", &Base1::_i_base1_public); printf("&Base1::_i_base1_protected: 0x%p/n", &Base1::_i_base1_protected); printf("&Base1::_i_base1_private : 0x%p/n", &Base1::_i_base1_private); }public: int _i_base1_public; protected: int _i_base1_protected; private: int _i_base1_private;}; // end of class Base1 class Base2{public: Base2(int a, int b, int c) : _i_base2_protected(a) , _i_base2_private(b) , _i_base2_public(c) { } static void show_layout() { printf("&Base2::_i_base2_protected: 0x%p/n", &Base2::_i_base2_protected); printf("&Base2::_i_base2_private : 0x%p/n", &Base2::_i_base2_private); printf("&Base2::_i_base2_public : 0x%p/n", &Base2::_i_base2_public); }protected: int _i_base2_protected; private: int _i_base2_private;public: int _i_base2_public; }; // end of class Base2 class Base3{public: public: Base3(int a, int b, int c) : _i_base3_private(a) , _i_base3_protected(b) , _i_base3_public(c) { } static void show_layout() { printf("&Base3::_i_base3_private : 0x%p/n", &Base3::_i_base3_private); printf("&Base3::_i_base3_protected: 0x%p/n", &Base3::_i_base3_protected); printf("&Base3::_i_base3_public : 0x%p/n", &Base3::_i_base3_public); }private: int _i_base3_private;protected: int _i_base3_protected; public: int _i_base3_public; }; // end of class Base3 class Derived1: public Base1, public Base2, public Base3{public: Derived1(int i1, int i2, int i3, int i4, int i5, int i6, int i7, int i8, int i9, int i0, int ia, int ib) : Base1(i1, i2, i3) , Base2(i4, i5, i6) , Base3(i7, i8, i9) , _i_derived1_public(i0) , _i_derived1_protected(ia) , _i_derived1_private(ib) { } void show_layout() { printf("&Derived1::_i_base1_public : 0x%p/n", &Derived1::_i_base1_public); printf(" &this->_i_base1_public : 0x%p -> 0x%01x/n", &_i_base1_public, _i_base1_public); printf("&Derived1::_i_base1_protected : 0x%p/n", &Derived1::_i_base1_protected); printf(" &this->_i_base1_protected : 0x%p -> 0x%01x/n", &_i_base1_protected, _i_base1_protected); printf(" Base1::_i_base1_private : 0x%p -> 0x%01x */n", (int*)this + 2, *((int*)this + 2)); printf("&Derived1::_i_base2_protected : 0x%p/n", &Derived1::_i_base2_protected); printf(" &this->_i_base2_protected : 0x%p -> 0x%01x/n", &_i_base2_protected, _i_base2_protected); printf(" Base2::_i_base2_private : 0x%p -> 0x%01x */n", (int*)this + 4, *((int*)this + 4)); printf("&Derived1::_i_base2_public : 0x%p/n", &Derived1::_i_base2_public); printf(" &this->_i_base2_public : 0x%p -> 0x%01x/n", &_i_base2_public, _i_base2_public); printf(" Base3::_i_base3_private : 0x%p -> 0x%01x */n", (int*)this + 6, *((int*)this + 6)); printf("&Derived1::_i_base3_protected : 0x%p/n", &Derived1::_i_base3_protected); printf(" &this->_i_base3_protected : 0x%p -> 0x%01x/n", &_i_base3_protected, _i_base3_protected); printf("&Derived1::_i_base3_public : 0x%p/n", &Derived1::_i_base3_public); printf(" &this->_i_base3_public : 0x%p -> 0x%01x/n", &_i_base3_public, _i_base3_public); printf("&Derived1::_i_derived1_public : 0x%p/n", &Derived1::_i_derived1_public); printf(" &this->_i_derived1_public : 0x%p -> 0x%01x/n", &_i_derived1_public, _i_derived1_public); printf("&Derived1::_i_derived1_protected: 0x%p/n", &Derived1::_i_derived1_protected); printf(" &this->_i_derived1_protected: 0x%p -> 0x%01x/n", &_i_derived1_protected, _i_derived1_protected); printf("&Derived1::_i_derived1_private : 0x%p/n", &Derived1::_i_derived1_private); printf(" &this->_i_derived1_private : 0x%p -> 0x%01x/n", &_i_derived1_private, _i_derived1_private); }public: int _i_derived1_public; protected: int _i_derived1_protected; private: int _i_derived1_private;}; // end of class Derived1 class Derived2: public Base3, public Base1, public Base2{public: Derived2(int i1, int i2, int i3, int i4, int i5, int i6, int i7, int i8, int i9, int i0, int ia, int ib) : Base3(i1, i2, i3) , Base1(i4, i5, i6) , Base2(i7, i8, i9) , _i_derived2_private(i0) , _i_derived2_public(ia) , _i_derived2_protected(ib) { } void show_layout() { printf(" Base3::_i_base3_private : 0x%p -> 0x%02x */n", (int*)this, *((int*)this)); printf("&Derived2::_i_base3_protected : 0x%p/n", &Derived2::_i_base3_protected); printf(" &this->_i_base3_protected : 0x%p -> 0x%02x/n", &_i_base3_protected, _i_base3_protected); printf("&Derived2::_i_base3_public : 0x%p/n", &Derived2::_i_base3_public); printf(" &this->_i_base3_public : 0x%p -> 0x%02x/n", &_i_base3_public, _i_base3_public); printf("&Derived2::_i_base1_public : 0x%p/n", &Derived2::_i_base1_public); printf(" &this->_i_base1_public : 0x%p -> 0x%02x/n", &_i_base1_public, _i_base1_public); printf("&Derived2::_i_base1_protected : 0x%p/n", &Derived2::_i_base1_protected); printf(" &this->_i_base1_protected : 0x%p -> 0x%02x/n", &_i_base1_protected, _i_base1_protected); printf(" Base1::_i_base1_private : 0x%p -> 0x%02x */n", (int*)this + 5, *((int*)this + 5)); printf("&Derived2::_i_base2_protected : 0x%p/n", &Derived2::_i_base2_protected); printf(" &this->_i_base2_protected : 0x%p -> 0x%02x/n", &_i_base2_protected, _i_base2_protected); printf(" Base2::_i_base2_private : 0x%p -> 0x%02x */n", (int*)this + 7, *((int*)this + 7)); printf("&Derived2::_i_base2_public : 0x%p/n", &Derived2::_i_base2_public); printf(" &this->_i_base2_public : 0x%p -> 0x%02x/n", &_i_base2_public, _i_base2_public); printf("&Derived2::_i_derived2_private : 0x%p/n", &Derived2::_i_derived2_private); printf(" &this->_i_derived2_private : 0x%p -> 0x%02x/n", &_i_derived2_private, _i_derived2_private); printf("&Derived2::_i_derived2_public : 0x%p/n", &Derived2::_i_derived2_public); printf(" &this->_i_derived2_public : 0x%p -> 0x%02x/n", &_i_derived2_public, _i_derived2_public); printf("&Derived2::_i_derived2_protected: 0x%p/n", &Derived2::_i_derived2_protected); printf(" &this->_i_derived2_protected: 0x%p -> 0x%02x/n", &_i_derived2_protected, _i_derived2_protected); }private: int _i_derived2_private;public: int _i_derived2_public; protected: int _i_derived2_protected; }; // end of class Derived2 int main(){ Base1::show_layout(); printf("/n"); Base2::show_layout(); printf("/n"); Base3::show_layout(); printf("/n"); Derived1 oDerived1(0x0, 0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8, 0x9, 0xa, 0xb); oDerived1.show_layout(); printf("/n"); Derived2 oDerived2(0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb); oDerived2.show_layout(); return 0; } // end of main  

输出：

&Base1::_i_base1_public : 0x00000000&Base1::_i_base1_protected: 0x00000004&Base1::_i_base1_private : 0x00000008 &Base2::_i_base2_protected: 0x00000000&Base2::_i_base2_private : 0x00000004&Base2::_i_base2_public : 0x00000008 &Base3::_i_base3_private : 0x00000000&Base3::_i_base3_protected: 0x00000004&Base3::_i_base3_public : 0x00000008 &Derived1::_i_base1_public : 0x00000000 &this->_i_base1_public : 0x0012FF34 -> 0x0&Derived1::_i_base1_protected : 0x00000004 &this->_i_base1_protected : 0x0012FF38 -> 0x1 Base1::_i_base1_private : 0x0012FF3C -> 0x2 *&Derived1::_i_base2_protected : 0x00000000 &this->_i_base2_protected : 0x0012FF40 -> 0x3 Base2::_i_base2_private : 0x0012FF44 -> 0x4 *&Derived1::_i_base2_public : 0x00000008 &this->_i_base2_public : 0x0012FF48 -> 0x5 Base3::_i_base3_private : 0x0012FF4C -> 0x6 *&Derived1::_i_base3_protected : 0x00000004 &this->_i_base3_protected : 0x0012FF50 -> 0x7&Derived1::_i_base3_public : 0x00000008 &this->_i_base3_public : 0x0012FF54 -> 0x8&Derived1::_i_derived1_public : 0x00000024 &this->_i_derived1_public : 0x0012FF58 -> 0x9&Derived1::_i_derived1_protected: 0x00000028 &this->_i_derived1_protected: 0x0012FF5C -> 0xa&Derived1::_i_derived1_private : 0x0000002C &this->_i_derived1_private : 0x0012FF60 -> 0xb Base3::_i_base3_private : 0x0012FEFC -> 0x00 *&Derived2::_i_base3_protected : 0x00000004 &this->_i_base3_protected : 0x0012FF00 -> 0x11&Derived2::_i_base3_public : 0x00000008 &this->_i_base3_public : 0x0012FF04 -> 0x22&Derived2::_i_base1_public : 0x00000000 &this->_i_base1_public : 0x0012FF08 -> 0x33&Derived2::_i_base1_protected : 0x00000004 &this->_i_base1_protected : 0x0012FF0C -> 0x44 Base1::_i_base1_private : 0x0012FF10 -> 0x55 *&Derived2::_i_base2_protected : 0x00000000 &this->_i_base2_protected : 0x0012FF14 -> 0x66 Base2::_i_base2_private : 0x0012FF18 -> 0x77 *&Derived2::_i_base2_public : 0x00000008 &this->_i_base2_public : 0x0012FF1C -> 0x88&Derived2::_i_derived2_private : 0x00000024 &this->_i_derived2_private : 0x0012FF20 -> 0x99&Derived2::_i_derived2_public : 0x00000028 &this->_i_derived2_public : 0x0012FF24 -> 0xaa&Derived2::_i_derived2_protected: 0x0000002C &this->_i_derived2_protected: 0x0012FF28 -> 0xbb 

结论：

1. 无论 acess section 是什么，以及它们出现的次序，data member 按其在类中声明的次序由低地址向高地址排；

2. 多继承时，各基类的 data member 按基类从左到右的次序先后由低地址向高地址排，派生类的 data member 出现在这些之后。

注：

从打印值可以看出，对派生类继承的基类的 data member 取指向 Data Member 的指针值，它们不是相对于派生类对象起始地址的，而是相对于各基类的 subobject。