C++对象内存布局：单继承，多继承，虚继承

0.前言

本文的讨论了类实例对象的内存分布，程序由visual studio2013编写。如果你想看懂 ，请认真看内存分布中的地址

1. 一个简单的例子

class A{ int a=1; char b=2;//内存对齐 char c=3;//内存对齐 }; A instance;

1.1 内存分布

地址变量名值0x0093f9a0instance-0x0093f9a0a0x000000010x0093f9a4b，c0xcccc0302(-859045118)

1.2说明

instance与a的地址相同

1.3 测试代码

int *sp = (int *)&instance; cout << *(sp++) << endl; cout << *(sp) << endl; 输出: 1 -859045118

2.单继承

class Base{ int a=1; char b=2; char c = 3; virtual void f1(){ cout << "Base::f1()" << endl; } }; class Derived : private Base{ int d=4; void f1(){ cout << "Derived::f1()" << endl; } virtual void f2(){ cout << "Derived::f2()" << endl; } void f3(){ cout << "Derived::f3()" << endl; } }; Derived d;

2.1 内存分布

地址变量名值0x00b8f7c0d-0x00b8f7c0_vfptr虚函数表地址-0x00b8f7c4Base::a0x000000010x00b8f7c8Base::b，Base::c0xcccc0302(-859045118)0x00b8f7ccDerived::d0x00000001

2.2说明

d与_vfptr 的地址相同，说明对象的起始位置存放是虚函数表地址不管是私有继承还是，私有变量，都能在2.3的测试代码中直接访问单继承只有1个虚函数表，虚函数依次放在表中

2.3 测试代码

int *dp = (int *)&d; //直接使用虚函数 typedef void(*fptr)(void); //f1 int vfptr_address = *dp;//虚函数表地址 int f1_address = ((int *)vfptr_address)[0];//虚函数表的第一个函数地址 fptr f1p = (fptr)f1_address;//函数f指针 f1p();//调用函数f，输出 "Derived::f1()" 原因 子类重写了父类的虚函数 //f2 int f2_address = ((int *)vfptr_address)[1];//虚函数表的第二个函数地址 fptr f2p = (fptr)f2_address;//函数f指针 f2p();//调用函数f，输出 "Derived::f2()" ， //fptr fp = (fptr)(((int*)(*sp))[0]);//简写 cout << hex << dp <<","<< hex << *(dp) << endl;//vfptr 虚函数表地址， dp++; cout << hex << dp << "," << hex << *(dp) << endl;//a dp++; cout << hex << dp << "," << hex << *(dp) << endl;//b,c dp++; cout << hex << dp << "," << hex << *(dp) << endl;//d

3.多继承

class Base1{ int a=1; char b=2; char c = 3; virtual void f1(){ cout << "Base1::f1()" << endl; } virtual void f2(){ cout << "Base1::f2()" << endl; } }; class Base2{ int a = 1; char b = 2; char c = 3; virtual void f1(){ cout << "Base2::f1()" << endl; } virtual void f2(){ cout << "Base2::f2()" << endl; } }; class Derived : private Base1, Base2{ int d=4; void f1(){ cout << "Derived::f1()" << endl; } void f3(){ cout << "Derived::f3()" << endl; } }; Derived d; //向上转型为Base1 int* b1p = (int *)(Base1*)&d; //向上转型为Base2 int* b2p = (int *)(Base2*)&d;

3.1 内存分布

地址变量名值0x0116f8b8&d，b1p , Base1::_vfptr虚函数表地址-0x0116f8bcBase1::a0x000000010x0116f8c0Base1::b，Base1::c0xcccc0302(-859045118)0x0116f8c4b2p , Base2::_vfptr虚函数表地址-0x0116f8c8Base2::a0x000000010x0116f8ccBase2::b，Base2::c0xcccc0302(-859045118)0x0116f8d0Derived::d0x00000001

3.2说明

可以认为派生类对象中包含基类对象，基类对象 按继承顺序放在派生类对象中函数重写时，可以看成单继承分别对基类函数中的虚函数重写每次继承都创建一个虚表向上转型 实质为对象偏移量的改变，即dynamic_cast的作用。 看：b1p ,b2p 在内存中的分布

3.3 测试代码

//Base1虚函数调用 int b1_vfptr_address = *b1p;//虚函数表地址 int b1_f1_address = ((int *)b1_vfptr_address)[0];//虚函数表的第一个函数地址 fptr b1_f1p = (fptr)b1_f1_address;//函数f指针 b1_f1p();//调用函数f，输出 "Derived::f1()" 原因 子类重写了父类的虚函数 int b1_f2_address = ((int *)b1_vfptr_address)[1];//虚函数表的第二个函数地址 fptr b1_f2p = (fptr)b1_f2_address;//函数f指针 b1_f2p();//输出 "Base1::f2()" ， int b1_f3_address = ((int *)b1_vfptr_address)[2];//虚函数表的第二个函数地址 fptr b1_f3p = (fptr)b1_f3_address;//函数f指针 b1_f3p();//输出 "Derived::f3()" //Base2虚函数调用 int b2_f1_address = ((int *)b2_vfptr_address)[0];//虚函数表的第一个函数地址 fptr b2_f1p = (fptr)b2_f1_address;//函数f指针 b2_f1p();//调用函数f，输出 "Derived::f1()" 原因 子类重写了父类的虚函数 int b2_f2_address = ((int *)b2_vfptr_address)[1];//虚函数表的第二个函数地址 fptr b2_f2p = (fptr)b2_f2_address;//函数f指针 b2_f2p();//调用函数f，输出 "Base2::f2()" ， int b2_f3_address = ((int *)b1_vfptr_address)[2];//虚函数表的第二个函数地址 fptr b2_f3p = (fptr)b1_f3_address;//函数f指针 b2_f3p();//调用函数f，输出 "Derived::f3()" ， cout << hex << b1p << "," << hex << *(b1p) << endl;//Base1::_vfptr 虚函数表地址， b1p++; cout << hex << b1p << "," << hex << *(b1p) << endl;//Base1::a b1p++; cout << hex << b1p << "," << hex << *(b1p) << endl;//Base2::b,Base2::c cout << hex << b2p << "," << hex << *(b2p) << endl;//Base2::_vfptr 虚函数表地址， b2p++; cout << hex << b2p << "," << hex << *(b2p) << endl;//Base2::a b2p++; cout << hex << b2p << "," << hex << *(b2p) << endl;//Base2::b,Base2::c b2p++; cout << hex << b2p << "," << hex << *(b2p) << endl;//Derived::d b2p++; cout << hex << b2p << "," << hex << *(b2p) << endl;//Derived::d

4.虚继承

class Base{ public: int a = 1; virtual void f1(){ cout << "Base::f1()" << endl; } virtual void f2(){ cout << "Base1::f2()" << endl; } }; class Base1 :public virtual Base { public: int b=2; virtual void f3(){ cout << "Base1::f3()" << endl; } }; class Base2 :public virtual Base { public: int c = 3; virtual void f3(){ cout << "Base2::f3()" << endl; } }; class Derived : public Base1, public Base2{ public: int d=4; void f1(){ cout << "Derived::f1()" << endl; } virtual void f4(){ cout << "Derived::f4()" << endl; } }; Derived d; int* dp = (int *)&d; int* bp = (int *)(Base*)&d; int* b1p = (int *)(Base1*)&d; int* b2p = (int *)(Base2*)&d;

4.1 内存分布

地址变量名值0x012ffc30dp ,b1p , Base1::_vfptr虚函数表地址指向Base1::f3()0x012ffc34指向的内存存着放当前地址到Base的偏移量[0xfffffffc,0x00000018] , 0x012ffc34+0x00000018==0x012ffc4c0x012ffc38Base1::b0x000000020x012ffc3cb2p , Base2::_vfptr虚函数表地址指向Base2::f3()0x012ffc40指向的内存存放着当前地址到Base的偏移[0xfffffffc,0x0000000c] , 0x012ffc40+0x0000000c==0x012ffc4c0x012ffc44Base2::c0x000000030x012ffc48Derived::d0x000000040x012ffc4cbp , Base2::_vfptr虚函数表地址指向Base::f1()，Base::f2()0x012ffc50Base::a0x00000001

4.2 说明

虚继承的共同在基类在内存中只有一份。派生类通过记录到 共同基类的偏移量来找到共同基类

4.3 测试代码

cout << hex << dp << "," << hex << *(dp) << endl;//Base1::_vfptr 地址 dp++; cout << hex << dp << "," << hex << *(dp) << endl;//指向的内存存放着 当前地址到Base的偏移量 cout << hex <<((int *)(*dp))[1]<<endl;//到Base的偏移量 输出18，即24 dp++; cout << hex << dp << "," << hex << *(dp) << endl;//Base1::b dp++; cout << hex << dp << "," << hex << *(dp) << endl;//Base2::_vfptr 地址 dp++; cout << hex << dp << "," << hex << *(dp) << endl;//指向的内存存放着 当前地址到Base的偏移量 cout << hex << ((int *)(*dp))[1] << endl;//到Base的偏移量 输出c，即12 dp++; cout << hex << dp << "," << hex << *(dp) << endl;//Base2::c dp++; cout << hex << dp << "," << hex << *(dp) << endl;//Derived::d dp++; cout << hex << dp << "," << hex << *(dp) << endl;//Base::_vfptr 地址 dp++; cout << hex << dp << "," << hex << *(dp) << endl;//Base::a