C++ 求解在虚继承情况下类的大小

（1）
#include
using namespace std;
class CommonBase
{
int co;
//virtual void fun(){ }
};

class Base1 : virtual public CommonBase
{
public:
virtual void print1() { }
virtual void print2() { }
private:
int b1;
};

class Base2 : virtual public CommonBase
{
public:
virtual void dump1() { }
virtual void dump2() { }
private:
int b2;
};

class Derived : public Base1, public Base2
{
public:
void print2() { }
void dump2() { }

// virtual void hello(){ }
private:
int d;
};

int main()
{
cout << sizeof(Derived) << endl; //输出32
system("pause");
}

（2）

#include
using namespace std;
class CommonBase
{
int co;
virtual void fun(){ }
};

class Base1 : virtual public CommonBase
{
public:
virtual void print1() { }
virtual void print2() { }
private:
int b1;
};

class Base2 : virtual public CommonBase
{
public:
virtual void dump1() { }
virtual void dump2() { }
private:
int b2;
};

class Derived : public Base1, public Base2
{
public:
void print2() { }
void dump2() { }

//virtual void hello(){ }

private:
int d;
};

int main()
{
cout << sizeof(Derived) << endl; //输出36
system("pause");
}

（3）

#include
using namespace std;
class CommonBase
{
int co;
// virtual void fun(){ }
};

class Base1 : virtual public CommonBase
{
public:
virtual void print1() { }
virtual void print2() { }
private:
int b1;
};

class Base2 : virtual public CommonBase
{
public:
virtual void dump1() { }
virtual void dump2() { }
private:
int b2;
};

class Derived : public Base1, public Base2
{
public:
void print2() { }
void dump2() { }

virtual void hello(){ }

private:
int d;
};

int main()
{
cout << sizeof(Derived) << endl; //输出32
system("pause");
}

为什么（3）在32位系统上输出的是32而不是36？

虚函数表，然后int是4字节，还要考虑内存对齐。

不同的编译器是不同的，具体你可以调试，并且看下内存（VS很容易看到这些）。