一段虚拟继承的代码——为何编译通过，连接报错！

[anantnt203120]

#include <iostream.h>
class furniture
{
public:
furniture(){}
void setweight(int i) {weight = i;}
int getweight() {return weight;}
protected:
int weight;
};

class sofa:virtual public furniture
{
public:
sofa(){}
void watch()
{
cout<<"watch\n";
}
};

class bad:virtual public furniture
{
public:
bad(){}
void sleep()
{
cout<<"sleep\n";
}
};

class sleepersofa:public sofa,public bad
{
public:
sleepersofa():sofa(),bad(){}
void foldout()
{
cout<<"foldout\n";
}
};

void mian()
{
sleepersofa sa;
sa.setweight(20);
cout<<sa.getweight()<<endl;
}

请问为什么编译没问题，连接却报有两个错！
LIBCD.lib(crt0.obj) : error LNK2001: unresolved external symbol _main
Debug/inherit040216.exe : fatal error LNK1120: 1 unresolved externals