笔试题总结

MFC中Sendmessage 和 Postmessage的区别

postmessage只把消息放入队列，不管消息是否处理就返回。而sendmessage被处理完了之后才返回。

为什么引入抽象基类和纯虚函数？

在很多情况下，基类本身声场对象是很不合理的。例如：动物作为基类派生出老虎、狮子等子类，但动物本身生成对象明显不合理。

虚析构函数是为了解决什么问题？用C++代码解释。

声明了两个类：ClaBase ClaDeriver。父类中声明虚析构函数。 虚析构函数是为了当使用一个基类指针去删除派生类对象时，系统会自动调用派生类对象。

<span style="font-size:14px;">class ClaBase{public:ClaBase(){}virtual ~ClaBase(){}virtual void  Dosomething(){cout <<"do something in ClaBase"<<endl;}};class ClaDeriver : public ClaBase{public:ClaDeriver(){cout << "calling constructor of ClaDeriver" <<endl;}~ClaDeriver(){cout << "calling destructor of ClaDeriver" <<endl;}  void Dosomething(){cout <<"do something in ClaDeriver"<<endl;}};int main(){ClaBase *p = new ClaDeriver;p->Dosomething();delete p;}</span>

如果将ClsBase中的virtual关键字去掉：

<span style="font-size:14px;">class ClaBase{public:ClaBase(){} ~ClaBase(){}virtual void  Dosomething(){cout <<"do something in ClaBase"<<endl;}};class ClaDeriver : public ClaBase{public:ClaDeriver(){cout << "calling constructor of ClaDeriver" <<endl;}~ClaDeriver(){cout << "calling destructor of ClaDeriver" <<endl;}  void Dosomething(){cout <<"do something in ClaDeriver"<<endl;}};int main(){ClaBase *p = new ClaDeriver;p->Dosomething();delete p;}</span>

系统会直接调用ClaBase中的析构函数而不去调用ClaDervier中的析构函数。

函数指针

Int * char * float *分别是指向int型char型float型的指针，那么通过类比也存在指向函数的指针--函数指针

int Add(int var_a,int var_b ){return var_a + var_b;}int Subtract(int var_a, int var_b){return var_a - var_b;}int Multiply(int var_a,int var_b){return var_a * var_b;}int main(){int (*ptrFun)(int ,int);int a,b,c,d,e;cin >> a;cin >> b;ptrFun = Add;c = (*ptrFun)(a,b);ptrFun = Subtract;d = (*ptrFun)(a,b);ptrFun = Multiply;e = (*ptrFun)(a,b);cout << c << " " <<d << " "<<e<< endl;}

指针函数

指针函数就是指针型函数，言外之意就是返回值为指针的函数。和整形函数、浮点型函数、字符型函数作类比更好理解。

int *Change(int *);int main(){int arra[5] = {0,1,2,3,4};int *ptr_arr = Change(arra);for(;ptr_arr < arra +5 ;ptr_arr++){printf("%d,",*ptr_arr);}printf("\n");return 0;}int *Change(int *ptr){int *ptr_temp = ptr;int i =0;while(i < 4){*ptr += 1;ptr++;i++;}return ptr_temp;}

什么是多态？请用C++代码说明。

class Person {//{}public:Person(){}~Person(){}virtual void Speak() =0;};class male : public Person{public:male() {}~male(){}void Speak(){cout<<"i'm a men"<<endl;}};class female :public Person {public:female(){}~female(){}void Speak(){cout<<"i'm a women"<<endl;}};int main(){Person *ptr = new male;ptr->Speak();ptr = new female;ptr->Speak();}