两道笔试题

[nwpulipeng]

Which of the following class DOES NOT need a copy constructor?
a.A matrix class in which the actual matrix is allocated dynamically
within the constructor and is deleted within its destructor.
b.A payroll class in which each object is provided with a unique ID.
c.A word class containing a string object and vector object of line
and column location pairs.
d.A library class containing a list of book object.

Which virtual function redeclarations of the Derived class are correct?
a. Base* Base::copy(Base*);
Base* Derived::copy(Derived*);
b. Base* Base::copy(Base*);
Derived* Derived::copy(Base*);
c. ostream& Base::print(int,ostream&=cout);
ostream& Derived::print(int,ostream&);
d. void Base::eval() const;
void Derived::eval();

[sankt]

1.题目本身有问题.一个空类里面都有默认的拷贝构造函数.
如要选择,B比较合适

2.
a 肯定不是,因为参数不一样.
b 正解.
c 这是友元函数,没有覆盖的说法.因为覆盖对应于成员函数.
d 错误.

证明:
/*
void Base::eval() const;
void Derived::eval();
*/

#include<iostream>
using namespace std;

class Base
{
public:
virtual void eval() const
{
cout<<"eval const"<<endl;
}
};

class Derived : public Base
{
public:
virtual void eval()
{
cout<<"eval "<<endl;
}
};

int main()
{
//Derived d;
//Base& b = d;
Base *b = new Derived();

b->eval();
delete b;

system("pause");
return 0;
}

[matrass_zhao]

2. b不对吧，应该是c才对。

[nwpulipeng]

本人也认为第二题应该选C

[逸学堂]

b. Base* Base::copy(Base*);
Derived* Derived::copy(Base*);
～～～～～
C＋＋的标准中提到过，虚函数的返回值可以是父子类（父类或者子类）。

[matrass_zhao]

ugg说的对，2.b是对的。covariant return type，大家可以查查看。

为什么c不对哪？

[yeahspyme]

2c的缺省参数有问题？

[铖邑]

b c？

[逸学堂]

一：b（一看薪水类，就不能有拷贝构造）
二：b. Base* Base::copy(Base*);
Derived* Derived::copy(Base*);
其他的俺不敢确定。

[Lemon_2000]

b c

[kingsun555]

第一题是问哪个类不需要一个CPOY CTOR,这个意思是不是可以利用编译器提供的COPY CTOR而不用程序员自己定义一个呢?如果这样的话, 我想B选项肯定得要自己手工定义一个COPY CTOR吧,还有A选项也必须自己定义一个COPY CTOR