Questions for C++

Questions for C++

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Time: 2 hours

1. What is overload? What is override? Can we overload functions by various kinds of return values? Why? (5)

a.

overload means there are several functions which have the same name in the same class, the function called operation is determined by the parameters number

override means use the same function name, parameter and return value to override the method of parent class in derived class

b.

No

c.

Overload function called is determined by the parameters, has not the relation to return value. So we can’t differ the overload functions by various kinds of return values

2. What is polymorphism in C++? You could draw a picture to depict your understand. (5)

Polymorphism points to the methods that override the parent class’s method with the same name, parameter and return value.

class CPerson

{

public:

void virtual AboutMe()

{

printf("I am CPerson");

}

}

class CMan : public CPerson

{

public:

void virtual AboutMe()

{

printf("I am CNan");

}

}

class CWoman : public CPerson

{

public:

void virtual AboutMe()

{

printf("I am CWoman");

}

}

void main()

{

CPerson * p = new CPerson();

p->AboutMe(); //I am CPerson

p = new CMan();

p->AboutMe(); //I am CNan

p = new CWoman();

p->AboutMe(); //I am CWoman

}

3. Write function “strcpy” without C++/C string library.(7)

char * strcpy(const char * pSrc, char * pDes)

{

assert(pSrc != NULL && pDes != NULL);

char * pTmp = pDes;

while((*pTmp++ = *pSrc++) != '/0');

return pTmp;

}

4. Describe the following data structures; arrays, linked lists, hash tables and binary trees. How are they different? What are the strengths and weaknesses of each? Give an example of a situation you would use each. (8)

Array is continued block in memory, it can only store the same type data and it’s size is determined while compiling.

Hash table can realize the map between key and value, It also requires a continued memory area when stores data. But it’s data locating utilizes hash code. Good hash code generated algorithm can make the memory utilized ratio very high.

Binary tree is a kind of data structure like tree figure. It can not be continual and it’s size is dynamic. We can realize data sort while utilizing it to store data; it’s time complexity is too low while we search, edit it.

5. Does C++ come with in-built threading support? If yes, how to do that? If no, how can you implement threading support in C++? Please write code piece if needed. (6)

No, C++ does not come with in-built threading support.

6. What is the result of the following program?(7)

#include <iostream.h>
class A
{
public:
A(){cout<<"A::A() called./n";}
virtual ~A(){cout<<"A::~A() called./n";}
};

class B:public A
{
public:
B(int i){
cout<<"B::B() called./n";
buf=new char[i];}
virtual ~B()
{
delete []buf;
cout<<"B::~B() called./n";
}
private:
char *buf;
};

void fun(A *a)
{
delete a;
}

void main()
{
A *a=new B(15);
fun(a);
}

直接执行得到输出结果。

7. What errors are caught at compile time vs link time? Please give at least 2 examples for each situation. (7)

8. The following codes are for 32 bits computers. Please calculate the value of sizeof. (8)

char str[] = "Hello" ;

char *p = str ;

int n = 10;

sizeof(str) = __6________

sizeof(p) = ___4_______

sizeof(n) = ____4______

void Func ( char str[100])

{

sizeof(str) = __4________

}

9. Implement Singleton design pattern with C++.(8)

class CSingleton

{

private:

CSingleton * instance;

CSingleton()

{

instance = NULL;

}

public:

CSingleton * getInstance()

{

if(instance == NULL)

instance = new CSingleton();

return instance;

}

}

10. Please analyze codes and tell what will be the results after running and why. (8)

Code 1:

void GetMemory(char *p)

{

p = (char *)malloc(100);

}

void Test(void)

{

char *str = NULL;

GetMemory(str);

strcpy(str, "hello world");

printf(str);

}

Answer 1:

程序会crash。因为GetMemory()并不能传递动态的内存，Test中的str一直都是NULL，strcpy(str, "hello world")这句将将使程序crash。

Code 2:

char *GetMemory(void)

{

char p[] = "hello world";

return p;

}

void Test(void)

{

char *str = NULL;

str = GetMemory();

printf(str);

}

Answer 2:

程序返回的结果可能是乱码。因为GetMemory()返回的是指向栈内存的指针，该指针的内容不是NULL，但其原先的内容已经被清除，新内容不可知。

11. Is it possible to keep 2 stacks in a single array, if one grows from position one of the array, and the other grows from the last position. Write a procedure PUSH(x, s) that pushes element x onto stack S, where S is one or the other of these two stacks. Include all necessary error checks (15)

12. class String

{

public:

String(const char *str = NULL); // constructor

String(const String &other); // copy constructor

~ String(void); // destructor

String & operate =(const String &other);

private:

char *m_data; // save the string

};

Please write the above four functions. (16)

a.

String::~String(void)

{

delete[] m_data;

}

b.

String::String(const char *str = NULL)

{

if(str == NULL)

{

m_data = new char[1];

m_data[0] = '/0';

}

else

{

m_data = new char[strlen(str) + 1];

strcpy(m_data, str);

}

}

c.

String::String(const String &other)

{

m_data = new char[strlen(other.m_data)+1];

strcpy(m_data, other.m_data);

}

d.

String & String::operate =(const String &other)

{

if(other == &otherL)

return *this;

delete[] m_data;

m_data = new char[strlen(other.m_data)+1];

strcpy(m_data, other.m_data);

return *this;

}