我写的第一个C++.
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分类目录归档：C++
C++ 练习
今天下午看书的时候发现一个很有趣的问题。
《C++ Primer Plus》
示例14.1(studentc.h)中，定义了两个非常奇怪的类成员函数
#ifndef STUDENTC_H_
#define STUDENTC_H_
#include <iostream>
#include <string>
#include <valarray>
class Student
{
private:
typedef std::valarray<double> ArrayDb;
std::string name;
ArrayDb scores;
std::ostream & arr_out(std::ostream & os) const;
public:
Student() :name("Null Student"), scores() {}
explicit Student(const std::string & s) :name(s), scores() {}
explicit Student(int n) :name("Nully"), scores(n) {}
Student(const std::string & s, int n) :name(s), scores(n) {}
Student(const std::string & s, const ArrayDb & a) : name(s), scores(a) {}
Student(const char * str, const double * pd, int n) :name(str), scores(pd,n) {}
~Student() {}
double Average() const;
const std::string & Name() const;
//[]重载函数1，似乎能匹配所有的调用
double & operator[](int i);
//[]重载函数2，在有上面的重载函数下，似乎永远匹配不到
double operator[](int i) const;
friend std::istream & operator>>(std::istream & is, Student & stu);
friend std::istream & getline(std::istream & is, Student & stu);
friend std::ostream & operator<<(std::ostream & os,const Student & stu);
};
#endif
在实现中分别添加两个重载函数的输出，查看调用的情况
#include "studentc.h"
using std::ostream;
using std::endl;
using std::istream;
using std::string;
double Student::Average() const
{
if (scores.size() > 0)
return scores.sum() / scores.size();
else
return 0;
}
const string & Student::Name() const
{
return name;
}
double & Student::operator[](int i)
{
std::cout << "Use double & " << std::endl;
return scores[i];
}
double Student::operator[](int i) const
{
std::cout << "Use double " << std::endl;
return scores[i];
}
ostream & Student::arr_out(ostream & os) const
{
int i;
int lim = scores.size();
if (lim > 0)
{
for(i = 0; i < lim; i++)
{
os << scores[i] << " ";
if ( i % 5 == 4)
os << endl;
}
if ( i % 5 != 0)
os << endl;
}
else
os << " empty array ";
return os;
}
istream & operator>>(istream & is, Student & stu)
{
is >> stu.name;
return is;
}
istream & getline(istream & is, Student & stu)
{
getline(is,stu.name);
return is;
}
ostream & operator<<(ostream & os, const Student & stu)
{
os << "Scores for " << stu.name << ":\n";
stu.arr_out(os);
return os;
}
#include "studentc.h"
#include <iostream>
using std::cin;
using std::cout;
using std::endl;
void set(Student & sa, int n);
const int pupils = 3;
const int quizzes = 5;
int main(void)
{
Student ada[pupils] = { Student(quizzes), Student(quizzes), Student(quizzes) };
int i;
for (i = 0; i < pupils; ++i)
set(ada[i], quizzes);
cout << "\nStudent List:\n";
for (i = 0; i < pupils; ++i)
cout << ada[i].Name() << endl;
cout << "\nResults:";
for (i = 0; i < pupils; ++i)
{
cout << endl << ada[i];
cout << "average: " << ada[i].Average() << endl;
}
cout << "Done.\n";
return 0;
}
void set(Student & sa, int n)
{
cout << "Please enter the student's name: ";
getline(cin,sa);
cout << "Please enter " << n << " quiz scores:\n";
for (int i = 0; i < n; i++)
cin >> sa[i];
while (cin.get() != '\n')
continue;
}
[code title="输出结果"]
Please enter the student's name: Gil Bayts
Please enter 5 quiz scores:
Use double &
92 94 96 93 95
Use double &
Use double &
Use double &
Use double &
Please enter the student's name: Pat Roone
Please enter 5 quiz scores:
Use double &
83 89 72 78 95
Use double &
Use double &
Use double &
Use double &
Please enter the student's name: Fleur O'Day
Please enter 5 quiz scores:
Use double &
92 89 96 74 64
Use double &
Use double &
Use double &
Use double &
Student List:
Gil Bayts
Pat Roone
Fleur O'Day
Results:
Scores for Gil Bayts:
92 94 96 93 95
average: 94
Scores for Pat Roone:
83 89 72 78 95
average: 83.4
Scores for Fleur O'Day:
92 89 96 74 64
average: 83
Done.
可以看到，const版本的operator[]完全没有匹配到。
C++ Class练习
《C++ Primer Plus》
第12章 课后练习2
#ifndef STRING_H_
#define STRING_H_
#include <iostream>
using std::ostream;
using std::istream;
class String
{
private:
char * str;
int len;
static int num_strings;
static const int CINLIM = 80;
public:
String(const char * s);
String();
String(const String &);
~String();
int length () const { return len; }
String & operator=(const String &);
String & operator=(const char *);
char & operator[](int i);
const char & operator[](int i) const;
void Stringlow();
void Stringup();
int has(const char c);
String operator+(const String & st1);
operator char *() const;
friend bool operator<(const String & st1, const String & st2);
friend bool operator>(const String & st1, const String & st2);
friend bool operator==(const String & st1, const String & st2);
friend String operator+(const char *, const String & st1);
friend ostream & operator<<(ostream & os, const String & st);
friend istream & operator>>(istream & is, String & st);
static int HowMany();
};
#endif
#include <cstring>
#include <cctype>
#include "String.h"
#include <iostream>
int String::num_strings = 0;
int String::HowMany()
{
return num_strings;
}
String::String(const char * s)
{
len = std::strlen(s);
str = new char[len+1];
std::strcpy(str,s);
num_strings++;
}
String::String()
{
len = 4;
str = new char[1];
str[0] = '\0';
num_strings++;
}
String::String(const String & st)
{
num_strings++;
len = st.len;
str = new char[len+1];
std::strcpy(str,st.str);
}
String::~String()
{
--num_strings;
delete [] str;
}
String & String::operator=(const String & st)
{
delete [] str;
len = strlen(st.str);
str = new char[len+1];
std::strcpy(str,st.str);
return *this;
}
String & String::operator=(const char * s)
{
delete [] str;
len = std::strlen(s);
str = new char[len+1];
std::strcpy(str, s);
return *this;
}
char & String::operator[](int i)
{
return str[i];
}
bool operator<(const String & st1, const String & st2)
{
return (std::strcmp(st1.str, st2.str) < 0);
}
bool operator>(const String & st1, const String & st2)
{
return st2 < st1;
}
bool operator==(const String & st1, const String & st2)
{
return (std::strcmp(st1.str, st2.str) == 0);
}
ostream & operator<<(ostream & os, const String & st)
{
os << st.str;
return os;
}
istream & operator>>(istream & is, String & st)
{
char temp[String::CINLIM];
is.get(temp,String::CINLIM);
if(is)
st = temp;
while (is && is.get() != '\n')
continue;
return is;
}
void String::Stringlow()
{
for (int i = 0;i < len; i++)
str[i] = tolower(str[i]);
}
void String::Stringup()
{
for (int i = 0;i < len; i++)
str[i] = toupper(str[i]);
}
/* 错误的重载函数，问题是构造temp的时候，已经使用new 分配了固定长度的内存。使用strcat的时候实际上已经内存溢出了.
String String::operator+(const String & st1)
{
String temp(str);
strcat(temp.str,st1.str);
temp.len = strlen(str) + strlen(st1.str) + 1;
return temp;
}
*/
String String::operator+(const String & st1)
{
int nlen = strlen(st1.str) + strlen(str) + 1;
char * nstr = new char[nlen];
strcpy(nstr,str);
strcat(nstr,st1.str);
String temp;
delete [] temp.str;
temp.str = nstr;
return temp;
}
String::operator char *() const
{
return str;
}
String operator+(const char * st1,const String & st2)
{
String temp;
strcat(temp.str,st1);
strcat(temp.str,st2.str);
temp.len = strlen(st1) + st2.len;
return temp;
}
int String::has(const char c)
{
int count = 0;
for(int i = 0; i < len; i++)
if (str[i] == c )
count++;
return count;
}
#include <iostream>
using namespace std;
#include "String2.h"
int main()
{
String s1(" and I am C++ stdudent.");
String s2 = "Pleas enter your name: ";
String s3;
cout << s2;
cin >> s3;
s2 = "My name is " + s3;
cout << s2 << ".\n";
s2 = s2 + s1;
s2.Stringup();
cout << "The string\n" << s2 << "\ncontains " << s2.has('A')
<< " 'A' characters in it.\n";
s1 = "red";
String rgb[3] = { String(s1), String("green"), String("blue") };
cout << "Enter the name of a primary color for mixing light: ";
String ans;
bool success = false;
while (cin >> ans)
{
ans.Stringlow();
for(int i = 0; i < 3; i++)
{
if(ans == rgb[i])
{
cout << "That's right!\n";
success = true;
break;
}
}
if (success)
break;
else
cout << "Try again!\n";
}
cout << "Bye\n";
return 0;
}
第12章 课后练习6
#ifndef QUEUE_H_
#define QUEUE_H_
class Customer
{
private:
long arrive;
int processtime;
public:
Customer() { arrive = processtime = 0; }
void set(long when);
long when() const { return arrive; }
int ptime() const { return processtime; }
};
typedef Customer Item;
class Queue
{
private:
struct Node { Item item; struct Node * next ; };
enum { Q_SIZE = 10 };
Node * front;
Node * rear;
int items;
const int qsize;
Queue(const Queue & q) : qsize(0) { }
Queue & operator=(const Queue & q) { return *this; }
public:
Queue( int qs = Q_SIZE );
~Queue();
bool isempty() const;
bool isfull() const;
int queuecount() const;
bool enqueue(const Item & item);
bool dequeue(Item & item);
};
#endif
#include "queue.h"
#include <cstdlib>
#include <stdio.h>
Queue::Queue(int qs) : qsize(qs)
{
front = rear = NULL;
items = 0;
}
Queue::~Queue()
{
Node * temp;
while(front != NULL)
{
temp = front;
front = front->next;
delete temp;
}
}
bool Queue::isempty() const
{
return items == 0;
}
bool Queue::isfull() const
{
return items == qsize;
}
int Queue::queuecount() const
{
return items;
}
bool Queue::enqueue(const Item & item)
{
if (isfull())
return false;
Node * add = new Node;
add->item = item;
add->next = NULL;
items++;
if(front == NULL)
front = add;
else
rear->next = add;
rear = add;
return true;
}
bool Queue::dequeue(Item & item)
{
if(front == NULL)
return false;
item = front->item;
items--;
Node * temp = front;
front = front->next;
delete temp;
if(items == 0)
rear = NULL;
return true;
}
void Customer::set(long when)
{
processtime = std::rand() % 3 + 1;
arrive = when;
}
[code lang="C" title="queue.cpp"]
#include <iostream>
#include <cstdlib>
#include <ctime>
#include "queue.h"
const int MIN_PER_HR = 60;
bool newcustomer(double x);
int main()
{
using std::cin;
using std::cout;
using std::endl;
using std::ios_base;
std::srand(std::time(0));
cout << "Case Study: Bank of Heather Automatic Teller\n";
cout << "Enter maximum size of queue: ";
int qs;
cin >> qs;
Queue line1(qs);
Queue line2(qs);
cout << "Enter the number of simulation hours: ";
int hours;
cin >> hours;
long cyclelimit = MIN_PER_HR * hours;
cout << "Enter the average number of customers per hours: ";
double perhour;
cin >> perhour;
double min_per_cust;
min_per_cust = MIN_PER_HR / perhour ;
Item temp1,temp2;
long turnaways = 0;
long customers = 0;
long served = 0;
long sum_line = 0;
int wait_time1,wait_time2;
wait_time1 = wait_time2 = 0;
long line_wait = 0;
for(int cycle = 0;cycle < cyclelimit; cycle++)
{
if (newcustomer(min_per_cust))
{
if ( line1.isfull() && line2.isfull() )
turnaways++;
else
{
customers++;
if ( line1.isfull() )
{
temp2.set(cycle);
line2.enqueue(temp2);
}
else
{
temp1.set(cycle);
line1.enqueue(temp1);
}
}
}
if (wait_time1 <= 0 && !line1.isempty())
{
line1.dequeue(temp1);
wait_time1 = temp1.ptime();
line_wait += cycle - temp1.when();
served++;
}
if (wait_time2 <= 0 && !line2.isempty())
{
line2.dequeue(temp2);
wait_time2 = temp2.ptime();
line_wait += cycle - temp2.when();
served++;
}
if(wait_time1 > 0)
wait_time1--;
if(wait_time2 > 0)
wait_time2--;
sum_line += ( line1.queuecount() + line2.queuecount() );
}
if (customers > 0)
{
cout << "customers accepted: " << customers << endl;
cout << " customers served: " << served << endl;
cout << " turnaways: " << turnaways << endl;
cout << "average queue size: ";
cout.precision(2);
cout.setf(ios_base::fixed, ios_base::floatfield);
cout << (double) sum_line / cyclelimit << endl;
cout << " average wait time: "
<< (double) line_wait / served << " minutes\n";
}
else
cout << "No customers!\n";
cout << "Done!\n";
return 0;
}
bool newcustomer(double x)
{
return (std::rand() * x / RAND_MAX < 1);
}
函数指针
今天看到了函数指针这一小结，发现自己对指针的认知还是不够深刻，特意贴下来，日后回顾。
《C++ Primer Plus》
第七章 7.19(arfupt.cpp)
#include <iostream>
const double * f1(const double ar[], int n);
const double * f2(const double [], int);
const double * f3(const double *, int);
int main()
{
using namespace std;
double av[3] = { 1112.3, 1542.6, 2227.9 };
//p1 指向 f1
const double * (*p1)(const double * ,int) = f1;
//自动声明类型
auto p2 = f2;
cout << "Using pointers to functions:\n";
cout << " Address Value\n";
cout << (*p1)(av,3) << ": " << *(*p1)(av,3) << endl;
cout << (*p2)(av,3) << ": " << *(*p2)(av,3) << endl;
//注意()优先级大于*，*p2(av,3) == *(p2(av,3))
cout << p2(av,3) << ": " << *p2(av,3) << endl;
//二级指针
const double * (*pa[3])(const double * , int) = { f1,f2,f3 };
auto pb = pa;
cout << "\nUsing an array of pointers to functions:\n";
cout << " Address Value\n";
for (int i = 0;i < 3;i++)
// (*(pa+i))(av,3) == pa[i](av,3) 似乎[]优先级高于()
cout << (*(pa + i))(av,3) << ": " << *(*(pa+i))(av,3) << endl;
cout << "\nUsing a pointer to a pointer to a function:\n";
cout << " Address Value\n";
for (int i = 0;i < 3; i++)
cout << pb[i](av,3) << ": " << *pb[i](av,3) << endl;
cout << "\nUsing pointers to an array of pointers:\n";
cout << " Address Value\n";
// 三级指针
const double *(*(*pd)[3])(const double *, int ) = & pa;
auto pc = &pa;
const double * pdb = (*pd)[1](av,3);
cout << (*pc)[0](av,3) << ": " << *(*pc)[0](av,3) << endl;
cout << pdb << ": " << *pdb << endl;
cout << (*(*pd)[2])(av,3) << ": " << *(*(*pd)[2])(av,3) << endl;
return 0;
}
const double * f1 (const double * ar, int n)
{
return ar;
}
const double * f2 (const double ar[] , int n)
{
return ar+1;
}
const double * f3 (const double * ar, int n)
{
return ar+2;
}
第七章 复习题-13
#include <iostream>
#include <cstring>
//声明一个结构体
struct applicant
{
char name[30];
int credit_rating[3];
};
using namespace std;
//声明一个show函数
void show(applicant *);
//声明一个empty函数
void empty(const applicant *);
//声明一个函数指针类型
typedef void (*tempty)(const applicant *);
//声明一个指向empty函数的函数指针p1
tempty p1 = empty;
//声明一个函数strscat
const char * strscat(const applicant *, const applicant *);
//声明一个函数指针p2,并指向strscat
const char * (*p2)(const applicant *,const applicant *) = strscat;
//声明一个包含5个函数指针的数组ap
tempty ap [5];
//声明一个包含10个函数指针的数组pb
const char * (*(pb[10]))(const applicant * ,const applicant *);
//同上,声明一个包含10个函数指针的数组pc
const char * (*pc[10])(const applicant * ,const applicant *);
//声明一个指向包含10个函数指针的数组的指针pa
char * (*(*pa[10]))(const applicant * ,const applicant *);
int main()
{
struct applicant test = { "=-=.",{ 0,1,2 } };
cout << "Use show function." << endl;
show(&test);
cout << "Use strscat function." << endl;
const char * pstr = strscat(&test,&test);
cout << pstr << endl;
//忘记添加的delete. =_=!
delete [] pstr;
return 0;
}
void show(struct applicant * app)
{
cout << app->name << endl;
for (int i = 0; i < 3; i++)
cout << app->credit_rating[i] << endl;
return;
}
void empty(const applicant * a)
{
return;
}
const char * strscat(const applicant * chars1, const applicant * chars2)
{
char pstr [20];
strcpy(pstr,chars1->name);
char * str = new char [40];
strcpy(str,strcat(pstr,chars2->name));
return str;
}