這是Acceleraated C++一書中第13章的代碼,中心思想是使用繼承和動態綁定來優化代碼,在實現該代碼的時候,學習了很多東西。附在代碼旁:
#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<iomanip> //這是對控制precesion精度的,控制輸出流的格式
#include<stdexcept> //異常處理,像domain_error runtime_error都包含在該頭文件中
using namespace std;
//輸出平均分
double grade(double midterm, double final,const vector<double>& homework)
{
double aver;
for(int i=0;i<homework.size();i++)
aver+=homework[i];
aver/=homework.size();
return 0.2*midterm+0.4*final+0.4*aver;
}
//讀入平常的作業分數,以0結束
istream& read_hw(istream& in, vector<double>& hw)
{
if(in)
{
hw.clear();
double temp;
while(in>>temp,temp)
hw.push_back(temp);
in.clear();
}
return in;
}
//基本的分數
class Core
{
friend class Student_info; //Student_info是將Core及Grad包裹起來,允許Student_info訪問Core的私有成員
public:
//constructor
Core():midterm(0),final(0){} //構造函數
Core(istream& is){ read(is); } //從輸入流中讀入成績
virtual ~Core(){} //虛構函數,設定爲虛擬的,但繼承Core的類不需要再設爲虛擬的了,如果繼承Core的類不需要在析構函數中定義特別的代碼,那它(繼承Core的類)也不必去定義虛構函數
string name() const;
virtual double grade() const; //const的定義代表其不改變任何數據,virtual代表其版本可由繼承類重載,繼承類的版本不需要加virtual關鍵字了
virtual istream& read(istream&);
protected:
istream& read_common(istream&);
double midterm, final;
vector<double> homework;
virtual Core* clone() const { return new Core(*this); } //這個就是深度複製了,重新申請內存,注意,this是一個指針,指向自己
private:
string n; //代表學生的序號
};
string Core::name() const
{
return n;
}
double Core::grade() const
{
return ::grade(midterm,final,homework);
}
istream& Core::read_common(istream& in)
{
in>>n>>midterm>>final;
return in;
}
istream& Core::read(istream& in)
{
read_common(in); //先讀入學生序號,期中成績,期末成績
read_hw(in,homework); //讀入平時的作業成績
return in;
}
//繼承自Core
class Grad:public Core
{
public:
Grad():thesis(0){}
Grad(istream& is){ read(is); }
double grade() const;
istream& read(istream&);
private:
double thesis;
protected:
Grad* clone() const { return new Grad(*this); }
};
istream& Grad::read(istream& in)
{
read_common(in);
in>>thesis;
read_hw(in,homework);
return in;
}
double Grad::grade() const
{
return min(Core::grade(),thesis);
}
//包含一個Core基類指針的類,它維護Core指針
class Student_info
{
public:
Student_info():cp(0){} //默認構造函數
Student_info(istream& is):cp(0){ read(is); } //從輸入流輸入,創建Student_info對象
Student_info(const Student_info&); //複製構造函數,從一個Student_info複製到本Student_info對象中
Student_info& operator=(const Student_info&); //賦值操作符函數
~Student_info(){ delete cp; } //析構函數,將cp指向的內存釋放掉
istream& read(istream&);
string name() const
{
if(cp) return cp->name(); //調用前先考慮下cp是否指向一個真正的Student_info對象
else throw std::runtime_error("uninitialized student");
}
double grade() const
{
if(cp) return cp->grade();
else throw std::runtime_error("uninitialized student");
}
static bool compare(const Student_info& s1, const Student_info& s2)
{
return s1.name()<s2.name();
}
private:
Core* cp;
};
Student_info::Student_info(const Student_info& s):cp(0)
{
if(s.cp) cp=s.cp->clone();
}
Student_info& Student_info::operator=(const Student_info& s)
{
if(&s!=this) //判斷別是自身複製了
{
delete cp;
if(s.cp)
{
cp=s.cp->clone();
}
else
cp=0;
}
return *this;
}
istream& Student_info::read(istream& is)
{
delete cp;
char ch;
is>>ch;
if(ch=='U')
cp=new Core(is);
else
cp=new Grad(is);
return is;
}
bool compare(const Core& c1,const Core& c2)
{
return c1.name()<c2.name();
}
bool compare_grades(const Core& c1, const Core& c2)
{
return c1.grade()<c2.grade();
}
bool compare_Core_ptrs(const Core* cp1, const Core* cp2)
{
return compare(*cp1,*cp2);
}
//使用裝Core對象的vector來測試下
int Core_test()
{
vector<Core> students;
Core record;
string::size_type maxlen=0;
while(record.read(cin),record.name()!="0")
{
maxlen=max(maxlen,record.name().size());
students.push_back(record);
}
sort(students.begin(),students.end(),compare);
for(vector<Core>::size_type i=0; i!=students.size(); ++i)
{
cout<<students[i].name()
<<string(maxlen+1-students[i].name().size(),' ');
try
{
double final_grade=students[i].grade();
streamsize prec=cout.precision();
cout<<setprecision(3)<<final_grade<<setprecision(prec)<<endl;
}
catch(exception e )
{
cout<<e.what()<<endl;
}
}
return 0;
}
//使用裝Grad對象的vector來測試下
int Grad_test()
{
vector<Grad> students;
Grad record;
string::size_type maxlen=0;
while(record.read(cin),record.name()!="0")
{
maxlen=max(maxlen,record.name().size());
students.push_back(record);
}
sort(students.begin(),students.end(),compare);
for(vector<Grad>::size_type i=0; i!=students.size();++i)
{
cout<<students[i].name()<<string(maxlen+1-students[i].name().size(),' ');
try
{
double final_grade = students[i].grade();
streamsize prec = cout.precision();
cout<<setprecision(3)<<final_grade<<setprecision(prec)<<endl;
}catch(exception e)
{
cout<<e.what()<<endl;
}
}
return 0;
}
//如果只是用放對象的Vector來測試的話,那我們要對Core和Grad類分別寫函數了,就像上面的一樣,而如果用指針或引用的話,就不一樣了,不需要再分別寫了,只寫一個函數就可以了
int Core_pointer_test()
{
vector<Core*> students;
Core* record;
char ch;
string::size_type maxlen=0;
while(cin>>ch)
{
if(ch=='U')
record=new Core;
else
record=new Grad;
record->read(cin);
if(record->name()=="0")
break;
maxlen=max(maxlen,record->name().size());
students.push_back(record);
}
sort(students.begin(),students.end(),compare_Core_ptrs);
for(vector<Core*>::size_type i=0; i!=students.size();++i)
{
cout<<students[i]->name()<<string(maxlen+1-students[i]->name().size(),' '); //這是爲了保證輸出整齊
try
{
double final_grade=students[i]->grade();
streamsize prec=cout.precision();
cout<<setprecision(3)<<final_grade<<setprecision(prec)<<endl;
}
catch(exception e)
{
cout<<e.what()<<endl;
}
delete students[i]; //別忘記釋放內存了
}
return 0;
}
//用Handle比上面的用指針有一個顯著的好處,那就是不用手動的申請,手動的釋放內存了。因爲這些操作都已經託管到Handle類裏了。
int Handle_test()
{
vector<Student_info> students;
Student_info record;
string::size_type maxlen=0;
while(record.read(cin),record.name()!="0")
{
maxlen=max(maxlen,record.name().size());
students.push_back(record);
}
sort(students.begin(),students.end(),Student_info::compare);
for(vector<Student_info>::size_type i=0; i!=students.size();++i)
{
cout<<students[i].name()<<string(maxlen+1-students[i].name().size(),' ');
try
{
double final_grade=students[i].grade();
streamsize prec=cout.precision();
cout<<setprecision(3)<<final_grade<<setprecision(prec)<<endl;
}
catch(domain_error e)
{
cout<<e.what()<<endl;
}
}
return 0;
}
int main()
{
Handle_test(); //還可以對上面的其它含test的函數進行測試,結果是一樣的
return 0;
}
該程序實現的功能是:輸入多條學生的序號,期中考試成績,期末考試成績,平時作業成績數據,按照計算規則,輸出每個學生的平均分。程序使用了C++的繼承,多態,動態類別識別,三原則(複製構造函數,賦值操作符,析構函數)等特性。