C++学习之友元函数，操作符重载
这是我写的第一个类，分数类型，虽然比较简单，但我犯的错也并不少，比方成员函数和全局函数打代码时总是搞混，输入负的分数时也出现各种变态的输出，还发现了一个以前不知道的知识点就是 a*1.0/b 和a/b*1.0不一样，收获不小，留下代码供复习用。
#include<iostream>#include<cmath>using namespace std;class fraction{public: fraction():mole(0),deno(1) {}; fraction(int,int); fraction(double); void print(); friend istream&operator>>(istream&,fraction&); friend ostream&operator<<(ostream&,const fraction&); friend fraction operator+(const fraction&,const fraction&); friend fraction operator-(const fraction&,const fraction&); friend fraction operator*(const fraction&,const fraction&); friend fraction operator/(const fraction&,const fraction&); friend fraction operator!(const fraction&); friend bool operator <=(const fraction&,const fraction&); friend bool operator >=(const fraction&,const fraction&); friend bool operator <(const fraction&,const fraction&); friend bool operator >(const fraction&,const fraction&); friend bool operator ==(const fraction&,const fraction&); friend bool operator !=(const fraction&,const fraction&); fraction& operator++(); const fraction operator++(int); fraction operator-()const; fraction& operator--(); const fraction operator--(int); operator double()const; fraction& operator+=(const fraction&); fraction& operator-=(const fraction&); fraction& operator*=(const fraction&); fraction& operator/=(const fraction&);private: int mole; int deno; void set(int,int); static int gcd(const int&,const int &);};fraction::fraction(int a,int b){ set(a,b);}fraction::fraction(double a){ mole=0,deno=1; int flag=a>0?1:-1; a=fabs(a); while(fabs(mole*1.0/deno-a)>0.001) { if(mole*1.0/deno<a)mole++; else deno++; } mole*=flag;}int fraction::gcd(const int& a,const int& b){ return b==0?a:gcd(b,a%b);}void fraction::print(){ cout<<mole<<"/"<<deno<<endl;}void fraction::set(int a,int b){ int t=gcd(a,b); mole=a/t; deno=b==0?1:b/t;}fraction fraction::operator -()const{ return fraction(-mole,deno);}fraction operator+(const fraction&a,const fraction&b){ int z=a.mole*b.deno+b.mole*a.deno; int m=a.deno*b.deno; return fraction(z,m);}fraction operator-(const fraction&a,const fraction&b){ int z=a.mole*b.deno-b.mole*a.deno; int m=a.deno*b.deno; return fraction(z,m);}fraction operator*(const fraction&a,const fraction&b){ int z=a.mole*b.mole; int m=a.deno*b.deno; return fraction(z,m);}fraction operator/(const fraction&a,const fraction&b){ int z=a.mole*b.deno; int m=a.deno*b.mole; return fraction(z,m);}fraction& fraction::operator++(){ mole+=deno; return *this;}fraction& fraction::operator--(){ mole-=deno; return *this;}const fraction fraction::operator--(int){ fraction t=*this; mole-=deno; return t;}const fraction fraction::operator++(int){ fraction t=*this; mole+=deno; return t;}fraction operator!(const fraction& a){ return fraction(a.deno,a.mole);}istream& operator>>(istream& in,fraction& a){ int mole=0,deno=1; in>>mole; in.ignore(); in>>deno; if(mole*1.0/deno<0) a.set(mole,fabs(deno)); else a.set(fabs(mole),fabs(deno)); return in;}ostream& operator<<(ostream& out,const fraction& b){ if(b.mole/b.deno<0)out<<'-'; out<<fabs(b.mole); if(b.mole&&b.deno!=1) { out<<'/'; out<<fabs(b.deno); } return out;}fraction::operator double()const{ return mole*1.0/deno;}bool operator <=(const fraction& a,const fraction& b){ return a.mole*b.deno<=b.mole*a.deno;}bool operator >=(const fraction&a,const fraction&b){ return a.mole*b.deno>=b.mole*a.deno;}bool operator <(const fraction&a,const fraction&b){ return a.mole*b.deno<b.mole*a.deno;}bool operator >(const fraction&a,const fraction&b){ return a.mole*b.deno>b.mole*a.deno;}bool operator ==(const fraction&a,const fraction&b){ return a.mole*b.deno==b.mole*a.deno;}bool operator !=(const fraction&a,const fraction&b){ return a.mole*b.deno!=b.mole*a.deno;}fraction& fraction::operator +=(const fraction& other){ mole=mole*other.deno+deno*other.mole; deno=deno*other.deno; int t=gcd(mole,deno); mole/=t; deno/=t; return *this;}fraction& fraction::operator -=(const fraction& other){ mole=mole*other.deno-deno*other.mole; deno=deno*other.deno; int t=gcd(mole,deno); mole/=t; deno/=t; return *this;}fraction& fraction::operator *=(const fraction& other){ mole*=other.mole; deno*=other.deno; int t=gcd(mole,deno); mole/=t,deno/=t; return *this;}fraction& fraction::operator /=(const fraction& other){ mole*=other.deno; deno*=other.mole; int t=gcd(mole,deno); mole/=t,deno/=t; return *this;}int main(){ fraction a,b; cout<<"求倒数："<<endl; cin>>a; cout<<"!a:"<<!a<<endl<<endl; cout<<"强制转换："<<endl; cin>>a; cout<<"double(a)"<<double(a)<<endl; cout<<"自加自减："<<endl; cin>>a; cout<<"a++:"<<a++<<endl; cout<<"a:"<<a<<endl; cout<<"++a:"<<++a<<endl; cout<<"a:"<<a<<endl<<endl; cout<<"基本运算："<<endl; cin>>a>>b; cout<<"a+b:"<<a+b<<endl; cout<<"a/b:"<<a/b<<endl; cout<<"a*b:"<<a*b<<endl; cout<<"a-b:"<<a-b<<endl; cout<<"a+=b"<<(a+=b)<<endl; cout<<"a="<<a<<"b="<<b<<endl; cout<<"a-=b"<<(a-=b)<<endl; cout<<"a="<<a<<"b="<<b<<endl; cout<<"a*=b"<<(a*=b)<<endl; cout<<"a="<<a<<"b="<<b<<endl; cout<<"a/=b"<<(a/=b)<<endl<<endl; cout<<"关系比较:"<<endl; cin>>a>>b; cout<<"a>=b:"<<(a>=b)<<endl; cout<<"a<=b:"<<(a<=b)<<endl; cout<<"a>b:"<<(a>b)<<endl; cout<<"a<b:"<<(a<b)<<endl; cout<<"a!=b:"<<(a!=b)<<endl; return 0;}