Effective c++ Item19代码练习

 先看如下代码：

例1：

#include <iostream>

using namespace std;

class rational {

public:

 explicit rational(int numerator = 0, int denominator = 1);

 int numerator() const;

 int denominator() const;

 const rational operator*(const rational& rhs) const;

private:

 int n,d;

};

rational::rational(int numerator, int denominator):n(numerator),d(denominator)

{

}

int rational::numerator()const

{

 return n;

}

int rational::denominator()const

{

 return d;

}

const rational rational::operator*(const rational& rhs) const

{

 return rational(this->numerator()*rhs.numerator(),this->denominator()*rhs.denominator());

}

int main()

{

 rational oneeighth(1, 8);

 rational result;

 result = oneeighth * 2;

 result = 2 * oneeighth;

 return 0;

}

以上代码在红色粗显的那两行报错:

error C2679: binary '*' : no operator defined which takes a right-hand operand of type 'const int' (or there is no acceptable conversion)”

报错原因：第一行是因为rational 的构造函数是explicit，此时不允许将整数2隐式转换成rational类，所以报错，第二行报错原因：

对象oneeighth 是一个包含operator*函数的类的实例，所以编译器调用了那个函数。而整数2没有相应的类，所以没有operator*成员函数，故报错。

将以上代码修改为如下：

例2：

#include <iostream>

using namespace std;

class rational {

public:

 rational(int numerator = 0, int denominator = 1);

 int numerator() const;

 int denominator() const;

 const rational operator*(const rational& rhs) const;

private:

 int n,d;

};

rational::rational(int numerator, int denominator):n(numerator),d(denominator)

{

}

int rational::numerator()const

{

 return n;

}

int rational::denominator()const

{

 return d;

}

const rational rational::operator*(const rational& rhs) const

{

 return rational(this->numerator()*rhs.numerator(),this->denominator()*rhs.denominator());

}

int main()

{

 rational oneeighth(1, 8);

 rational result;

 result = oneeighth * 2;

 result = 2 * oneeighth;

 return 0;

}

此时只有一行报错，那如何修改这个错误呢？

例3：

#include <iostream>

using namespace std;

class rational {

public:

 rational(int numerator = 0, int denominator = 1);

 int numerator() const;

 int denominator() const;

private:

 int n,d;

};

rational::rational(int numerator, int denominator):n(numerator),d(denominator)

{

}

int rational::numerator()const

{

 return n;

}

int rational::denominator()const

{

 return d;

}

const rational operator*(const rational& lhs,const rational& rhs)

{

 return rational(lhs.numerator() * rhs.numerator(),

  lhs.denominator() * rhs.denominator());

}

int main()

{

 rational oneeighth(1, 8);

 rational result;

 result = oneeighth * 2;

 result = 2 * oneeighth;

 return 0;

}

呵呵，通过三个例子你明白了吗?