先看如下代碼：
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;
}
以上代碼在紅色粗顯的那兩行報錯:
error C2679: binary '*' : no operator defined which takes a right-hand operand of type 'const int' (or there is no acceptable conversion)”
報錯原因：第一行是因爲rational 的構造函數是explicit，此時不允許將整數2隱式轉換成rational類，所以報錯，第二行報錯原因：
對象oneeighth 是一個包含operator*函數的類的實例，所以編譯器調用了那個函數。而整數2沒有相應的類，所以沒有operator*成員函數，故報錯。
將以上代碼修改爲如下：
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;
}
此時只有一行報錯，那如何修改這個錯誤呢？
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，通過三個例子你明白了嗎?