[008]C/C++系列 | 轉換函數(二)conversion function VS non-explicit-one-argument ctor

備註：

one-argument：Fraction(int num, int den=1)一個參數 

class Fraction
{
public:
    Fraction(int num, int den=1)
        : m_numerator(num), m_denominator(den) {}
    Fraction operator+(const Fraction& f) {
        return Fraction(...);        
    }
private:
    int m_numerator; // 分子
    int m_denominator; // 分母
};

Fraction f(3,5);
Fraction d2 = f + 4; // 調用non-explict ctor將4轉爲Fraction(4,1)，然後調用operator+

一、VS conversion function

class Fraction
{
public:
    Fraction(int num, int den=1)
        : m_numerator(num), m_denominator(den) {}
    operator double() const {
        return (double)( m_numerator / m_denominator );
    }    
    Fraction operator+(const Fraction& f) {
        return Fraction(...);        
    }
private:
    int m_numerator; // 分子
    int m_denominator; // 分母
};

Fraction f(3,5);
Fraction d2 = f + 4; // [Error]ambiguous

 

二、explicit

class Fraction
{
public:
    explicit Fraction(int num, int den=1)
        : m_numerator(num), m_denominator(den) {}
    operator double() const {
        return (double)( m_numerator / m_denominator );
    }    
    Fraction operator+(const Fraction& f) {
        return Fraction(...);        
    }
private:
    int m_numerator; // 分子
    int m_denominator; // 分母
};

Fraction f(3,5);
Fraction d2=f+4; // [Error]conversion from 'double' to 'Fraction' requested

備註：

不允許4轉換成爲Fraction！！！

 

三、標準庫例子 

template<class Alloc>
class vector<bool, Alloc>
{
public:
    typedef __bit_reference reference;
protected:
    reference operator[](size_type n) {
        return *(begin() + difference_type(n));
...

struct __bit_reference {
    unsigned int* p;
    unsigned int mask;
    ...
public:
    operator bool() const { return !(!(*p & mask)); }
...