Re: [閒聊] VOID POINTER OPERATOR

[LoganChien (簡子翔)]

其實這個叫作 unspecified_bool，有很多種其怪的寫法，光是 BOOST
函式庫就有三種。我覺得還頗有趣的。不過這種東西好像是在幫編譯
器出考題一樣。


簡單版： cast to void const *

class Even
{
    int i;
public:
    Even(int i_) : i(i_) {}

    operator void const * () const { return (i % 2 == 0) ? this : 0 ; }
};



進階版： cast to member selector

class Even
{
    int i;
    typedef int (Even::*member_selector);
public:
    Even(int i_) : i(i_) {}
    operator member_selector () const { return (i % 2 == 0) ? &Even::i : 0 ; }
};


高階版： cast to member function selector

class Even
{
    int i;

    typedef int (Even::*unspecified_bool_t)() const;
public:
    Even(int i_) : i(i_) {}

    int get() const { return i; }

    operator unspecified_bool_t () const
    {
        return (i % 2 == 0) ? &Even::get : 0 ;
    }
};



非常強大版： cast to static member function selector

class Even
{
    int i;

private:
    static void unspecified_bool(Even ***) {}
    typedef void (*unspecified_bool_t)(Even ***);

public:
    Even(int i_) : i(i_) {}

    operator unspecified_bool_t ()
    {
        return (i % 2 == 0) ? unspecified_bool : 0;
    }
};


ref. Boost SmartPtr Lib http://tinyurl.com/yh5vh8r

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