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Restrain Conversion Operators with the "Indirect Conversion" Idiom-3 : Page 3


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booling Around
In the pre-bool era, C++ libraries often used void* as the target type of a Boolean conversion operator. The <fstream> library is an example of this. Will void* save the day? Indeed, a void* conversion operator prevents nonsensical expressions such as p1+p2 from compiling:

template <class T> class Ptr { public: operator void*() const { return ((void*) rawptr); } }; cout<<"p1 + p2 = "<< p1+p2 <<endl; //compilation error Ptr <File> pf; Ptr <Query> pq; // Query and File are unrelated types if(pf==pq) // so so, compares raw addresses

And yet, a void* conversion operator introduces new surprises:

cout << pf <<endl;//prints raw address, do you want that? delete pf; //undefined behavior

The Pointer to Member Hack
Recently, C++ pundits have come up with a better idea. Instead of using bool or void*, classes such as std::tr1::shared_ptr use pointers to members as the target type of a Boolean conversion operator.



Author's Note: The terms "conversion to bool" and "conversion to Boolean" aren't interchangeable. In the former, bool is the target type. The latter however uses an unspecified type that's convertible to bool.

Let's look at a concrete implementation of this technique.

First, add a nested struct that has one data member to class Ptr (code changes are highlighted):

template <class T> class Ptr { private: struct PtrConversion { int valid; }; };

Next, declare a typedef that serves as a synonym for "pointer to an int member of PtrConversion":

template <class T> class Ptr { private: struct PtrConversion { int valid; }; public: typedef int PtrConversion::*pmi; };

Finally, define a public conversion operator whose target type is pmi:

public: operator pmi() const //conversion to Boolean { return rawptr? &PtrConversion::valid : 0; }



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