24.1. operator int() const

Prior to the introduction of the bool keyword to the language, the most obvious form of such a thing might have been to provide operator int() const, as in:

Listing 24.1.

class ExpressibleThing


{


public:


  operator int() const;


};





ExpressibleThing  e;





if(e)   // Evaluates whether e "is"


{}


if(!e)  // Evaluates whether e "is not"


{}

The big problem with this is that an int is readily convertible to many other types, and it can be meaninglessly used in the construction of an entirely unrelated type:

ExpressibleThing    e;


std::vector<String> vs(e); // What on earth does this mean?!

This is meaningless, and also a potential source of undefined behavior. Since Expressible Thing need only return any non-zero value, it could be correctly implemented to return a different non-zero value each time it is called. This could result in a silent failure, or cause memory exhaustion, or, worse, could actually work correctly in most cases: operator int() const might return 1,000, which is always large enough for the subsequent uses of vs, throughout your testing but not enough when your product goes live.