relational::operator bool()
Douglas Gregor
gregod at cs.rpi.edu
Wed Nov 14 15:44:43 CET 2001
Hello,
This is just a C++ nitpick, but operator bool() is often considered harmful
because of the plethora of implicit conversions (bool->int being the worst of
these implicit conversions, IMHO). I'm not suggesting that operator bool() be
removed from relational, but instead replace it with a safer construct.
Within the Boost C++ libraries (http://www.boost.org), we've adopted a
"safe_bool" conversion using of a pointer-to-member function. The trick can
be illustrated concisely:
// Add to class relational
private:
struct dummy {
void nonnull() {};
};
typedef void (dummy::*safe_bool)();
safe_bool make_safe_bool(bool cond) const
{ return cond? &dummy::nonnull : 0; }
Then in relational::operator safe_bool() const, whereever there is a
"return <bool-expression>", it should be replaced with
"return make_safe_bool(<bool-expression>)".
The use of the pointer-to-member function eliminates implicit conversions,
but the relational class can still be used in a boolean context (since it is
evaluated as "is the pointer-to-member function null?"). All meaningless
operations that are allowed by a "bool" conversion except for == and != are
eleminated by this "safe_bool".
I can submit a patch against CVS if needed.
Doug Gregor
gregod at cs.rpi.edu
More information about the GiNaC-devel
mailing list