I'm looking at all these "operator unspecified_bool_type"s in Boost. They vary: It's hard to get all of these right, and various Boost libraries have made changes in the past that got it wrong: type-safety compiler workarounds zero runtime overhead (a.k.a. more compiler workarounds) In fact I haven't previously seen an implementation that obviously achieves safety in the general case. Even pointer-to-members have an operator== and operator!=, which will cause trouble if the boolean-convertible type doesn't have its own operator== and operator!= (although it often does). But it is possible! I thought it required macros at first, but it actually works better(in some ways) with the Curiously Recurring Template Pattern. If you, the user, make a class C (such as shared_ptr) that you want to have an operator safe-bool, derive from a boost::convertible_to_bool<C> (and define a member of C "bool operator_bool() const"). Boost shall have, in order to generate errors for invalid operations, the following: template<typename T>operator==(boost::convertible_to_bool<T>const&, boost::unspecified_bool_type); template<typename T>operator==(boost::unspecified_bool_type, boost::convertible_to_bool<T>const&); and similar for operator!=, and (when unspecified_bool_type is a function pointer) for <, >, <=, >=. (The return type is something clever like boost::TYPES_MAY_NOT_BE_COMPARED_IMPLICITLY_VIA_BOOLEAN_CONVERSIONS to improve error messages.) When the compiler has support for the C++0x "explicit operator bool", none of these hacks are needed, and the library can adjust to that too. There are several more details I've been looking at too... ADL, base-chaining like Boost.Operators, writing tests/docs for weird inheritance situations... :-) Perhaps this should be provided as a Boost library in its own right, and Boost should use it everywhere that Boost currently uses idiosyncratic operator safe-bools. Are there any pitfalls I'm missing? (are there important compilers that can't deal with the overhead or correctness of using a base class for this purpose?) Does this sound like a good idea? I can post my code soon if there's interest. -Isaac