Re: [Boost-users] concept_check question.

sr kumar writes:

Hi, I am having trouble understanding why the following concept_check, which checks an inheritance relationship, doesn't work but the BOOST_STATIC_ASSERT does.

Just to make sure that the concept does actually work, the LessThanComparableConcept was implemented and then commented out.

Would appreciate getting the missing link in my understanding.

thanks in advance for all help, -sr

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#include #include #include

using namespace boost;

template <typename T> struct MyConcept { void constraints() { (bool)(boost::is_base_and_derived::value == true);

According to the library docs, the constraints() function is supposed to "exercise the valid expressions of the concept." The idea is that if any of the expressions is invalid, instantiating the constraints() will cause a compilation error. All you've done above is test that is_base_and_derived::value == true is syntactically valid and can be converted to bool (using a c-style cast, no less -- almost anything can be converted that way!). That expression is valid for any T. Since nothing is derived from int, it is always false, but the value of the expression isn't what's being tested here. If you want to enforce that T is derived from some type X, you need an expression that will fail to compile if X isn't a base of T. For example, you could use the STATIC_ASSERT from below in the constraints() function. However, I recommend BOOST_MPL_ASSERT((is_base_and_derived)); instead, because you'll get better error messages.

// (bool)(a

class X { public: X() { function_requires (); BOOST_STATIC_ASSERT( (is_base_and_derived::value == true)); } ~X(){} // bool operator<(X& rhs) { return (x < rhs.x); } int x; };

int main() { return 0;}

HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com