Demo of using an 'invoke' shim with SFINAE and enable_if<> to conditionally specialize templates based on whether a given expression is defined for a parameter class type

invoke.cpp

#include <boost/utility/enable_if.hpp>
#include <boost/typeof/std/utility.hpp>

using boost::false_type;
using boost::true_type;
using boost::enable_if;

// always evaluates to true, as long as expression that generated 'X' compiled
template <typename X> struct invoke : public true_type {};


// this default will always match, if nothing else does
template <typename X, typename Enabled=void>
struct dispatch {
    static const int value = 37;
};

// If the expression 'X().v' is defined, this specialization will match.
// otherwise, it will silently not match via SFINAE, with no compiler err
template <typename X>
struct dispatch<X, typename enable_if<invoke<BOOST_TYPEOF_TPL(X().v)> >::type> {
    static const int value = 73;
};

// define a test class having a member variable 'v'
struct ttt {
    float v;
};

int main(int argc, char** argv) {
    std::cout << dispatch<ttt>::value << "\n";   // value is 73
    std::cout << dispatch<int>::value << "\n";   // value is 37
    return 0;
}