Hi folks, I would like to define a function in a class only if a child defines it. In the below example, I would like the function f() to exist only in an object bar if it defines a function i(). So far, my approach with boost::enable_if failed because bar is not a complete type during examination. T is clearly incomplete at the time when b is defined. What would be the correct approach to solve this problem? #include <boost/type_traits/is_function.hpp> #include <boost/utility/enable_if.hpp> template <typename T> struct foo { typename boost::enable_if< typename boost::is_function<typename T::i>::type, void >::type f() { } }; struct bar : foo<bar> { void i() { } }; int main() { bar b; b.f(); // should only compile since bar defines i() return 0; } enable_if.cc: In instantiation of ‘foo<bar>’: enable_if.cc:16: instantiated from here enable_if.cc:10: error: invalid use of undefined type ‘struct bar’ enable_if.cc:16: error: forward declaration of ‘struct bar’ enable_if.cc: In function ‘int main()’: enable_if.cc:25: error: ‘struct bar’ has no member named ‘f’ Matthias -- Matthias Vallentin vallentin@icsi.berkeley.edu http://matthias.vallentin.cc