c++ - Initialize base subobject with dependent type name in derived class -

c++ - Initialize base subobject with dependent type name in derived class -

given sample code class c deriving a or b depending on policy

#include <iostream> #include <type_traits> struct { a(int a) { std::cout << << "\n"; } }; struct b { b(int a) { std::cout << -a << "\n"; } }; template<bool> struct policy; template<> struct policy<true> { typedef type; }; template<> struct policy<false> { typedef b type; }; template<typename t> struct c : public policy<std::is_polymorphic<t>::value>::type { c() : /* ????? */(5) {} }; int main() { c<int> a; // should print -5 c<std::ostream> b; // should print 5 }

how initialize base of operations class of c? (or, if it's not possible, there workaround?)

do way found base of operations class of c:

template<typename t> struct c : public policy<std::is_polymorphic<t>::value>::type { c() : policy<std::is_polymorphic<t>::value>::type(5) {} };

of course, create more readable, can add together typedef (maybe need base of operations class multiple times within c, it's worth it). this:

template<typename t> struct c : public policy<std::is_polymorphic<t>::value>::type { typedef typename policy<std::is_polymorphic<t>::value>::type base; c() : base(5) {} };

to create little less ugly, add together layer of indirection, e.g. template class baseforc yields right base of operations baseforc<t>::type , encapsulates policy , is_polymorphic doing 1 class.

c++ c++11 typetraits