Value wrapper inspired by State monad, designed to solve http://stackoverflow.com/questions/1516919/declaring-and-initializing-a-variable-in-a-conditional-or-control-statement-in-c

control-state.cpp

#include <iostream>

template<typename T, typename U> class wrapped_value {
   T t;
   U u;
   wrapped_value(T t, U u): t(std::move(t)), u(std::move(u)) {}
public:
   explicit operator bool() const { return u; }
   friend T &unwrap(wrapped_value &w) { return w.t; }
   template<typename T2, typename V> friend class wrapped_value;
   template<typename F> auto bind(F &&f)
   -> wrapped_value<T, decltype(f(std::declval<U>()))> {
      return {std::move(t), f(u)}; }
};

template<typename T> wrapped_value<T, T> wrap(T &&t) { return {t}; }
template<typename T> class wrapped_value<T, T> {
   T t;
   wrapped_value(T t): t(std::move(t)) {}
public:
   friend wrapped_value wrap<>(T &&t);
   template<typename F> auto bind(F &&f)
   -> wrapped_value<T, decltype(f(std::declval<T>()))> {
      auto &&v = f(t); return {std::move(t), v}; }
};

template<typename T, typename U, typename V>
auto operator>(wrapped_value<T, U> &&w, V &&v)
-> wrapped_value<T, decltype(std::declval<U>() > v)> {
      return w.bind([v](const U &u) { return u > v; }); }

int main() {
   for (auto i: {9, 10, 11, 12}) {
      if (auto value = wrap(i * 10).bind([](int x) { return x > 100; }))
         std::cout << i << ' ' << unwrap(value) << '\n';
      if (auto value = wrap(i * 11) > 99)
         std::cout << i << ' ' << unwrap(value) << '\n';
   }
}

http://stackoverflow.com/questions/1516919/declaring-and-initializing-a-variable-in-a-conditional-or-control-statement-in-c