optional instance created by placement-new with stack-memory

optional_instance.h

/*
 * optional instance created by placement-new with stack-memory
 *
 * ex.
 * struct ThreadContextScope {
 *    ThreadContextScope() { // setup thread-context }
 *    ~ThreadContextScope() { // teardown thread-context }
 * };
 *
 * void some_function(void) {
 *    OPTIONAL_INSTANCE(ThreadContextScope, HasThreadContext() == false);
 *    ...
 */
#include <new> /* 'operator new' : function does not take X arguments */

#define CONCATENATE_DIRECT(s1, s2) s1##s2
#define CONCATENATE(s1, s2) CONCATENATE_DIRECT(s1, s2)
#define ANONYMOUS_VARIABLE(str) CONCATENATE(str, __LINE__)

#define ALIGN(DECL, ALIGNMENT) __declspec(align(ALIGNMENT)) DECL
#define ALIGN_OF(...) __alignof(__VA_ARGS__)

#define OPTIONAL_INSTANCE(klass, condition, ...) \
   ALIGN(char ANONYMOUS_VARIABLE(STACKMEMORY_OPTIONAL_INSTANCE)[sizeof(klass)], ALIGN_OF(klass)); \
   CallDestructor<klass> ANONYMOUS_VARIABLE(on_exit_scope)((condition) ? (new (ANONYMOUS_VARIABLE(STACKMEMORY_OPTIONAL_INSTANCE)) klass(##__VA_ARGS__)) : 0)

template <typename T>
struct CallDestructor {
   CallDestructor(T *ptr) : p(ptr) {}
   ~CallDestructor() {
      if (p)
         p->~T();
   }
   T *p;
};