How can I make compile time assertions without C++11

Question

In a job interview, I was asked to write a metafunction that determined whether a type was a pointer. This is what I presented:

template <typename T>
struct is_pointer
{ static const bool value = false; }

template <typename T>
struct is_pointer<T *>
{ static const bool value = true; }

Then I was asked to write a meta-assert, that will fail during compile time if my is_pointer function is not doing the right thing.

When I used static_assert, he explicitly told me that I may only use C++98 standard. How can I achieve this?

Answer 1

There are different approaches, a common one trying to typedef an invalid type:

#define static_assert(condition) \
        typedef char assert ## __LINE__ [((condition)?1:-1)]

This can be used in mostly any context and will trip the compiler if the condition is false, since it would try to typedef an invalid type (array of negative number of elements). It can be used in different contexts:

// namespace level:
static_assert(sizeof(int)==4);
struct type {
   // class level:
   static_assert(sizeof(int)==4);
   void f() {
       // function level
       static_assert(sizeof(int)==4);
   }
};