Question

C++11 includes a wonderful amount of great features when it comes to type deduction and management altogether. For example auto and decltype -keywords have proven themselves to be a worthy addition to the language.

As I adopted these simple, yet effective features, I begin to think of implementing some kind of reflection system. Here’s what I’ve managed to pull off this far:

/// ------------------------------------------------------------
/// @class  Reflection
/// @brief  General-purpose reflection class.
/// @exmpl  Get type id:
///             auto a = Reflection::get_id_type<int>();
///             auto b = Reflection::get_id_type<Object>();
///         Get type via received id:
///             decltype(Reflection::get_type(a)) d;
/// @note   It is forbidden to create an instance of this class.
/// ------------------------------------------------------------
class Reflection{
public:
    /// Static member functions:
    template<typename T>
    static inline long get_id_type(void){
        return reinterpret_cast<long>(&Database<T>::id);
    }
    static auto get_type(long const type_id) -> decltype(/* UNFINISHED! */){ // This is where I'm having problems.
        // This function body is intentionally left empty.
        // All that matters is the return type.
    }
private:
    /// Inner structures:
    template<typename T>
    struct Database{
        static void* id; // Created void pointer here, because only the address of this variable matters.
    };
    /// Constructors & destructors:
    Reflection(void) = delete;
    ~Reflection(void) = delete;
    /// Member functions (overloaded operators):
    Reflection& operator=(Reflection&&) = delete;
    Reflection& operator=(Reflection const&) = delete;
};

The code should be easy enough to understand. If you read the overall code with the comments, you should realize how to use this class. But the question is:

“How do I return an expression from function “get_type” in order to
turn this expression into usable type by decltype-specifier?”

Thanks in advance.

Answer 1

You can’t have a return type that depends on the runtime value of an argument. Period. What you’re trying to do is not feasible.