Question

I need help getting the broken part of this code working.

How do I tag dispatch two functions (that return different value-types) based on a string?

If the overall code can be simplified with the intent of dispatching with strings, please do make recommendations. TY.

Requirements:

1. Dispatch based on a string
2. Rectangle overload needs to return int, while Circle overload needs to return std::string
3. The mapping from Rectangle_Type to int and Circle_Type to std::string is fixed and known at compile time. Part of my problem is std::map is a run-time construct: I don’t know how to make the std::string to tag mapping a compile-time construct.
4. If necessary, run-time resolution is okay: however, the dispatch must allow for different return types based on the enum/type resolved to.

CODE

#include <map>
#include <string>
#include <iostream>

struct Shape    { };
struct Rectangle_Type : public Shape { using value_type=int;         };
struct Circle_Type    : public Shape { using value_type=std::string; };

Rectangle_Type Rectangle;
Circle_Type    Circle;

static std::map<std::string,Shape*> g_mapping =
{
    { "Rectangle", &Rectangle },
    { "Circle",    &Circle    }
};

int tag_dispatch( Rectangle_Type )
{
    return 42;
}

std::string tag_dispatch( Circle_Type )
{
    return "foo";
}

int
main()
{
    std::cerr << tag_dispatch( Circle    ) << std::endl;   // OK
    std::cerr << tag_dispatch( Rectangle ) << std::endl;   // OK

#define BROKEN
#ifdef BROKEN
    std::cerr << tag_dispatch( (*g_mapping["Rectangle"]) ) << std::endl;
    std::cerr << tag_dispatch( (*g_mapping["Circle"])    ) << std::endl;
#endif
}

Answer 1

Unless C++11 has changed this.. The problem is that you are dereferencing a Shape* pointer, which means the resulting datatype (Shape&) does not have a valid overload of tag_dispatch.

You can do something like g_mapping["Rectangle"]->tag_dispatch(). Or more cleanly rewrite as below.

std::string tag_dispatch( Shape& shape)
{
    return shape->tag_dispatch();
}

This way you can support non Shape objects with an identical interface. Both need you to make tag_dispatch as a virtual function of Shape as well.