C++ Registry Pattern

A self-registering map of functions, allowing for dynamic dispatching based on some identifier. Example usages may be constructing a subclass or using an appropriate I/O function based on an enum value or a string key. All functions in the map need to have the same signature in this implementation. Multiple threads can read from the map using Dispatch() at the same time, but do not call Register() and Dispatch() from different threads as the same time.

Since the Registry template instantiation can be quite verbose and registration is a bit of boilerplate, it is recommended that users create a type alias and a macro for registration. For example:

using BaseRegistry = Registry<std::string,
                              std::unique_ptr<Base>(int, double)>;
#define REGISTER_BASE_SUBCLASS(subclass)                         \
    static bool _registered_##subclass = BaseRegistry::Register( \
        #subclass, [](int a, double b) {                         \
            return std::unique_ptr<Base>(new subclass(a, b));    \
        });

The following policies for missing keys are supported:

1. MissingKeyPolicy::exception - throws the std::out_of_range from unordered_map's at() when the key does not exist.
2. MissingKeyPolicy::default_construct - returns a default-constructed object when the key does not exist.
3. MissingKeyPolicy::optional - Dispatch now returns a std::optional of Func's return type. This is only supported when C++17 is enabled.

NOTE: If the registration is taking place in a STATIC library linked to the executable, then all of the headers with the registrations must be included or the library must be linked with with the platform-specific whole library flags, such as

-Wl,--whole-archive -lmylib -Wl,--no-whole-archive

on Max OS X, this is done with the flags

-Wl,-force_load -lmylib

Template Parameters

• Key - The identifier type for the function map
• Func- The function signature type for the function map
• MKP - The behavior policy for what to do in the case of a missing key
• Hash - The hash function to use for the function map
• KeyEqual - The key equality function for the function map
• Allocator - The allocator to use for the function map

Examples

See the examples directory for an example with CMake.