08. Operator Services - gracicot/kangaru GitHub Wiki

In kangaru, there are built-in services that their purpose is to make specific operations on the container. They help encapsulate which jobs on the container a class is able to do.

They generaly wrap the container and allow only an operation to do with the container.


There's a service called kgr::container_service. It's a special service that inject a reference to the container itself.

You can call it directly, use it in invoke or as dependency:

struct Type {
    kgr::container& container;

struct TypeService : kgr::service<Type, kgr::dependency<kgr::container_service>> {};

kgr::container container1;
kgr::container& container2 = conatiner1.service<container_service>();

assert(&container1 == &container2); // passes, both are the same

container1.invoke([&](kgr::container& container3, Type type) {
    // Both passes
    assert(&container1 == &container3);
    assert(&container3 == &type.container);


There's another service called kgr::fork_service. It's a service that will call container.fork(), and then inject the fork.

Just like the container service, we can use it as a depencency and in parameter injection:

kgr::container container1;

// equivalent to:
// kgr::container container2 = container1.fork();
kgr::container container2 = container1.service<fork_service>();

container.invoke([](kgr::container fork) {
    // fork is a forked container, we recieved the container by value.

The fork service has another version called kgr::filtered_fork_service<F>, where F is a predicate to filter services:

kgr::container container1;

// equivalent to:
// kgr::container container2 = container1.fork<kgr::none_of<Service1, Service2>>();
// Every service from container1 are observed by the fork except Service1 and Service2
kgr::container container2 = container1.service<filtered_fork_service<kgr::none_of<Service1, Service2>>>();


A generator is an object that it's purpose is to create a specific service. It's kind of a factory that internally uses the container. The class generator<T> is a template class where T is a service definition.

The generator class has a service definition named generator_service.

It has one member function: operator(...). This function is equivalent to calling the service<T>(...) function on the container. It will forward every arguments to the service function.

Here's an example:

// SceneService is a non-single service.
kgr::generator<SceneService> sceneGenerator = container.service<kgr::generator_service<SceneService>>();

auto scene1 = sceneGenerator();
auto scene2 = sceneGenerator();
auto scene3 = sceneGenerator("special parameter");

There's another version of the generator for a forked container. It's called forked_generator, and it's definition forked_generator_service.


As it's name says, this class serves the purpose of calling the invoke function from the container. Why using kgr::invoker when you can just use container.invoke? Because again, it encapsulate what you can do with the container, and express your intent of only calling invoke on it.

Here's a code snippet using kgr::invoker:

int send_message(MessageBus&, Window&, double timeout);

kgr::invoker invoker = container.service<kgr::invoker_service>();

invoker(send_message, 10); // calls send_message with 10 as it's timeout

When using the default invoker, you can send a map as the first parameter of the function object. This can be used to specify which map to use for each call:

struct custom_map;
kgr::invoker invoke = container.service<kgr::invoker_service>();

invoke(kgr::map<custom_map>{}, send_message, 10);

If sending the map as first parameter is not the ideal, there is a way to specify which maps to use in advance. This is effectively a curried kgr::invoker:

kgr::mapped_invoker<kgr::map<MyMap1, MyMap2>> invoker =
    container.service<kgr::mapped_invoker_service<kgr::map<MyMap1, MyMap2>>>();

Also, just like the generator, another version of the invoker for a forked container is provided, called kgr::forked_invoker, and it's definition kgr::forked_invoker_service.


The lazy class is used to represent a service that will be constructed later. The type lazy<T> takes a service definition as parameter. That service definition will be used with the container to instantiate your object.

The lazy class has the dereference operator * and the arrow operator -> to access the contained object. Additionally, it has a get() method that returns a reference to the contained object.

The object is first instantiated when the first access operator of function is called. The instance is then reused after.

Lazy is provided with a service definition named lazy_service<T> where T is a service definition.

You can use it like this:

// The contained 'MessageBus&' is not constructed yet.
kgr::lazy<MessageBusService> lazy_message_bus = container.service<kgr::lazy_service<MessageBusService>>();

// MessageBusService is constructed here, the operator* is used.
std::cout << *lazy_message_bus;

// The same instance is reused again and returned by operator->

And again, there's the equivalent of lazy with a forked container, named forked_lazy<T> and forked_lazy_service<T>

Override Range

The v4.2.0 version added a new operator service named kgr::override_range_service<T> where T is a polymorphic service. It injects a range of services that overrides T. It can be useful to iterate and do operation on a multiple services that offers the same interface.

It return a kgr::override_range<X> where X is implementation defined.

This type should be only used to iterate on services immediately. Adding a new override in the container will invalidate the range.

Here's an example of use:

struct Base {
    virtual void print() { std::cout << "Base\n" << std::endl; }

struct Derived : Base {
    void print() override { std::cout << "Derived\n" << std::endl; }

struct BaseService : kgr::single_service<Base>, kgr::polymorphic {};
struct DerivedService : kgr::single_service<Derived>, kgr::overrides<BaseService> {};

kgr::container container{};


auto base_range = container.service<kgr::override_range_service<BaseService>>();

for (Base& service : range) {
    service.print(); // prints Base and Derived

In this example, we defined two services, one overrides the other. Then we add them both in the container. After that we ask the container for a range of every service that overrides BaseService. The base serivce instance is included in the range.

The loop will pass over both services and will call print.


While you can use the container directly everywhere, you can also be more fined grained over what a particular piece of code should be able to do with the container. It eventually reduces coupling with kangaru and help expressing your intent with other programmer about what you will do with the container.

Also, some special built in service exposes features of the container that is only available through them. They are great tools that can help simplify code using the container.

Please visit example7 to see more of operator service usage.

Next chapter: Custom Service Definitions

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