superbuild/modules/entt/docs/md/locator.md

Crash Course: service locator

Table of Contents

• Introduction
• Service locator

Introduction

Usually service locators are tightly bound to the services they expose and it's hard to define a general purpose solution.
This tiny class tries to fill the gap and to get rid of the burden of defining a different specific locator for each application.

Service locator

The service locator API tries to mimic that of std::optional and adds some extra functionalities on top of it such as allocator support.
There are a couple of functions to set up a service, namely emplace and allocate_emplace:

entt::locator<interface>::emplace<service>(argument);
entt::locator<interface>::allocate_emplace<service>(allocator, argument);

The difference is that the latter expects an allocator as the first argument and uses it to allocate the service itself.
Once a service has been set up, it's retrieved using the value function:

interface &service = entt::locator<interface>::value();

Since the service may not be set (and therefore this function may result in an undefined behavior), the has_value and value_or functions are also available to test a service locator and to get a fallback service in case there is none:

if(entt::locator<interface>::has_value()) {
    // ...
}

interface &service = entt::locator<interface>::value_or<fallback_impl>(argument);

All arguments are used only if necessary, that is, if a service doesn't already exist and therefore the fallback service is constructed and returned. In all other cases, they are discarded.
Finally, to reset a service, use the reset function.