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# hololink::module::Service

> Service provides a get_service method which provides a type-safe way to fetch a <Derived> service implementation from a module.

```cpp showLineNumbers={false}
template <typename Derived>
class Service
```

`Service` provides a [`get_service`](#getservice) method which provides a type-safe way to fetch a \<Derived> service implementation from a module.

Note that the same service instance may implement any number of `Service` interfaces, so an interface is provided allowing a visitor to see the list of service names that these objects support.

```cpp showLineNumbers={false}
#include <hololink/module/service.hpp>
```

**Template parameters**

**`Derived`** `typename`

---

**Inherits from:** `hololink::module::ServiceBase` (public)

---

## Methods

### module \[#module]

```cpp showLineNumbers={false}
std::shared_ptr<Module> hololink::module::Service::module() const
```

The [Module](module) that this service was published from.

The service holds no strong reference to its [Module](module), so this does not extend the [Module](module)'s lifetime.

**Returns:** The owning [Module](module), or an empty pointer if the service has not been published or its [Module](module) no longer exists.

---

## Static methods

### get_service \[#getservice]

```cpp showLineNumbers={false}
static std::shared_ptr<Derived> hololink::module::Service<Derived>::get_service(
    std::shared_ptr<Module> module,
    const char *instance_id,
    bool allow_null = false
)
```

Type-safe way to fetch a std::shared_ptr to an instance of \<Derived> from a [Module](module).

Note that [Module](module) usually caches instances, it's likely that calls to get_service with the same locator_id will return pointers to the same implementation object.

**Returns:** A shared pointer to the service instance.

**Parameters**

**`module`** `std::shared_ptr<Module>`

Is the shared object that owns the instance we wish to fetch

---

**`instance_id`** `const char *`

Names the specific object, usually expressed as a set of "name=value;..." pairs, as defined by the implementations themselves. This name is normally fetched by calling `Derived::locator_id(enumeration_metadata)`.

---

**`allow_null`** `bool` — default: false

The default value of `false` causes this routine to throw a std::runtime_error if the service cannot be found within the module; this would be expected to terminate the calling program. If the application wants to handle the error, pass in `true` here and get_service will return `nullptr` when this module doesn't provide the requested service.

---

### for_each_type_id \[#foreachtypeid]

```cpp showLineNumbers={false}
template <typename Callback>
static bool hololink::module::Service<Derived>::for_each_type_id(
    Callback &&cb
)
```

Iterate over every type_id supported by this instance.

The callback signature is `bool(const char*)` — returning false halts the walk; true continues. Iteration is depth-first, so you'll see the most specific implementation types first.