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# hololink::module::ConfigurableService

> A service whose instance is constructed from EnumerationMetadata.

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

A service whose instance is constructed from [EnumerationMetadata](enumerationmetadata).

Services are instances addressed by instance_id; `ConfigurableService` constructs the instance_id from fields in EnumerationData, via Derived::locator_id, (frequently of the form "serial=X;instance=Y" or another string uniquely identifying the implementation object). When the [Module](module) doesn't yet have an instance of that service, [EnumerationMetadata](enumerationmetadata) will be used to initialize it.

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

**Template parameters**

**`Derived`** `typename`

The service class being resolved.

---

**Inherits from:** `hololink::module::Service< Derived >` (public)

---

## Methods

### configure \[#configure]

```cpp showLineNumbers={false}
virtual void hololink::module::ConfigurableService<Derived>::configure(
    const EnumerationMetadata &
)
```

Use `metadata` contents to initialize this instance.

This routine is guaranteed to be called the first time a call to get_service(metadata) is called on a specific service instance.

### module \[#module]

```cpp showLineNumbers={false}
std::shared_ptr<Module> hololink::module::ServiceBase::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]

#### Fetch a service

```cpp showLineNumbers={false}
static std::shared_ptr<Derived> hololink::module::ConfigurableService<Derived>::get_service(
    const EnumerationMetadata &metadata,
    bool allow_null = false
)
```

Fetch a service, configured by [EnumerationMetadata](enumerationmetadata).

[Module](module) usually caches service objects; so when a specific service instance isn't found, it will be constructed with the current values in [EnumerationMetadata](enumerationmetadata). Later calls to get_service are likely to return that cached instance, ignoring changes to any enumeration metadata that aren't specifically related to service addressing.

**Returns:** A shared pointer to the requested service (\<Derived>).

**Throws:** `std::runtime_error` when allow_null==false and the service lookup (or construction) fails.

**Parameters**

**`metadata`** `const EnumerationMetadata &`

Addressing and configuration for the service.

---

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

Use `true` to return nullptr when the service cannot be resolved; the default `false` value raises a std::runtime_error.

---

#### Overload 2

inline

```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.