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122 lines
2.6 KiB
Markdown
122 lines
2.6 KiB
Markdown
---
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title: "singleton"
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---
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{{< callout type="info">}}
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Header: `singleton.h`
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{{< /callout >}}
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Allows creation of a singleton, with deterministic construction and destruction.
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```cpp
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etl::singleton<typename T>
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```
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`T` The type to use as a singleton.
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Before create Is called, the singleton is in the invalid state.
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See also `etl::singleton_base`.
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## Static functions
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**C++03 and below.**
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```cpp
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static void create()
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```
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Creates the singleton. Default constructs the instance.
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Sets the singleton to the valid state.
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Only the first call to create is effective.
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Must be called before any calls to `instance()`.
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---
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```cpp
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template <typename T1>
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static void create(const T1& p1)
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```
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Construct from 1 parameter.
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---
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```cpp
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template <typename T1, typename T2>
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static void create(const T1& p1, const T2& p2)
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```
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Construct from 2 parameters.
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---
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```cpp
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template <typename T1, typename T2, typename T3>
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static void create(const T1& p1, const T2& p2, const T3& p3)
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```
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Construct from 3 parameters.
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---
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```cpp
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template <typename T1, typename T2, typename T3, typename T4>
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static void create(const T1& p1, const T2& p2, const T3& p3, const T4& p4)
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```
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Construct from 4 parameters.
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---
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**C++11 and above.**
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```cpp
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template <typename... TArgs>
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static void create(TArgs&&... args)
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```
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Creates the singleton and constructs the type using the supplied parameters.
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Sets the singleton to the valid state.
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Only the first successful call to create is effective. Does nothing if the singleton is already valid.
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Must be called before any calls to `instance()`.
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---
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```cpp
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static T& instance()
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```
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Returns a reference to the one instance of `T`.
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Asserts an `etl::singleton_not_created` if create has not been called.
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---
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```cpp
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static void destroy()
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```
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Calls the destructor of the instance.
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Sets the singleton to the invalid state.
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Only the first successful call to destroy is effective. Does nothing if the singleton is already invalid.
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---
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```cpp
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static bool is_valid()
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```
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Returns `true` if the instance is valid.
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Returns `false` before `create` and after `destroy` are called.
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---
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## Example
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```cpp
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class MyType;
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using MySingleton = etl::singleton<MyType>;
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bool is_valid;
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is_valid = MySingleton::is_valid(); // false
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MyType& mt1 = MySingleton::instance(); // Raises ETL_ASSERT etl::singleton_not_created
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MySingleton::create(1, "Hello World"); // Construct with parameters
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is_valid = MySingleton::is_valid(); // true
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MyType& mt2 = MySingleton::instance(); // Get the instance
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is_valid = MySingleton::is_valid(); // true
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MySingleton::destroy(); // Destruct the instance
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is_valid = MySingleton::is_valid(); // false
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```
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