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title: "delegate"
weight: 1
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etl::delegate is a type-safe, generic, and efficient delegate implementation. A delegate is essentially a type-safe function pointer. It can be used to encapsulate a callable unit (like a function, a function pointer, member function, or a callable object like a functor or a lambda function) and call it later.

etl::delegate is similar in a lot of ways to std::function_ref introduced with C++26.

The delegate functions may be defined at compile time and/or runtime, depending on the function type.
Most delegates can only be constructed via a create function, except lambdas and functors, which can be created by the delegate constructor.

This may be used to implement a platform abstraction layer that allows an embedded application to interface with multiple hardware platforms.

Here's a high-level overview of how etl::delegate works:
Creation: 
When you create a delegate, you specify the function signature that it should match. This is done using template parameters. For example, etl::delegate<void(int)> creates a delegate that can hold references to functions that take an integer as an argument and return void.
Assignment: 
You can assign a function to the delegate using the create method. The function you assign must match the delegate's function signature. For example, if you have a delegate of type etl::delegate<void(int)>, you could assign a function void myFunction(int) to it like this: 
auto myDelegate = etl::delegate<void(int)>::create<myFunction>();
Invocation: 
Once a function has been assigned to the delegate, you can call that function through the delegate just like you would call a regular function. For example, if myDelegate is a delegate that holds a reference to myFunction, you could call myFunction through the delegate like this: myDelegate(123);
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Notes: 
<=20.21.0 
This class only supports >=C++11.

>=20.22.0 
This class is automatically selects C++03 or C++11 versions based on the value of ETL_USING_CPP11
The C++03 variant only supports delegates taking 0 or 1 parameter.

etl::delegate is non-owning. You cannot use lambdas that capture variables that will have gone out of scope when the delegate is called. This would result in  dangling references. Lambdas are not copied; The delegate merely keeps pointers to lambdas. This also applies to functors.

>=20.40.0
Move constructors from lambdas or functors are disabled, thereby stopping a delegate being created from a temporary defined in aparameter.

>=20.42.0
etl::delegate is noexcept
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etl::delegate<TReturn(TParams…)>

20.22.0
For C++03 
etl::delegate<TReturn(void)>
etl::delegate<TReturn(TParam)>

For the examples below, assume the following definitions.

class Test
{
public:
  
  int member_function(int);
  static int member_static(int);
  int operator()(int);
  int operator()(int) const;
};

Test test;

int global(int);

auto lambda = [](int i){ return i; };
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Public types

return_type    TReturn                     20.43.0
argument_types etl::type_list<TParams...>  20.43.0
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Run time construction

Lambdas
etl::delegate<int(int)> d(lambda);
etl::delegate<int(int)> d = lambda;
etl::delegate<int(int)> d = etl::delegate<int(int)>::create(lambda);

d.set(lambda);

Functors
etl::delegate<int(int)> d(test);
etl::delegate<int(int)> d = test;
etl::delegate<int(int)> d = etl::delegate<int(int)>::create(test);

d.set<test>();

Member functions
etl::delegate<int(int)> d = etl::delegate<int(int)>::create<Test, &Test::member_function>(test);

d.set<Test, &Test::member_function>(test);
d.set<Test, test, &Test::member_function>();
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Compile time construction

Global functions
auto d = etl::delegate<int(int)>::create<global>();

Member functions (if the instance is known at compile time)
auto d = etl::delegate<int(int)>::create<Test, test, &Test::member_function>(); // Deprecated
auto d = etl::delegate<int(int)>::create<Test, &Test::member_function, test>(); // New

Functors (if the instance is known at compile time)
auto d = etl::delegate<int(int)>::create<Test, test>();
auto d = etl::delegate<int(int)>::create<const Test, test>();
Note: These are disabled for GCC <= v8.1 as it generates an 'Internal Compiler Error'.

Static member functions
auto d = etl::delegate<int(int)>::create<Test::member_static>();
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Constexpr

Most delegates can be declared as constexpr. (C++11 and above)
static Test test;
constexpr auto d = etl::delegate<int(int)>::create<Test, test, &Test::member_function>();

All create functions are constexpr under C++14.
All create functions are [[nodiscard]] under C++17.
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Calling the delegate

The delegate may be called as a function with the defined parameter signature.

etl::delegate<int(int)> d;

int r = d(3);
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Operations

ETL_CONSTEXPR14 delegate()
Constructs an uninitialised delegate.
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ETL_CONSTEXPR14 delegate(const delegate& other)
Copy constructs a delegate.
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ETL_CONSTEXPR14 TReturn operator()(TParams... args) const ETL_NOEXCEPT
Executes the delegate.
constexpr from 20.42.1
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20.17.0
ETL_CONSTEXPR14 bool call_if(TParams... args) const ETL_NOEXCEPT
For delegates returning void.
Calls the delegate if valid.
Returns true if valid, otherwise false.
constexpr from 20.42.2

ETL_CONSTEXPR14 etl::optional<TReturn> call_if(TParams... args) const ETL_NOEXCEPT
For delegates returning TReturn.
Calls the delegate if valid.
Returns a valid etl::optional<TReturn> containing the return value, if valid.
constexpr from 20.42.2
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20.17.0
template <typename TAlternative>
ETL_CONSTEXPR14 TReturn call_or(TAlternative alternative, TParams... args) const ETL_NOEXCEPT
Calls the delegate if valid, otherwise calls alternative.
constexpr from 20.42.2

template <TReturn(*Method)(TParams...)>
ETL_CONSTEXPR14 TReturn call_or(TParams... args) const ETL_NOEXCEPT
Calls the delegate if valid, otherwise calls Method.
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void clear() ETL_NOEXCEPT
Sets the delegate back to the uninitialised state.
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ETL_NODISCARD ETL_CONSTEXPR14 bool is_valid() const ETL_NOEXCEPT
ETL_CONSTEXPR14 operator bool() const
Returns true if the delegate has been initialised, otherwise false.
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ETL_CONSTEXPR14 operator =() ETL_NOEXCEPT
Assigns from another delegate or lambda.
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ETL_CONSTEXPR14 operator ==() ETL_NOEXCEPT
ETL_CONSTEXPR14 operator !=() ETL_NOEXCEPT
Compares delegates.
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make_delegate
For C++17 and above.
20.39.5
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template <auto Function>
constexpr auto make_delegate() noexcept
Make a delegate from a free function.
Returns etl::delegate<function_type>::create<Function>();
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template <typename TLambda>
constexpr auto make_delegate(TLambda& instance) noexcept
Make a delegate from a functor or lambda function.
Returns etl::delegate<function_type>(instance);
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template <typename T, T& Instance>
constexpr auto make_delegate() noexcept
Make a delegate from a functor, compile time.
Returns etl::delegate<function_type>::create<T, Instance>();
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template <typename T, auto Method, T& Instance>
constexpr auto make_delegate() noexcept
Make a delegate from a member function at compile time.
Returns etl::delegate<function_type>::create<T, Method, Instance>();
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template <typename T, auto Method, const T& Instance>
constexpr auto make_delegate() noexcept
Make a delegate from a const member function at compile time.
Returns etl::delegate<function_type>::create<T, Method, Instance>();
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template <typename T, auto Method>
constexpr auto make_delegate(T& instance) noexcept
Make a delegate from a member function at run time.
Returns etl::delegate<function_type>::create<T, Method>(instance);
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template <typename T, auto Method>
constexpr auto make_delegate(const T& instance) noexcept
Make a delegate from a member function at run time.
Returns etl::delegate<function_type>::create<T, Method>(instance);