

## method

### Synopsis

```c++
namespace boost::openmethod {

template<
    typename Method, typename ReturnType,
    class Policy = BOOST_OPENMETHOD_DEFAULT_POLICY>
class method;

template<typename Name, typename... Parameters, typename ReturnType, class Policy>
class method<Name(Parameters...), ReturnType, Policy> {
  public:
    using function_type = ReturnType (*)(CallParameters...);

    auto operator()(CallParameters... args) const -> ReturnType;

    static method fn;

    template<auto... Functions>
    struct override;

    template<auto Overrider>
    static function_type next;

  private:
    method();
    method(const method&) = delete;
    method(method&&) = delete;
    ~method();
};

}
```

### Description

`method` implements an open-method that takes a parameter list - `Parameters` -
and returns a `ReturnType`. `Name` can be any type. Its purpose is to make it
possible to have multiple methods with the same signature. Typically, `Name` is
a class whose name reflects the method's purpose.

`Parameters` must contain at least one virtual parameter, i.e. a parameter that
has a type in the form `virtual_ptr<T,{nbsp}Policy>` or `virtual_<T>`. The
dynamic types of the virtual arguments (the arguments corresponding to virtual
parameters in the method's signature) are taken into account to select the
overrider to call.

A `method` is attached to a `Policy`, which influences several parts of the
dispatch mechanism - for example, how to obtain a v-table pointer for an object,
how to report errors, whether to perform sanity checks, etc.

### Members

#### constructor

```c++
method();
```

Add the method to the list of methods registered in `Policy`.

The constructor is private. The only instance is the static member variable
`fn`.

#### destructor

```c++
~method();
```

Remove the method from the list of methods registered in `Policy`.

#### operator()

```c++
auto operator()(CallParameters... args) const -> ReturnType;
```

Call the method with the arguments `args`.

`CallParameters` are the `Parameters` without the `virtual_` decorators. Note
that `virtual_ptr`{empty}s are preserved.

The overrider is selected in a process similar to overloaded function
resolution, with extra rules to handle ambiguities. It proceeds as follows:

1. Form the set of all applicable overriders. An overrider is applicable if it
   can be called with the arguments passed to the method.

2. If the set is empty, call the error handler (if present in the policy), then
   terminate the program with `abort`

3. Remove the overriders that are dominated by other overriders in the set.
   Overrider A dominates overrider B if any of its virtual formal parameters is
   more specialized than B's, and if none of B's virtual parameters is more
   specialized than A's.

4. If the resulting set contains only one overrider, call it.

5. If the return type is a registered polymorphic type, remove all the
   overriders that return a less specific type than the others.

6. If the resulting set contains only one overrider, call it.

7. Otherwise, call one of the remaining overriders. Which overrider is selected
   is not specified, but it is the same across calls with the same arguments
   types.

For each virtual argument `arg`, the dispatch mechanism calls
`virtual_traits::peek(arg)` and deduces the v-table pointer from the `result`,
using the first of the following methods that applies:

1. If `result` is a `virtual_ptr`, get the pointer to the v-table from it.

2. If a function named `boost_openmethod_vptr` that takes `result` and returns a
   `vptr_type` exists, call it.

3. Call `Policy::dynamic_vptr(result)`.

#### fn

```c++
static method fn;
```

The `method`{empty}'s unique instance. The method is called via the call
operator on `fn`: `method::fn(args...)`.

#### override

```c++
template<auto... Functions>
struct override;
```

Add _Functions_ to the overriders of `method`.

#### next

```c++
template<auto Overrider>
static function_type next;
```

Pointer to the next most specialized overrider after _Overrider_, i.e. the
overrider that would be called for the same tuple of virtual arguments if
_Overrider_ was not present. Set to `nullptr` if no such overrider exists.
