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## Design Overview

### Type Erasure

"Type erasure", where static type information is eliminated by the use of dynamically dispatched interfaces, is used extensively within the Boost.Signals library to reduce the amount of code generated by template instantiation. Each signal must manage a list of slots and their associated connections, along with a `std::map` to map from group identifiers to their associated connections. However, instantiating this map for every token type, and perhaps within each translation unit (for some popular template instantiation strategies) increase compile time overhead and space overhead.

To combat this so-called "template bloat", we use Boost.Function and Boost.Any to store unknown types and operations. Then, all of the code for handling the list of slots and the mapping from slot identifiers to connections is factored into the class `signal_base` that deals exclusively with the `any` and `function` objects, hiding the actual implementations using the well-known pimpl idiom. The actual `signalN` class templates deal only with code that will change depending on the number of arguments or which is inherently template-dependent (such as connection).

### `connection` class

The `connection` class is central to the behavior of the Boost.Signals library. It is the only entity within the Boost.Signals system that has knowledge of all objects that are associated by a given connection. To be specific, the `connection` class itself is merely a thin wrapper over a `shared_ptr` to a `basic_connection` object.

`connection` objects are stored by all participants in the Signals system: each `trackable` object contains a list of `connection` objects describing all connections it is a part of; similarly, all signals contain a set of pairs that define a slot. The pairs consist of a slot function object (generally a Boost.Function object) and a `connection` object (that will disconnect on destruction). Finally, the mapping from slot groups to slots is based on the key value in a `std::multimap` (the stored data in the `std::multimap` is the slot pair).

### Slot Call Iterator

The slot call iterator is conceptually a stack of iterator adaptors that modify the behavior of the underlying iterator through the list of slots. The following table describes the type and behavior of each iterator adaptor required. Note that this is only a conceptual model: the implementation collapses all these layers into a single iterator adaptor because several popular compilers failed to compile the implementation of the conceptual model.

Iterator Adaptor | Purpose
--- | ---
Slot List Iterator | An iterator through the list of slots connected to a signal. The `value_type` of this iterator will be `std::pair<any, connection>`, where the `any` contains an instance of the slot function type.
Filter Iterator Adaptor | This filtering iterator adaptor filters out slots that have been disconnected, so we never see a disconnected slot in later stages.
Projection Iterator Adaptor | The projection iterator adaptor returns a reference to the first member of the pair that constitutes a connected slot (e.g., just the `boost::any` object that holds the slot function).
Transform Iterator Adaptor | This transform iterator adaptor performs an `any_cast` to extract a reference to the slot function with the appropriate slot function type.
Transform Iterator Adaptor | This transform iterator adaptor calls the function object returned by dereferencing the underlying iterator with the set of arguments given to the signal itself, and returns the result of that slot call.
Input Caching Iterator Adaptor | This iterator adaptor caches the result of dereferencing the underlying iterator. Therefore, dereferencing this iterator multiple times will only result in the underlying iterator being dereferenced once; thus, a slot can only be called once but its result can be used multiple times.
Slot Call Iterator | Iterates over calls to each slot.

### `visit_each` function template

The `visit_each` function template is a mechanism for discovering objects that are stored within another object. Function template `visit_each` takes three arguments: an object to explore, a visitor function object that is invoked with each subobject, and the `int` 0.

The third parameter is merely a temporary solution to the widespread lack of proper function template partial ordering. The primary `visit_each` function template specifies this third parameter type to be `long`, whereas any user specializations must specify their third parameter to be of type `int`. Thus, even though a broken compiler cannot tell the ordering between, e.g., a match against a parameter `T` and a parameter `A<T>`, it can determine that the conversion from the integer 0 to `int` is better than the conversion to `long`. The ordering determined by this conversion thus achieves partial ordering of the function templates in a limited, but successful, way. The following example illustrates the use of this technique:

    template<typename> class A {};
    template<typename T> void foo(T, long);
    template<typename T> void foo(A<T>, int);
    A<T> at;
    foo(at, 0);

In this example, we assume that our compiler can not tell that `A<T>` is a better match than `T`, and therefore assume that the function templates cannot be ordered based on that parameter. Then the conversion from 0 to `int` is better than the conversion from 0 to `long`, and the second function template is chosen.

| Copyright © 2001-2003 Douglas Gregor

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