Implementation

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## Implementation

The following paragraphs describe issues that had to be considered during the implementation of the circular_buffer:

#### Thread-Safety

The thread-safety of the `circular_buffer` is the same as the thread-safety of containers in most STL implementations. This means the `circular_buffer` is not fully thread-safe. The thread-safety is guaranteed only in the sense that simultaneous accesses to distinct instances of the `circular_buffer` are safe, and simultaneous read accesses to a shared `circular_buffer` are safe.

If multiple threads access a single `circular_buffer`, and at least one of the threads may potentially write, then the user is responsible for ensuring mutual exclusion between the threads during the container accesses. The mutual exclusion between the threads can be achieved by wrapping operations of the underlying `circular_buffer` with a lock acquisition and release. (See the Bounded Buffer example code at circular_buffer_bound_example.cpp)

#### Overwrite Operation

Overwrite operation occurs when an element is inserted into a full `circular_buffer` - the old element is being overwritten by the new one. There was a discussion what exactly "overwriting of an element" means during the formal review. It may be either a destruction of the original element and a consequent inplace construction of a new element or it may be an assignment of a new element into an old one. The `circular_buffer` implements assignment because it is more effective.

From the point of business logic of a stored element, the destruction/construction operation and assignment usually mean the same. However, in very rare cases (if in any) they may differ. If there is a requirement for elements to be destructed/constructed instead of being assigned, consider implementing a wrapper of the element which would implement the assign operator, and store the wrappers instead. It is necessary to note that storing such wrappers has a drawback. The destruction/construction will be invoked on every assignment of the wrapper - not only when a wrapper is being overwritten (when the buffer is full) but also when the stored wrappers are being shifted (e.g. as a result of insertion into the middle of container).

#### Writing to a Full Buffer

There are several options how to cope if a data source produces more data than can fit in the fixed-sized buffer:

- Inform the data source to wait until there is room in the buffer (e.g. by throwing an overflow exception).
- If the oldest data is the most important, ignore new data from the source until there is room in the buffer again.
- If the latest data is the most important, write over the oldest data.
- Let the producer to be responsible for checking the size of the buffer prior writing into it.

It is apparent that the `circular_buffer` implements the third option. But it may be less apparent it does not implement any other option - especially the first two. One can get an impression that the `circular_buffer` should implement first three options and offer a mechanism of choosing among them. This impression is wrong.

The `circular_buffer` was designed and optimized to be circular (which means overwriting the oldest data when full). If such a controlling mechanism had been enabled, it would just complicate the matters and the usage of the `circular_buffer` would be probably less straightforward.

Moreover, the first two options (and the fourth option as well) do not require the buffer to be circular at all. If there is a need for the first or second option, consider implementing an adaptor of e.g. std::vector. In this case the `circular_buffer` is not suitable for adapting, because, contrary to std::vector, it bears an overhead for its circular behaviour.

#### Reading/Removing from an Empty Buffer

When reading or removing an element from an empty buffer, the buffer should be able to notify the data consumer (e.g. by throwing underflow exception) that there are no elements stored in it. The `circular_buffer` does not implement such a behaviour for two reasons:

- It would introduce a performance overhead.
- No other std container implements it this way.

It is considered to be a bug to read or remove an element (e.g. by calling `front()` or `pop_back()`) from an empty std container and from an empty `circular_buffer` as well. The data consumer has to test if the container is not empty before reading/removing from it by testing `empty()`. However, when reading from the `circular_buffer`, there is an option to rely on the `at()` method which throws an exception when the index is out of range.

#### Iterator Invalidation

An iterator is usually considered to be invalidated if an element, the iterator pointed to, had been removed or overwritten by an another element. This definition is enforced by the Debug Support and is documented for every method. However, some applications utilizing `circular_buffer` may require less strict definition: an iterator is invalid only if it points to an uninitialized memory.

Consider following example:

    #define BOOST_CB_DISABLE_DEBUG // The Debug Support has to be disabled, otherwise the code produces a runtime error.

    #include <boost/circular_buffer.hpp>
    #include <boost/assert.hpp>
    #include <assert.h>

    int main(int /*argc*/, char* /*argv*/[])
    {

      boost::circular_buffer<int> cb(3);

      cb.push_back(1);
      cb.push_back(2);
      cb.push_back(3);

      boost::circular_buffer<int>::iterator it = cb.begin();

      assert(*it == 1);

      cb.push_back(4);

      assert(*it == 4); // The iterator still points to the initialized memory.

      return 0;
    }

The iterator does not point to the original element any more (and is considered to be invalid from the "strict" point of view) but it still points to the same valid place in the memory. This "soft" definition of iterator invalidation is supported by the `circular_buffer` but should be considered as an implementation detail rather than a full-fledged feature. The rules when the iterator is still valid can be inferred from the code in soft_iterator_invalidation.cpp.

#### Move emulation and rvalues

Since Boost 1.54.0 support for move semantics was implemented using the Boost.Move library. If rvalue references are available `circular_buffer` will use them, but if not it uses a close, but imperfect emulation. On such compilers:

- Non-copyable objects can be stored in the containers. They can be constructed in place using `emplace`, or if they support Boost.Move, moved into place.
- The containers themselves are not movable.
- Argument forwarding is not perfect.

`circular_buffer` will use rvalues and move emulations for value types only if move constructor and move assignment operator of the value type do not throw; or if the value type has no copy constructor.

Some methods won't use move constructor for the value type at all, if the constructor throws. This is required for data consistency and avoidance of situations, when aftrer an exception `circular_buffer` contains moved away objects along with the good ones.

See documentation for `is_copy_constructible`, `is_nothrow_move_assignable` and `is_nothrow_move_constructible` type triats. There you'll find information about how to make constructor of class noexcept and how to make a non-copyable class in C++03 and C++98.

Performance of `circular_buffer` will greatly improve if value type has noexcept move constructor and noexcept move assignment.

#### Exceptions of move_if_noexcept(T&)

Reference documentation of the `circular_buffer` contains notes like "Throws: See Exceptions of `move_if_noexcept(T&)`". That note means the following: `move_if_noexcept(T& value)` does not throws exceptions at all, but it returns `value` as rvalue reference only if class `T` have noexcept move constructor and noexcept move assignment operator; or if it has no copy constructor. Otherwise `move_if_noexcept(T& value)` returns `value` as const reference.

This leads us to the following situation:

- If `value` has a noexcept move constructor and noexcept move assignment operator, then no exceptions will be thrown at all.
- If `value` has a throwing move constructor and some copy constructor, then method may throw exceptions of copy constructor.
- If `value` has no copy constructor, then method may throw exceptions of move constructor.

`move_if_noexcept(T&)` uses Boost.Move, `is_copy_constructible`, `is_nothrow_move_assignable` and `is_nothrow_move_constructible` type triats.

#### Caveats

The `circular_buffer` should not be used for storing pointers to dynamically allocated objects. When a circular buffer becomes full, further insertion will overwrite the stored pointers - resulting in a memory leak. One recommend alternative is the use of smart pointers, for example Boost Smart pointers.

std::auto_ptr

| Caution
Any container of `std::auto_ptr` is considered particularly hazardous.

| Tip
Never create a circular buffer of `std::auto_ptr`. Refer to Scott Meyers' excellent book Effective STL for a detailed discussion. (Meyers S., Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library. Addison-Wesley, 2001.)

While internals of a `circular_buffer` are circular, iterators are not. Iterators of a `circular_buffer` are only valid for the range `\[begin(), end()\]`, so for example: iterators `(begin() - 1)` and `(end() + 1)` are both invalid.

#### Debug Support

In order to help a programmer to avoid and find common bugs, the `circular_buffer` contains a kind of debug support.

The `circular_buffer` maintains a list of valid iterators. As soon as any element gets destroyed all iterators pointing to this element are removed from this list and explicitly invalidated (an invalidation flag is set). The debug support also consists of many assertions (`BOOST_ASSERT` macros) which ensure the `circular_buffer` and its iterators are used in the correct manner at runtime. In case an invalid iterator is used, the assertion will report an error. The connection of explicit iterator invalidation and assertions makes a very robust debug technique which catches most of the errors.

Moreover, the uninitialized memory allocated by `circular_buffer` is filled with the value `0xcc` in the debug mode. When debugging the code, this can help the programmer to recognize the initialized memory from the uninitialized. For details refer the source code circular_buffer/debug.hpp.

The debug support is enabled only in the debug mode (when the `NDEBUG` is not defined). It can also be explicitly disabled (only for `circular_buffer`) by defining macro BOOST_CB_DISABLE_DEBUG.

#### Compatibility with Interprocess library

The `circular_buffer` is compatible with the Boost.Interprocess library used for interprocess communication. Considering that the circular_buffer's debug support relies on 'raw' pointers (which is not permited by the Interprocess library) the code has to compiled with `-DBOOST_CB_DISABLE_DEBUG` or `-DNDEBUG` (which disables the Debug Support). Not doing that will cause the compilation to fail.