Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

PrevUpHomeNext

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:

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 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:

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:

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] Caution

Any container of std::auto_ptr is considered particularly hazardous.

[Tip] 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.


PrevUpHomeNext