...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
C++11 allows to delete some implicitly generated functions as constructors and assignment using '= delete' as in
public: thread(thread const&) = delete;
On compilers not supporting this feature, Boost.Thread relays on a partial simulation, it declares the function as private without definition.
private: thread(thread &);
The emulation is partial as the private function can be used for overload resolution for some compilers and prefer it to other overloads that need a conversion. See below the consequences on the move semantic emulation.
In order to implement Movable classes, move parameters and return types Boost.Thread
uses the rvalue reference when the compiler support it. On compilers not
supporting it Boost.Thread uses either the emulation provided by Boost.Move
or the emulation provided by the previous versions of Boost.Thread depending
whether BOOST_THREAD_USES_MOVE
is defined or not. This macros is unset by default when BOOST_THREAD_VERSION
is 2. Since BOOST_THREAD_VERSION
3, BOOST_THREAD_USES_MOVE
is defined.
Previous to version 1.50, Boost.Thread make use of its own move semantic emulation which had more limitations than the provided by Boost.Move. In addition, it is of interest of the whole Boost community that Boost.Thread uses Boost.Move so that boost::thread can be stored on Movable aware containers.
To preserve backward compatibility at least during some releases, Boost.Thread allows the user to use the deprecated move semantic emulation defining BOOST_THREAD_DONT_USE_MOVE.
Many aspects of move semantics can be emulated for compilers not supporting rvalue references and Boost.Thread legacy offers tools for that purpose.
Next follows the interface of the legacy move semantic helper class and function.
namespace boost { namespace detail { template<typename T> struct thread_move_t { explicit thread_move_t(T& t_); T& operator*() const; T* operator->() const; private: void operator=(thread_move_t&); }; } template<typename T> boost::detail::thread_move_t<T> move(boost::detail::thread_move_t<T> t); }
We can write a MovableOny class as follows. You just need to follow these simple steps:
detail::thread_move_t<classname>
detail::thread_move_t<classname>
detail::thread_move_t<classname>
For example the thread class defines the following:
class thread { // ... private: thread(thread&); thread& operator=(thread&); public: detail::thread_move_t<thread> move() { detail::thread_move_t<thread> x(*this); return x; } operator detail::thread_move_t<thread>() { return move(); } thread(detail::thread_move_t<thread> x) { thread_info=x->thread_info; x->thread_info.reset(); } thread& operator=(detail::thread_move_t<thread> x) { thread new_thread(x); swap(new_thread); return *this; } // ... };
BOOST_THREAD_NO_COPYABLE(CLASS)
BOOST_THREAD_MOVABLE(CLASS)
BOOST_THREAD_MOVABLE_ONLY(CLASS)
BOOST_THREAD_COPYABLE_AND_MOVABLE(CLASS)
BOOST_THREAD_RV_REF(TYPE)
,
BOOST_THREAD_RV_REF_BEG
and BOOST_THREAD_RV_REF_END
BOOST_THREAD_RV(V)
BOOST_THREAD_MAKE_RV_REF(RVALUE)
BOOST_THREAD_DCL_MOVABLE
, BOOST_THREAD_DCL_MOVABLE_BEG(T1)
and
BOOST_THREAD_DCL_MOVABLE_END
In order to make the library code portable Boost.Thread uses some macros that will use either the ones provided by Boost.Move or the deprecated move semantics provided by previous versions of Boost.Thread.
See the Boost.Move documentation for a complete description on how to declare new Movable classes and its limitations.
BOOST_THREAD_RV_REF(TYPE)
is the equivalent of BOOST_RV_REF(TYPE)
BOOST_THREAD_RV_REF_BEG
is the equivalent of BOOST_RV_REF_BEG(TYPE)
BOOST_THREAD_RV_REF_END
is the equivalent of BOOST_RV_REF_END(TYPE)
BOOST_THREAD_FWD_REF(TYPE)
is the equivalent of `BOOST_FWD_REF(TYPE)
In addition the following macros are needed to make the code portable:
BOOST_THREAD_RV(V)
macro to access the rvalue from a BOOST_THREAD_RV_REF(TYPE),
BOOST_THREAD_MAKE_RV_REF(RVALUE)
makes a rvalue.
BOOST_THREAD_DCL_MOVABLE(CLASS)
to avoid conflicts with Boost.Move
BOOST_THREAD_DCL_MOVABLE_BEG(T1)
and BOOST_THREAD_DCL_MOVABLE_END
are variant of BOOST_THREAD_DCL_MOVABLE
when the parameter is a template instantiation.
Other macros are provided and must be included on the public section:
BOOST_THREAD_NO_COPYABLE
declares a class no-copyable either deleting the copy constructors
and copy assignment or moving them to the private section.
BOOST_THREAD_MOVABLE(CLASS)
declares all the implicit conversions
to an rvalue-reference.
BOOST_THREAD_MOVABLE_ONLY(CLASS)
is the equivalent of BOOST_MOVABLE_BUT_NOT_COPYABLE(CLASS)
BOOST_THREAD_COPYABLE_AND_MOVABLE(CLASS)
is the equivalent of BOOST_COPYABLE_AND_MOVABLE(CLASS)
This macro marks a class as no copyable, disabling copy construction and assignment.
This macro marks a class as movable, declaring all the implicit conversions to an rvalue-reference.
This macro marks a type as movable but not copyable, disabling copy construction and assignment. The user will need to write a move constructor/assignment to fully write a movable but not copyable class.
This macro marks a type as copyable and movable. The user will need to write a move constructor/assignment and a copy assignment to fully write a copyable and movable class.
This macro is used to achieve portable syntax in move constructors and
assignments for classes marked as BOOST_THREAD_COPYABLE_AND_MOVABLE
or BOOST_THREAD_MOVABLE_ONLY
.
BOOST_THREAD_RV_REF_BEG
and BOOST_THREAD_RV_REF_END
are used when the parameter end with a >
to avoid the compiler error.
While Boost.Move emulation allows to access an rvalue reference BOOST_THREAD_RV_REF(TYPE)
using the dot operator, the legacy defines the operator->
. We need then a macro BOOST_THREAD_RV
that mask this difference.
E.g.
thread(BOOST_THREAD_RV_REF(thread) x) { thread_info=BOOST_THREAD_RV(x).thread_info; BOOST_THREAD_RV(x).thread_info.reset(); }
The use of this macros has reduced considerably the size of the Boost.Thread move related code.
While Boost.Move is the best C++03 move emulation there are some limitations that impact the way the library can be used. For example, with the following declarations
class thread { // ... private: thread(thread &); public: thread(rv<thread>&); // ... };
This could not work on some compilers even if thread is convertible to
rv<thread>
because the compiler prefers the private copy constructor.
thread mkth() { return thread(f); }
On these compilers we need to use instead an explicit conversion. The library provides a move member function that allows to workaround the issue.
thread mkth() { return thread(f).move(); }
Note that ::boost::move
can not be used in this case as thread is not implicitly convertible
to thread&
.
thread mkth() { return ::boost::move(thread(f)); }
To make the code portable Boost.Thread the user needs to use a macro
BOOST_THREAD_MAKE_RV_REF
that can be used as in
thread mkth() { return BOOST_THREAD_MAKE_RV_REF(thread(f)); }
Note that this limitation is shared also by the legacy Boost.Thread move emulation.
As Boost.Move defines also the boost::move
function we need to specialize the has_move_emulation_enabled_aux
metafunction.
template <> struct has_move_emulation_enabled_aux<thread> : BOOST_MOVE_BOOST_NS::integral_constant<bool, true> {};
so that the following Boost.Move overload is disabled
template <class T> inline typename BOOST_MOVE_BOOST_NS::disable_if<has_move_emulation_enabled_aux<T>, T&>::type move(T& x);
The macros BOOST_THREAD_DCL_MOVABLE(CLASS)
, BOOST_THREAD_DCL_MOVABLE_BEG(T1)
and BOOST_THREAD_DCL_MOVABLE_END
are used for this purpose. E.g.
BOOST_THREAD_DCL_MOVABLE(thread)
and
BOOST_THREAD_DCL_MOVABLE_BEG(T) promise<T> BOOST_THREAD_DCL_MOVABLE_END
Locks provide an explicit bool conversion operator when the compiler provides them.
explicit operator bool() const;
The library provides un implicit conversion to an undefined type that can be used as a conditional expression.
#if defined(BOOST_NO_EXPLICIT_CONVERSION_OPERATORS) operator unspecified-bool-type() const; bool operator!() const; #else explicit operator bool() const; #endif
The user should use the lock.owns_lock() when a explicit conversion is required.
If owns_lock()
would return true
, a
value that evaluates to true
in boolean contexts, otherwise a value that evaluates to false
in boolean contexts.
Nothing.
!
owns_lock()
.
Nothing.
Some of the enumerations defined in the standard library are scoped enums.
On compilers that don't support them, the library uses a class to wrap the underlying type. Instead of
enum class future_errc { broken_promise, future_already_retrieved, promise_already_satisfied, no_state };
the library declare these types as
BOOST_SCOPED_ENUM_DECLARE_BEGIN(future_errc) { broken_promise, future_already_retrieved, promise_already_satisfied, no_state } BOOST_SCOPED_ENUM_DECLARE_END(future_errc)
These macros allows to use 'future_errc' in almost all the cases as an scoped enum.
There are however some limitations:
Instead of
switch (ev) { case future_errc::broken_promise: // ...
use
switch (boost::native_value(ev)) { case future_errc::broken_promise:
And instead of
#ifdef BOOST_NO_SCOPED_ENUMS template <> struct BOOST_SYMBOL_VISIBLE is_error_code_enum<future_errc> : public true_type { }; #endif
use
#ifdef BOOST_NO_SCOPED_ENUMS template <> struct BOOST_SYMBOL_VISIBLE is_error_code_enum<future_errc::enum_type> : public true_type { }; #endif