task Class


Base class for tasks.


class task;


#include "tbb/task.h"


Class task is the base class for tasks. You are expected to derive classes from task, and at least override the virtual method task* task::execute().

Each instance of task has associated attributes, that while not directly visible, must be understood to fully grasp how task objects are used. The attributes are described in the table below.

Task Attributes




Either null, or a pointer to another task whose refcount field will be decremented after the present task completes. Typically, the successor is the task that allocated the present task, or a task allocated as the continuation of that task.

Methods of class task call the successor "parent" and its preceding task the "child", because this was a common use case. But the library has evolved such that a child-parent relationship is no longer required between the predecessor and successor.


The number of Tasks that have this as their parent. Increments and decrement of refcount are always atomic.


Always allocate memory for task objects using special overloaded new operators provided by the library, otherwise the results are undefined. Destruction of a task is normally implicit. The copy constructor and assignment operators for task are not accessible. This prevents accidental copying of a task, which would be ill-defined and corrupt internal data structures.


Some member descriptions illustrate effects by diagrams such as in the figure below.

Example Effect Diagram

Conventions in these diagrams are as follows:

  • The big arrow denotes the transition from the old state to the new state.

  • Each task's state is shown as a box divided into parent and refcount sub-boxes.

  • Gray denotes state that is ignored. Sometimes ignored state is left blank.

  • Black denotes state that is read.

  • Blue denotes state that is written.


In the description below, types proxy1...proxy5 are internal types. Methods returning such types should only be used in conjunction with the special overloaded new operators, as described in Section task Allocation.

namespace tbb {
        class task {
            virtual ~task() {}
            virtual task* execute() = 0;
            // Allocation
            static proxy1 allocate_root();
            static proxy2 allocate_root( task_group_context& );
            proxy3 allocate_continuation();
            proxy4 allocate_child();
            static proxy5 allocate_additional_child_of( task& );
            // Explicit destruction
            static void destroy( task& victim );
            // Recycling
            void recycle_as_continuation();
            void recycle_as_safe_continuation();
            void recycle_as_child_of( task& new_parent );
            // Synchronization
            void set_ref_count( int count );
            void increment_ref_count();
            int add_ref_count( int count );
            int decrement_ref_count();
            void wait_for_all();
            static void spawn( task& t );
            static void spawn( task_list& list );
            void spawn_and_wait_for_all( task& t );  
            void spawn_and_wait_for_all( task_list& list );  
            static void spawn_root_and_wait( task& root );
            static void spawn_root_and_wait( task_list& root );
            static void enqueue( task& );
            // Task context
            static task& self();
            task* parent() const;
            void set_parent( task *p );
            bool is_stolen_task() const;
            task_group_context* group();
            void change_group( task_group_context& ctx );
            // Cancellation
            bool cancel_group_execution();
            bool is_cancelled() const;
            // Priority
            static void enqueue( task&, priority_t );
            void set_group_priority ( priority_t );
            priority_t group_priority () const;
            // Affinity
            typedef implementation-defined-unsigned-type affinity_id;
            virtual void note_affinity( affinity_id id );
            void set_affinity( affinity_id id );
            affinity_id affinity() const;
            // Debugging
            enum state_type {
            int ref_count() const;
            state_type state() const;
    } // namespace tbb
    void *operator new( size_t bytes, const proxy1& p );
    void operator delete( void* task, const proxy1& p );
    void *operator new( size_t bytes, const proxy2& p );
    void operator delete( void* task, const proxy2& p );
    void *operator new( size_t bytes, const proxy3& p );
    void operator delete( void* task, const proxy3& p );
    void *operator new( size_t bytes, proxy4& p );
    void operator delete( void* task, proxy4& p );
    void *operator new( size_t bytes, proxy5& p );
    void operator delete( void* task, proxy5& p );


Prior to Intel® Threading Building Blocks (Intel® TBB) 3.0, methods allocate_additional_child_of, destroy, and spawn were non-static. Evolution of the library made the this argument superfluous for these calls. The change preserves source compatibility except in cases where the address of the method was taken. Executables compiled with the older headers that had the non-static form will continue to work when linked against the current Intel TBB 3.0 run-time libraries.

For more complete information about compiler optimizations, see our Optimization Notice.