task Allocation

Always allocate memory for task objects using one of the special overloaded new operators. The allocation methods do not construct the task. Instead, they return a proxy object that can be used as an argument to an overloaded version of operator new provided by the library.

In general, the allocation methods must be called before any of the tasks allocated are spawned. The exception to this rule is allocate_additional_child_of(t), which can be called even if task t is already running. The proxy types are defined by the implementation. The only guarantee is that the phrase "new(proxy) T(...)" allocates and constructs a task of type T.


Allocating tasks larger than 216 bytes might be significantly slower than allocating smaller tasks. In general, task objects should be small lightweight entities.

Because these methods are used idiomatically, the members in the following table show the idiom, not the declaration. The argument this is typically implicit, but shown explicitly in the headings to distinguish instance methods from static methods.

Member Description
new( task::allocate_root( task_group_context& group ) ) T

Allocate a task of type T with the specified cancellation group. The figure below summarizes the state transition.

Effect of task::allocate_root()

Use method spawn_root_and_wait to execute the task.

new( task::allocate_root() ) T

Like new(task::allocate_root(task_group_context&)) except that cancellation group is the current innermost cancellation group.

new( x.allocate_continuation() ) T

Allocates and constructs a task of type T, and transfers the successor from x to the new task. No reference counts change. The figure below summarizes the state transition.

Effect of allocate_continuation()

new( x.allocate_child() ) T

Allocates a task with this as its successor. The figure below summarizes the state transition.

Effect of allocate_child()

If using explicit continuation passing, then the continuation, not the successor, should call the allocation method, so that successor is set correctly.

If the number of tasks is not a small fixed number, consider building a task_list of the predecessors first, and spawning them with a single call to task::spawn. If a task must spawn some predecessors before all are constructed, it should use task::allocate_additional_child_of(*this) instead, because that method atomically increments refcount, so that the additional predecessor is properly accounted. However, if doing so, the task must protect against premature zeroing of refcount by using a blocking-style task pattern.

new(task::allocate_additional_child_of( y )) T

Allocates a task as a predecessor of another task y. Task y may be already running or have other predecessors running. The figure below summarizes the state transition.

Effect of allocate_additional_child_of(successor)

Because y may already have running predecessors, the increment of y.refcount is atomic (unlike the other allocation methods, where the increment is not atomic). When adding a predecessor to a task with other predecessors running, it is up to the programmer to ensure that the successor's refcount does not prematurely reach 0 and trigger execution of the successor before the new predecessor is added.

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