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2102. Why is std::launch an implementation-defined type?

Section: 30.6.1 [futures.overview] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-20 Last modified: 2016-02-10

Priority: Not Prioritized

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30.6.1 [futures.overview] says std::launch is an implementation-defined bitmask type, which would usually mean the implementation can choose whether to define an enumeration type, or a bitset, or an integer type. But in the case of std::launch it's required to be a scoped enumeration type,

enum class launch : unspecified {
  async = unspecified,
  deferred = unspecified,

so what is implementation-defined about it, and what is an implementation supposed to document about its choice?

[2011-12-02 Moved to Tentatively Ready after 6 positive votes on c++std-lib.]

Proposed resolution:

This wording is relative to the FDIS.

Change 30.6.1 [futures.overview] paragraph 2 as indicated:

The enum type launch is an implementation-defineda bitmask type ( [bitmask.types]) with launch::async and launch::deferred denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched by async() applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload of async() by adding their extensions to the launch policy under the “as if” rule. — end note ]