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

Section: 33.10.1 [futures.overview] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

33.10.1 [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,
>       implementation-defined
>     };

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 33.10.1 [futures.overview] paragraph 2 as indicated:

> The enum type `launch` is an implementation-defined a bitmask type (16.3.3.3.4 [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 ]