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2276. Missing requirement on std::promise::set_exception

Section: 33.10 [futures] Status: C++17 Submitter: Jonathan Wakely Opened: 2013-07-30 Last modified: 2017-07-30

Priority: Not Prioritized

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Discussion:

The standard does not specify the behaviour of this program:

#include <future>
#include <cassert>

struct NonTrivial
{
  NonTrivial() : init(true) { }
  ~NonTrivial() { assert(init); }
  bool init;
};

int main()
{
  std::promise<NonTrivial> p;
  auto f = p.get_future();
  p.set_exception(std::exception_ptr());
  f.get();
}

The standard doesn't forbid making the state ready with a null exception_ptr, so what should get() return? There's no stored exception to throw, but it can't return a value because none was initialized.

A careful reading of the standard shows 33.10.5 [futures.state] p8 says "A shared state is ready only if it holds a value or an exception ready for retrieval." One can infer from the fact that set_exception() makes the state ready that it must store a value or exception, so cannot store "nothing", but that isn't explicit.

The promise::set_exception() and promise::set_exception_at_thread_exit() members should require p != nullptr or should state the type of exception thrown if p is null.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

SG1 provides P/R and requests move to SG1-OK status: Add Requires clauses for promise (30.6.5 [futures.promise]) set_exception (before p18) and set_exception_at_thread_exit (before p24): Requires: p is not null.

[2015-10, Kona issue prioritization]

Priority 0, move to Ready

Proposed resolution:

This wording is relative to N4431.

1. Change 33.10.6 [futures.promise] as depicted:

   void set_exception(exception_ptr p);
   

   -??- Requires: p is not null.

   -18- Effects: atomically stores the exception pointer p in the shared state and makes that state ready (30.6.4).

   void set_exception_at_thread_exit(exception_ptr p);
   

   -??- Requires: p is not null.

   -24- Effects: Stores the exception pointer p in the shared state without making that state ready immediately. […]