May 6th, 2021

Is std::exception_ptr nothrow constructible and assignable?

Raymond Chen
Raymond Chen

When I was writing my series on awaitable signals and coroutine promises, I had to investigate a few details of the language specification, like why coroutine_handle::resume() is potentially-throwing. Another detail I had to investigate was whether std::exception_ptr was potentially-throwing in its constructors and assignment operators.

The answer is No, but you have to chase through the language specification a bit.

The exception_ptr is introduced in the C++ language specification under [propagation], and it says that exception_ptr is a NullablePointer¹ but otherwise says nothing about exceptions.

Chase through to [nullablepointer.requirements], which spells out the various operations that must be supported. It says, for example, that the object must satisfy Cpp17CopyConstructible (or CopyConstructible, as cppreference.com calls it), but if you follow that definition, there is still no mention of exceptions.

But the magic sentence is right there. You just overlooked it.

Back in the definition of NullablePointer, it says,

4 No operation which is part of the NullablePointer requirements shall exit via an exception.

So there it is. The exception_ptr, or more generally, anything that satisfies NullablePointer, is nothrow comparable, constructible, assignable, destructible, swappable, and testable.

¹ Starting in C++20, these requirements have been renamed Cpp17NullablePointer, Cpp17CopyConstructible, etc.

Category
Old New Thing
Topics
Code

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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  • IS4

    It’s like the subtle sentence in XML Schema that makes xs:hexBinary and xs:base64Binary semantically disjoint.

  • Sumit Bhardwaj

    Now, that’s some excellent language-lawyering. 🙂 Thank you!

    • Erik Fjeldstrom

      And now to see how many implementations follow the rules to the letter! (Compilers and libraries have gotten a lot better at this over time.)

      • Jonathan Wilson

        The people doing the C++ implementation for Visual C++ seem to do everything possible to meet the standard (AFAIK the only times they deviate from the standard is because of limitations in Windows that prevent proper standard implementation or because they need to remain ABI compatible)

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