[llvm] r264467 - [Support] Add Error::errorForOutParameter helper.

[David Blaikie via llvm-commits]

On Fri, Mar 25, 2016 at 2:56 PM, Lang Hames via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: lhames
> Date: Fri Mar 25 16:56:35 2016
> New Revision: 264467
>
> URL: http://llvm.org/viewvc/llvm-project?rev=264467&view=rev
> Log:
> [Support] Add Error::errorForOutParameter helper.
>
> This helper method creates a pre-checked Error suitable for use as an out
> parameter in a constructor. This avoids the need to have the constructor
> check a known-good error before assigning to it.
>

Hmm, thinking about it - how does this work to ensure safety?

If the callee only assigns to the parameter on failure cases, won't the
code fail to assert on successful cases?

(ie: shouldn't it actually be unchecked by default as it was before - the
caller should check it on return regardless of what the function did)


>
>
> Modified:
>     llvm/trunk/include/llvm/Support/Error.h
>     llvm/trunk/unittests/Support/ErrorTest.cpp
>
> Modified: llvm/trunk/include/llvm/Support/Error.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Error.h?rev=264467&r1=264466&r2=264467&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/Support/Error.h (original)
> +++ llvm/trunk/include/llvm/Support/Error.h Fri Mar 25 16:56:35 2016
> @@ -143,6 +143,13 @@ public:
>    /// constructor, but should be preferred for readability where possible.
>    static Error success() { return Error(); }
>
> +  /// Create a 'pre-checked' success value suitable for use as an
> out-parameter.
> +  static Error errorForOutParameter() {
> +    Error Err;
> +    (void)!!Err;
> +    return Err;
> +  }
> +
>    // Errors are not copy-constructable.
>    Error(const Error &Other) = delete;
>
>
> Modified: llvm/trunk/unittests/Support/ErrorTest.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/ErrorTest.cpp?rev=264467&r1=264466&r2=264467&view=diff
>
> ==============================================================================
> --- llvm/trunk/unittests/Support/ErrorTest.cpp (original)
> +++ llvm/trunk/unittests/Support/ErrorTest.cpp Fri Mar 25 16:56:35 2016
> @@ -105,6 +105,12 @@ TEST(Error, UncheckedSuccess) {
>  }
>  #endif
>
> +// Test that errors to be used as out parameters are implicitly checked (
> +// and thus destruct quietly).
> +TEST(Error, ErrorAsOutParameter) {
> +  Error E = Error::errorForOutParameter();
> +}
> +
>  // Check that we abort on unhandled failure cases. (Force conversion to
> bool
>  // to make sure that we don't accidentally treat checked errors as
> handled).
>  // Test runs in debug mode only.
>
>
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