I just landed support for MOZ_DBG for nsIFile (shows the path of the file)
and nsIURI (shows the uri spec) onto inbound.
On Sat, Mar 30, 2019 at 8:04 AM Dave Townsend <dtowns...@mozilla.com> wrote:
> This sounds excellent. I think on Monday I'll go right to work making this
> work for URIs which are probably the things I end up logging the most from
> C++.
>
> On Fri, Mar 29, 2019 at 11:16 PM Cameron McCormack <c...@mcc.id.au> wrote:
>
>> Lately I've been finding Rust's dbg!() macro[1] useful for quick
>> debugging. Its main usefulness is in avoiding the need to extract out an
>> expression into a separate variable just so that you can print out its
>> value and then use the value in its original context.
>>
>> I wanted something similar for C++, so in bug 1538081, which just landed
>> on autoland, I've added the MOZ_DBG macro.
>>
>> MOZ_DBG can be added around almost any kind of expression[2], as long as
>> there is an operator<<(ostream&, ...) defined for its type. I added
>> operator<< definitions for nsAString, nsACString, mozilla::Span, nsTArray,
>> mozilla::Array, and T[N], since they seemed useful. And as a special case,
>> if you wrap MOZ_DBG around a pointer value, then it will use the operator<<
>> of the dereferenced object (if the operator<< exists, and the pointer is
>> non-null), and otherwise just prints out the pointer value. The output
>> goes to stderr.
>>
>> The macro is defined in mfbt/DbgMacro.h, but I've included it into
>> nsDebug.h, so that it should be available in most files without needing to
>> explicitly #include <mozilla/DbgMacro.h>. It's available in non-DEBUG
>> builds, but not in MOZILLA_OFFICIAL builds.
>>
>> Example:
>>
>> nsTArray<int> numbers;
>> MOZ_DBG(numbers = { MOZ_DBG(123 * 1), MOZ_DBG(123 * 2) });
>> MOZ_DBG(numbers[MOZ_DBG(numbers.Length() - 1)]);
>> MOZ_DBG(numbers) = { 789 };
>> MOZ_DBG(numbers);
>>
>> Output:
>>
>> [/path/to/file.cpp:319] 123 * 1 = 123
>> [/path/to/file.cpp:319] 123 * 2 = 246
>> [/path/to/file.cpp:319] numbers = { MOZ_DBG(123 * 1), MOZ_DBG(123 * 2)
>> } = [123, 246]
>> [/path/to/file.cpp:320] numbers.Length() - 1 = 1
>> [/path/to/file.cpp:320] numbers[MOZ_DBG(numbers.Length() - 1)] = 246
>> [/path/to/file.cpp:321] numbers = [123, 246]
>> [/path/to/file.cpp:322] numbers = [789]
>>
>> There is also a macro MOZ_DEFINE_DBG, which can be used to define an
>> operator<< for a class. It's like a poor imitation of #[derive(Debug)].
>>
>> Example:
>>
>> struct Point {
>> int x;
>> int y;
>>
>> MOZ_DEFINE_DBG(Point, x, y)
>> };
>>
>> Point p{10, 20};
>> MOZ_DBG(p);
>>
>> Output:
>>
>> [/path/to/file.cpp:100] p = Point { x = 10, y = 20 }
>>
>>
>> [1] https://doc.rust-lang.org/stable/std/macro.dbg.html
>> [2] One specific case where it won't work is if you are wrapping it
>> around a prvalue being used to initialize an object, and the type of that
>> value is something that doesn't have a copy or move constructor available.
>> Such types should be rare, though, and you'll get a compiler error if you
>> try.
>> _______________________________________________
>> dev-platform mailing list
>> dev-platform@lists.mozilla.org
>> https://lists.mozilla.org/listinfo/dev-platform
>>
>
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