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.
> _______________________________________________
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> https://lists.mozilla.org/listinfo/dev-platform
>
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