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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