问题
I recently came across some code that looked like:
if(sizeof(var,2) == 4) { ... }
(where var is a type)
I was quite surprised to see what appeared to be two arguments to the sizeof operator. A quick scan of the ISO/ANSI C99 standard did not yield any secrets. I couldn't come up with any reading of the grammar that allowed a comma there.
Searching Google Code, I was able to find an example of this syntax in some PPC code.
Is this some PPC-specific syntax? What does it mean?
EDIT: It turns out that both what I was looking at--as well as the linked code--is syntax specific to the WindRiver Diab compiler:
sizeof(type, int-const):
If int-const is 0 sizeof returns the size in bytes of type.
If int-const is 1 sizeof returns the alignment of type.
If int-const is 2 sizeof returns an integer constant designating the type of type. Look up "sizeof operator" in the Diab C/C++ User's Guide for values.
Wow, they've really overloaded the meaning of the sizeof operator.
EDIT2: Full documentation is here: http://www.vxdev.com/docs/vx55man/diab5.0ppc/c-additi.htm#3001432
回答1:
On further research, I discovered that this is behavior specific to the WindRiver Diab compiler. Please see the EDIT in the question for details.
回答2:
It looks to me like a simple application of the comma operator, which evaluates its first argument, throws away the result, then evaluates its second argument.
In this case, it's determining whether the literal 2 has size 4. The "var" part is irrelevant.
回答3:
Looks like a red herring. My guess is that you are accidentally using the comma operator and sizeof is being applied to the last value.
回答4:
As mentioned, the comma operator is being applied and sizeof is returning the size of an integer literal. Offhand this looks like an error on the author's part, but there could be some sinister coding happening.
sizeof expressions are not evaluated so they can be used for a number of tricky things. One example is to provide a reference for an otherwise unreferenced variable without causing the compiler to generate any code. See this article on creating a better assert macro for an example. Alexandrescu has some other examples of sizeof trickery in Modern C++ Design, if memory serves. It's possible, but not likely, that one of these non-obvious usages is intended.
Whatever the usage, if it's not commented in this situation then it's clearly not worth the trade-off in readability and should be changed.
回答5:
Important disclaimer: Below code is pseudo code. The arguments to sizeof are never actually be evaluated, with the meaning of "executed", it is always a compile time construct (and therefore one of the tools preferred by template authors, like enums).
note that I borrowed auto below from C++ (0x); it tells the compiler to deduce the type from the initializer expression and makes the examples a bit simpler
What many don't know is that you can invoke sizeof validly like this:
auto s = sizeof int;
I.e., no parentheses needed. Therefore, if you pass (x,y) to sizeof, it is equivalent to
auto c = (x,y);
auto s = sizeof c;
or just
auto c = x,y;
auto s = sizeof c;
x,y is a sequence, where each part is evaluated left to right, and the sequence gets the value of the last part, in this case this is y. So, the original code is roughly equivalent to
auto s = sizeof y;
So it looks to me that the compiler in question does something really, really braindead, in that it introduces an extension that also compiles on other compilers, but with a totally different meaning. This is bad.
来源：https://stackoverflow.com/questions/966034/why-call-sizeof-operator-with-two-arguments