The do ... while
and if ... else
are there to make it so that a
semicolon after your macro always means the same thing. Let's say you
had something like your second macro.
#define BAR(X) f(x); g(x)
Now if you were to use BAR(X);
in an if ... else
statement, where the bodies of the if statement were not wrapped in curly brackets, you'd get a bad surprise.
if (corge)
BAR(corge);
else
gralt();
The above code would expand into
if (corge)
f(corge); g(corge);
else
gralt();
which is syntactically incorrect, as the else is no longer associated with the if. It doesn't help to wrap things in curly braces within the macro, because a semicolon after the braces is syntactically incorrect.
if (corge)
{f(corge); g(corge);};
else
gralt();
There are two ways of fixing the problem. The first is to use a comma to sequence statements within the macro without robbing it of its ability to act like an expression.
#define BAR(X) f(X), g(X)
The above version of bar BAR
expands the above code into what follows, which is syntactically correct.
if (corge)
f(corge), g(corge);
else
gralt();
This doesn't work if instead of f(X)
you have a more complicated body of code that needs to go in its own block, say for example to declare local variables. In the most general case the solution is to use something like do ... while
to cause the macro to be a single statement that takes a semicolon without confusion.
#define BAR(X) do { \
int i = f(X); \
if (i > 4) g(i); \
} while (0)
You don't have to use do ... while
, you could cook up something with if ... else
as well, although when if ... else
expands inside of an if ... else
it leads to a "dangling else", which could make an existing dangling else problem even harder to find, as in the following code.
if (corge)
if (1) { f(corge); g(corge); } else;
else
gralt();
The point is to use up the semicolon in contexts where a dangling semicolon is erroneous. Of course, it could (and probably should) be argued at this point that it would be better to declare BAR
as an actual function, not a macro.
In summary, the do ... while
is there to work around the shortcomings of the C preprocessor. When those C style guides tell you to lay off the C preprocessor, this is the kind of thing they're worried about.
No, because the macro invocation MAKE_TEMPLATE()
does not have zero arguments at all; it has one argument comprising zero tokens.
Older preprocessors, apparently including GCC at the time this answer was originally written, sometimes interpreted an empty argument list as you'd hope, but the consensus has moved toward a stricter, narrower extension which more closely conforms to the standard.
To get the answer below to work, define an additional macro parameter before the ellipsis:
#define MAKE_TEMPLATE(UNUSED, ...) template <typename T, ## __VA_ARGS__ >
and then always put a comma before the first argument when the list is not empty:
MAKE_TEMPLATE(, foo )
Old answer
According to http://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html, GCC does support this, just not transparently.
Syntax is:
#define MAKE_TEMPLATE(...) template <typename T, ## __VA_ARGS__ >
Anyway, both also support variadic templates in C++0x mode, which is far preferable.
Best Answer
As for the updated question, by the use of auxiliary macro
VA_ARGS
like the following, the arguments will be expanded as expected.