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c++macrosescapingc-preprocessorpreprocessor-directive
Giffyguy
Giffyguy

Reputation: 21302

How can I use parentheses in C++ macro parameters?

I need to use parentheses in my actual parameters for a macro, but the parentheses appear to be changing the behavior of the comma separating the macro parameters.

I had my preprocessor dump its output into a text file, so I could see what it was producing.
Then I performed a basic test to confirm the behavior.

#define MACRO_TEST_1( X , Y ) X && Y

MACRO_TEST_1 ( A , B )
// Desired result: A && B
// Actual result:  A && B

MACRO_TEST_1 ( ( C , D ) )
// Desired result: ( C && D )
// Actual result:  ( C , D ) &&
// Warning: "not enough actual parameters for macro 'MACRO_TEST_1'"

It appears that adding an opening parenthese to the first parameter, and a closing parenthese to the second parameter, causes the preprocessor to treat the comma as part of the first parameter, and therefore assumes that I did not supply a second parameter at all.
This is evidenced by the warning, as well as the preprocessor output showing nothing after the &&.

So my question is, how can I tell the preprocessor that the comma seperates the parameters, even though the parameters have parentheses in them?

I tried escaping the parentheses or the comma, but this made no difference.
(Same results, just with the escape character inserted into the preprocessor output.)

Upvotes: 3

Views: 5549

Answers (2)

Giffyguy
Giffyguy

Reputation: 21302

I technically found a solution, albeit ugly.
You have to define symbols for the parentheses characters.

#define OP (
#define CP )

#define MACRO_TEST_1( X , Y ) X && Y

MACRO_TEST_1 ( A , B )
// Desired result: A && B
// Actual result:  A && B

MACRO_TEST_1 ( OP C , D CP )
// Desired result: ( C && D )
// Actual result:  ( C && D )

Upvotes: 5

Edgar Rokjān
Edgar Rokjān

Reputation: 17483

So my question is, how can I tell the preprocessor that the comma separates the parameters, even though the parameters have parentheses in them?

I think that there is no way, at least in such macro implementation.

From the gcc.gnu.org:

To invoke a macro that takes arguments, you write the name of the macro followed by a list of actual arguments in parentheses, separated by commas.

Leading and trailing whitespace in each argument is dropped, and all whitespace between the tokens of an argument is reduced to a single space. Parentheses within each argument must balance; a comma within such parentheses does not end the argument.

You cannot leave out arguments entirely; if a macro takes two arguments, there must be exactly one comma at the top level of its argument list.

Upvotes: 2

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