Weird C++ preprocessor macro syntax

Question

I've distilled my problem down to this code snippet - but it is a part of a larger program so I don't want a different way to do this - I need a way to make this work!

When I generate a preprocessed file from this code:

#define OUTER(a, b) \
    a##b
#define INNER(c, d) \
    c##d

enum foo {
    OUTER(INNER(x, y), z)
}; // line 108

int APIENTRY _tWinMain(...)
{
    foo bar = xyz; // line 112
}

I get:

enum foo {
    xyz
}; // line 108

int __stdcall wWinMain(...)
{
    foo bar = xyz; // line 112
}

which is what I want. However, if I try to compile the code I get:

error C2146: syntax error : missing '}' before identifier 'z' line 108
error C2143: syntax error : missing ';' before '}' line 108
error C2143: syntax error : missing ';' before '}' line 108
error C2059: syntax error : '}' line 108
error C2065: 'xyz' : undeclared identifier line 112

I can't work it out! The problem seems to be caused by the ## in the:

#define OUTER(a, b) \
    a##b

but why (and how to fix it) is beyond me...

@Jack Could you explain what a##b is supposed to do? Never saw this before. — InsertNickHere, Aug 01 '10 at 17:45
@Insert It's a preprocessor operator that literally concatenates its two arguments. For instance, `OUTER(test, string)` in the OP code snippet is replaced by `teststring` by the preprocessor. — Mehrdad Afshari, Aug 01 '10 at 17:47
OK well I can confirm that this happens in gcc too, and their error is a bit more descriptive. It's saying that it can't paste `)` and `z` together. It looks to me like the preprocessor can't do nested pastes like this, but this is guesswork at this point. Hope that helps a bit.. — Blindy, Aug 01 '10 at 17:48
@InsertNickHere: http://msdn.microsoft.com/en-us/library/09dwwt6y%28VS.80%29.aspx It is the Token-Pasting Operator — abelenky, Aug 01 '10 at 17:48
@InsertNickHere It's token concatenation, part of any C preprocessor. http://en.wikipedia.org/wiki/C_preprocessor#Token_concatenation — nos, Aug 01 '10 at 17:49
Can you provide a more realistic example of usage or the macros? Right now, the macros are essentially useless. — strager, Aug 01 '10 at 17:50

score 9 · Accepted Answer · answered Aug 01 '10 at 17:49

9

Use this instead:

#define CONCAT(X,Y) X##Y
#define OUTER(a, b) CONCAT(a,b)
#define INNER(a, b) CONCAT(a,b)

enum foo {
    OUTER(INNER(x, y),z)
}; // line 108

int main(...)
{
    foo bar = xyz; // line 112
}

answered Aug 01 '10 at 17:49

Nordic Mainframe

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Woo hoo it works! But can you explain to me why it works? Why is my preprocessed output correct? I see from the gcc error described below that it appears to be keeping a ")" and trying to concatenate the "z" to "xy)"? But this doesn't appear in my preprocessed output! – Jack Aug 01 '10 at 17:59
Read Jonathan Leffler's canonical answer: http://stackoverflow.com/questions/1489932/c-preprocessor-and-concatenation – Nordic Mainframe Aug 01 '10 at 18:04
Ah! Is OUTER getting "INNER(c, d), z" and not "xy, z" as I thought? That would make sense. But I still don't get why my preprocessed output is correct! – Jack Aug 01 '10 at 18:05
No need to declare two separate OUTER/INNER macros, this works also: OUTER(OUTER(x, y),z) – Agnius Vasiliauskas Aug 28 '10 at 07:47

score 2 · Answer 2 · answered Aug 01 '10 at 17:51

2

Preprocessing your example with gcc results in:

enum foo {
t.c:7:1: error: pasting ")" and "z" does not give a valid preprocessing token
    xy z
};

which should give you a clue of why Luther's solution works and yours doesn't.

answered Aug 01 '10 at 17:51

Employed Russian

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score 1 · Answer 3 · answered Aug 01 '10 at 17:50

1

If you are using gcc, then you can give it the -E option to see the preprocessed output. Then you can easily see what the preprocessor has output and how to further debug your macros. Other compilers also have similar options.

answered Aug 01 '10 at 17:50

zvrba

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Weird C++ preprocessor macro syntax

3 Answers3