| Topic: |
DEVELOP > c-Plus-Plus |
| User: |
"cppcraze" |
| Date: |
17 Jan 2008 08:58:48 PM |
| Object: |
A problem about concatenation in macro |
Hi,
I am just stumbled by a problem about concatenation in macro. See
below code snippet:
// there're some contants definition in this class
struct X
{
enum {A, B, C};
};
// and here I want to define a utility macro to help me generate some
functions
#define MK_FUNC(arg) \
int get##arg() \
{ \
return X::##arg; \
}
MK_FUNC(A)
MK_FUNC(B)
MK_FUNC(C)
#undef MKFUNC
// then I can use getA(), getB() .... in my program.
But the preprocessor always complains:
warning: pasting "::" and "A" does not give a valid preprocessing
token
warning: pasting "::" and "B" does not give a valid preprocessing
token
warning: pasting "::" and "C" does not give a valid preprocessing
token
I really don't why this will happen. Isn't this usage in the macro
"X::##arg" an invalid ? Hope someone can help me out.
- Martin
.
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| User: "James Kanze" |
|
| Title: Re: A problem about concatenation in macro |
18 Jan 2008 07:11:15 AM |
|
|
On Jan 18, 3:58 am, cppcraze <cppcr...@gmail.com> wrote:
I am just stumbled by a problem about concatenation in macro. See
below code snippet:
// there're some contants definition in this class
struct X
{
enum {A, B, C};
};
// and here I want to define a utility macro to help me generate some
// functions
#define MK_FUNC(arg) \
int get##arg() \
{ \
return X::##arg; \
}
MK_FUNC(A)
MK_FUNC(B)
MK_FUNC(C)
#undef MKFUNC
// then I can use getA(), getB() .... in my program.
But the preprocessor always complains:
warning: pasting "::" and "A" does not give a valid preprocessing
token
warning: pasting "::" and "B" does not give a valid preprocessing
token
warning: pasting "::" and "C" does not give a valid preprocessing
token
I really don't why this will happen. Isn't this usage in the macro
"X::##arg" an invalid?
It's invalid. The string "X::##arg" breaks down into the tokens
X, ::, ## and arg. You're trying to paste :: and arg to get a
single token, and that isn't a valid token in C++.
Why do you paste at all here? Isn't the token you want
precisely the expansion of arg?.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34
.
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| User: "jalina" |
|
| Title: Re: A problem about concatenation in macro |
18 Jan 2008 01:56:15 AM |
|
|
cppcraze a écrit :
Hi,
I am just stumbled by a problem about concatenation in macro. See
below code snippet:
// there're some contants definition in this class
struct X
{
enum {A, B, C};
};
// and here I want to define a utility macro to help me generate some
functions
#define MK_FUNC(arg) \
int get##arg() \
{ \
return X::##arg; \
}
MK_FUNC(A)
MK_FUNC(B)
MK_FUNC(C)
#undef MKFUNC
// then I can use getA(), getB() .... in my program.
But the preprocessor always complains:
warning: pasting "::" and "A" does not give a valid preprocessing
token
warning: pasting "::" and "B" does not give a valid preprocessing
token
warning: pasting "::" and "C" does not give a valid preprocessing
token
I really don't why this will happen. Isn't this usage in the macro
"X::##arg" an invalid ? Hope someone can help me out.
- Martin
THe ## is used to make a new *single* token from two others. Since
::##arg gives (e.g. for A) ::A which is not a single token.
.
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|
| User: "red floyd" |
|
| Title: Re: A problem about concatenation in macro |
17 Jan 2008 09:54:57 PM |
|
|
cppcraze wrote:
Hi,
I am just stumbled by a problem about concatenation in macro. See
below code snippet:
// there're some contants definition in this class
struct X
{
enum {A, B, C};
};
// and here I want to define a utility macro to help me generate some
functions
#define MK_FUNC(arg) \
int get##arg() \
{ \
return X::##arg; \
}
MK_FUNC(A)
MK_FUNC(B)
MK_FUNC(C)
#undef MKFUNC
// then I can use getA(), getB() .... in my program.
But the preprocessor always complains:
warning: pasting "::" and "A" does not give a valid preprocessing
token
warning: pasting "::" and "B" does not give a valid preprocessing
token
warning: pasting "::" and "C" does not give a valid preprocessing
token
I really don't why this will happen. Isn't this usage in the macro
"X::##arg" an invalid ? Hope someone can help me out.
The token pasting operator ## creates a new identifier. Since in the
X:: sequence, arg is an independent identifier, and not pasted to
anything (like "get"), just use X::arg, and it will expand properly.
.
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| User: "James Kanze" |
|
| Title: Re: A problem about concatenation in macro |
18 Jan 2008 07:13:51 AM |
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On Jan 18, 4:54 am, red floyd <no.s...@here.dude> wrote:
[...]
The token pasting operator ## creates a new identifier.
Not necessarily a new identifier. For example, I've used it for
things like:
op ## =3D
where the legal values of op were +, -, etc.
The only real requirement is that the results form a legal
token.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34
.
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