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A problem about concatenation in macro

cppcraze
Guest
 
Posts: n/a
#1: Jan 18 '08
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

red floyd
Guest
 
Posts: n/a
#2: Jan 18 '08

re: A problem about concatenation in macro


cppcraze wrote:
Quote:
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.

jalina
Guest
 
Posts: n/a
#3: Jan 18 '08

re: A problem about concatenation in macro


cppcraze a écrit :
Quote:
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.
James Kanze
Guest
 
Posts: n/a
#4: Jan 18 '08

re: A problem about concatenation in macro


On Jan 18, 3:58 am, cppcraze <cppcr...@gmail.comwrote:
Quote:
I am just stumbled by a problem about concatenation in macro. See
below code snippet:
Quote:
// there're some contants definition in this class
struct X
{
enum {A, B, C};
};
Quote:
// 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; \
}
Quote:
MK_FUNC(A)
MK_FUNC(B)
MK_FUNC(C)
Quote:
#undef MKFUNC
Quote:
// then I can use getA(), getB() .... in my program.
Quote:
But the preprocessor always complains:
Quote:
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
Quote:
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 orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
James Kanze
Guest
 
Posts: n/a
#5: Jan 18 '08

re: A problem about concatenation in macro


On Jan 18, 4:54 am, red floyd <no.s...@here.dudewrote:

[...]
Quote:
The token pasting operator ## creates a new identifier.
Not necessarily a new identifier. For example, I've used it for
things like:
op ## =
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 orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
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