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 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 me out.

- Martin

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 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 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.

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 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 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.

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 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 orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

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 ## =
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