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};
};
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// 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; \
}
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MK_FUNC(A)
MK_FUNC(B)
MK_FUNC(C)
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// then I can use getA(), getB() .... in my program.
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But the preprocessor always complains:
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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
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