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A preprocessor strobe operator with literary string prefixes

So, I want to do the traditional string statement thing in a macro:

#define FOO(x) foo(#x, (x))

However, I need to use the string literal prefix: http://en.cppreference.com/w/cpp/language/string_literal
This is a problem, because if I need a UTF-32 string literal, I try to do this:

#define FOO(x) foo(U#x, (x))

But gcc 4.9.2 complains:

error: 'U' was not declared in this scope

Is there a way to force the compiler to treat U

as a prefix a string macro variable?

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1 answer


Yes, use concatenation :

#define CONCAT2(x, y) x ## y
#define CONCAT(x, y) CONCAT2(x, y)
#define STRINGIZE(x) #x

#define FOO(x) foo(CONCAT(U, STRINGIZE(x)), (x))


Additional indirection, besides being good, if you pass a macro to be evaluated first, is "necessary" because of N3936 §16.3.2 [cpp.stringize] / 2, which says:

The order of evaluation of the # and ## operators is not specified.

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