Using the only the preprocessor for string concatenation?

Question

Is it possible to concatenate quoted string literals outside of the language (C++, in this case)?

That is, can I define MY_MACRO(a,b,c) and use it thus:

MY_MACRO("one", "two", "three")

and have it expand to: "onetwothree"?

The use case is to apply an attribute and its message to, say, a function signature, like so:

MY_ATTRIBUTE_MACRO("this", "is", "the reason") int foo() { return 99; }

and it would result in:

[[nodiscard("thisisthe reason")]] int foo() { return 99; }

Answer 1

The language already does string concatenation!

This:

"hi" "James"

becomes just one string literal.

That means you do not need any preprocessor tricks for this at all.

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You need only employ this in the output of your macro:

#define MY_ATTRIBUTE_MACRO(x,y,z) [[nodiscard(x y z)]]

Now this:

MY_ATTRIBUTE_MACRO("this", "is", "the reason") int foo() { return 99; }

is this:

[[nodiscard("this" "is" "the reason")]] int foo() { return 99; }

which is actually already what you wanted, because of the implicit string concatenation (which happens after macro expansion):

[[nodiscard("thisisthe reason")]] int foo() { return 99; }

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Translation phase 4:

[lex.phases]/4: Preprocessing directives are executed, macro invocations are expanded, and _­Pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal-character-name is produced by token concatenation, the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted.

Translation phase 6:

[lex.phases]/6: Adjacent string literal tokens are concatenated.