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The concatenation "?" concatenates one character or one character string to another within a macro body.
At macro expansion, the character or character string on the left of the concatenation is concatenated to the character or character string on the right of the sign. The "?" itself disappears after concatenating the strings.

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The symbols before and after the concatenation symbol "?" in the symbols of a macro definition can be recognized as formal parameters or local symbols, and concatenation symbols can also be used as delimiter symbols. At macro expansion, formal parameters or local symbols before and after "?" in the symbols are evaluated before they are concatenated in the symbols.

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Cautions