identifier ## identifier

The ## operator is a binary operator, sometimes called the concatenation operator because it concatenates preprocessor tokens. It can be used only in the macro replacement text of a #define directive. It must not appear at the start or end of the macro replacement text. The operands of the ## operator must be parameter names for the macro being defined. They are replaced by the corresponding macro arguments, which are not expanded. The tokens immediately adjacent to the ## operator are concatenated to form a single token. If the result is not a valid token, the behavior is undefined; otherwise, the token is expanded normally.

The evaluation order of # and ## operators is undefined. If the order is important, you can control the order by using multiple macros.

Example

The following example prints std to cout because the concat macro assembles the token std from s, t, and d:

#define s this is not expanded by the concatenation operator
#define t nor is this, so the result is the token std
#define concat(part1, part2, part3) part1 ## part2 ## part3
concat(s, t, d)::cout << "std";

See Also

# operator, #define directive