[C/C++] # and ## in macro definition

Macros in C and C++ belong to the category of compiler preprocessing.

Single pound sign # operator

Single pound sign # (stringification operator) is used to convert macro parameter variable name into a string (Stringfication).

Here's an example:

#include <stdio.h>

#define PRINT_VARIABLE(x)  printf(#x " = %d\n", x)

int main() {
    
    
    int num = 10;
    PRINT_VARIABLE(num);
    return 0;
}

In the above example, #x will convert the parameter x received by the macro PRINT_VARIABLE into a string constant. In this example, the parameter variable name received by PRINT_VARIABLE is num, so #x is equivalent to "num", which will be displayed when outputting:

num = 10

Double pound sign ## operator

Double pound sign ## (concatenator, concatenator) is used to connect two tokens (Token) together
For example:

#include <stdio.h>
#define CONCAT(x, y)  x##y

int main() {
    
    
    int result = CONCAT(3, 5);
    printf("Result: %d\n", result);
    return 0;
}

The output is:

Result: 35

Another example:

#define LINK_MULTIPLE(a,b,c,d) a##_##b##_##c##_##d
typedef struct _record_type LINK_MULTIPLE(name,company,position,salary);

When expanded, this statement is equivalent to:

typedef struct _record_type name_company_position_salary;

Notice

It should be noted that the # and ## operators can only be used in macro definitions, and their use needs to be careful to avoid unexpected behavior.

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Reference: https://www.cnblogs.com/lixiaofei1987/p/3171439.html