如何在宏中使用##？

Question

Here is a simple demo. But Xcode shows me the error 'pasting formed '->client_port', an invalid preprocessing token expanded from macro 'PARSE_COLUMN' cfg->##column = value'

I just want to set value for my struct quickly, and what is the problem with 'cfg->##column = value'

#include <stdio.h>

#define PARSE_COLUMN( column, value ) \
    printf("parse column:%s\n",#column);\
    cfg->##column = value

typedef struct {
    int client_port;
} server_config;

int main(void) {
    server_config *cfg = new server_config;
    PARSE_COLUMN(client_port,123);
    return 0;
}

Answer 1

## is used to concatenate two macro arguments, for example column##value would be exchanged with client_port123 in the code above.

You don't need to use it here, you can just write cfg->column = value .

Answer 2

## is used to concatenate two valid pre-processor tokens into a single one. cfg-> is not a valid pre-processor token, but rather cfg . Therefore you get a compiler error.

In this case you can simply drop the ## and it will work: cfg->column = value .

Please note that macros like these are usually bad practice and functions are preferable.

Answer 3

## is used to split tokens, then concatenate them.

Example:

#define A1(name, type)  type name_##type##_type
#define A2(name, type)  type name##_##type##_type

A1(a1, int);  /*  int name_int_type; */
A2(a1, int);  /*  int a1_int_type;   */

In macro A1, name_##type##_type is split into 3 parts: { name_ , type , _type }, only part 2 type is a valid argument and can be replaced.

In macro A2, name##_##type##_type is split into 4 parts: { name , _ , type , _type }, both part 1 name and part 3 type are valid arguments and can be replaced.