title: How to use ## in a macro?
body: <p>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'</p>
  
  <p>I just want to set value for my struct quickly, and what is the problem with 'cfg->##column = value'</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  
  #define PARSE_COLUMN( column, value ) \
      printf("parse column:%s\n",#column);\
      cfg-&gt;##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;
  }
  </code></pre>