title: How I can check if a string is defined or not?
body: <p>I have codes as below.<br>
  I wanna check if a string is defined or not using <strong><code>isdefined</code></strong> function. <br>
  How I do that?</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;stdbool.h&gt;
  #define string char*
  
  bool isdefined(string str){
       /*do something*/
  }
  
  int main(){
      string hw;                 //print NO if I write this line
      string hw = "Hello World"; //print YES if I write this line
      if(isdefined(hw))
          printf("YES");
      else
          printf("NO");
      return 0;
  }
  </code></pre>