title: Have C++ standard library ifdef or ifndef preprocessor instructions?
body: <p>I'm building my own terminal app project in C++ and I'm asking myself if standard library has ifdef or ifndef preprocessors instructions. I want to know that because I need to create different header files which need some standard library headers such as "string" and some others, i don't want to include the same library 3 or more times because it makes the program heavier.<br/>
  For example i wrote on my header files something like this to prevent the .h file to be included more than once:<br/></p>
  
  <pre><code>#ifndef myheader_h
  #define myheader_h
      // my file code here
  #endif
  </code></pre>
  
  <p>I tried compiling but the compiler say me nothing about errors or warnings.<br/>
  I also tried to read the standard-library source code (<a href="https://en.cppreference.com/w/cpp/header" rel="nofollow noreferrer">https://en.cppreference.com/w/cpp/header</a>) and I haven't found any preprocessor rule like ifdef or ifndef.<br/>
  Should i include standard library headers like this?<br/></p>
  
  <pre><code>#ifndef string_h
  #define string_h
      #include &lt;string&gt;
  #endif
  </code></pre>
  
  <p>I hope my question isn't already asked because I haven't found it while searching it.</p>
  
  <h3>Updates</h3>
  
  <p>To some who said <i>"you're not in the position where you need to worry about"</i> and who said <i>"it costs very little if it has proper include guards"</i>, I meant: program's heaviness is important, I want to make it slighter so I don't want to entirely include the same file multiple times. Have std lib files proper include guards? (my header files have them, didn't know std lib files)</p>