title: double dash before a variable in C++
body: <p>Sorry if this is a beginner question, but I could not find an answer anywhere - I came across a class destructor in C++, in which there is</p>
  
  <pre><code>--num_strings;
  </code></pre>
  
  <p>I am wondering what the double dash means. Any help is appreciated, thanks. Below are parts of the header and cpp files:</p>
  
  <pre><code>// strngbad.h
  #include &lt;iostream&gt;
  class StringBad
  {
  private: 
      char * str;
      int len;
      static int num_strings;
  
  public:
      StringBad(const char * s);
      StringBad();
      ~StringBad();
  };
  </code></pre>
  
  <hr>
  
  <pre><code>//  strngbad.cpp
  #include &lt;cstring&gt;
  #include "strngbad.h"
  using std::cout;
  ....
  ....
  ....
  
  StringBad::StringBad(const char *s)
  {
      len = std::strlen(s);
      str = new char[len + 1];
      std::strcpy(str, s);
      num_strings++;
  }
  
  StringBad::~StringBad()
  {
      --num_strings;
      delete [] str;
  }
  </code></pre>