title: C++ performance of accessing member variables versus local variables
body: <p>Is it more efficient for a class to access member variables or local variables? For example, suppose you have a (callback) method whose sole responsibility is to receive data, perform calculations on it, then pass it off to other classes. Performance-wise, would it make more sense to have a list of member variables that the method populates as it receives data? Or just declare local variables each time the callback method is called?</p>
  
  <p>Assume this method would be called hundreds of times a second...</p>
  
  <p>In case I'm not being clear, here's some quick examples:</p>
  
  <pre><code>// use local variables
  class thisClass {
      public:
          void callback( msg&amp; msg )
          {
              int varA;
              double varB;
              std::string varC;
              varA = msg.getInt();
              varB = msg.getDouble();
              varC = msg.getString();
  
              // do a bunch of calculations
           }
  
  };
  
  // use member variables
  class thisClass {
      public:
          void callback( msg&amp; msg )
          {
               m_varA = msg.getInt();
               m_varB = msg.getDouble();
               m_varC = msg.getString();
  
               // do a bunch of calculations
          }
  
      private:
          int m_varA;
          double m_varB;
          std::string m_varC;
  
  };
  </code></pre>