title: class header+ implementation
body: <p>what am I doing wrong here? I keep on getting a compilation error when I try to run this in codelab (turings craft)</p>
  
  <p>Instructions:</p>
  
  <p>Write the implementation (.cpp file) of the  GasTank class of the previous exercise. The full specification of the class is: 
  A data member named  amount of type double. 
  A constructor that no parameters. The constructor initializes the data member  amount to 0. 
  A function named  addGas that accepts a parameter of type  double . The value of the  amount instance variable is increased by the value of the parameter. 
  A function named  useGas that accepts a parameter of type  double . The value of the  amount data member is decreased by the value of the parameter. 
  A function named  getGasLevel that accepts no parameters.  getGasLevel returns the value of the  amount data member. </p>
  
  <pre><code>class GasTank{
  double amount;
  GasTank();
  void addGas(double);
  void useGas(double);
  double getGasLevel();};
  
  GasTank::GasTank(){
  amount=0;}
  double GasTank::addGas(double a){
  amount+=a;}
  double GasTank::useGas(double a){
  amount+=a;}
  double GasTank::getGasLevel(){
  return amount;}
  </code></pre>