Implementing Addition and Subtraction

We can now easily write the rules for addition and subtraction, since the dimensions of the arguments must always match.

template 
quantity
operator+(quantity x, quantity y)
{
  return quantity(x.value() + y.value());
}

template 
quantity
operator-(quantity x, quantity y)
{
  return quantity(x.value() - y.value());
}

These operators enable us to write code like:

quantitylength> len1( 1.0f );
quantitylength> len2( 2.0f );

len1 = len1 + len2;   // OK

but prevent us from trying to add incompatible dimensions:

len1 = len2 + quantitymass>( 3.7f ); // error