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From: "kevin  cline" <kevin.cline@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: why learn C?
Date: 1 Apr 2007 23:16:23 -0700
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On Apr 2, 12:03 am, Brian May <b...@snoopy.apana.org.au> wrote:
> >>>>> "kevin" == kevin cline <kevin> writes:
>
>     kevin> "The big difference is that C++ templates allow type
>     kevin> checking during compile-time [...]
>
> How is this any different from Ada generics?

Because Ada generics have to be explicitly instantiated.  Ada requires
an explicit instantiation for every combination of types that appears
in the program.  C++ template functions are implicitly instantiated,
and can be defined once to cover all combinations of physical units,
like this:

template <int M, int L, int T> class Unit
// Mechanical unit with mass dimension M, length dimension L,
// and time dimension T.
{
private:
  double value;
public:
  explicit Unit(double v): value(v) {}
  Unit operator+(Unit rhs) { return Unit(v + rhs.v); }
  Unit operator-(Unit rhs) { return Unit(v - rhs.v); }
  Unit<M+M2,L+L2,T+T2> operator*(Unit<M2,L2,T2>& rhs)
  { return Unit<M+M2,L+L2,T+T2>(v * rhs.v); }
  ...
};

typedef Unit<0,0,0> Scalar;
typedef Unit<1,0,0> Mass;
typedef Unit<0,1,0> Length;
typedef Unit<0,0,1> Time;
typedef Unit<1,1,-2> Force;
typedef Unit<1,-1,-2> Pressure;
typedef Unit<1,-3,0> Density;

Mass gram(1);
Length meter(1);
Time second(1);

Pressure water_pressure(Length head)
{
  Force g_earth = Scalar(9.8)*meter/second/second;
  Density water_density = Scalar(1000000)*gram/meter/meter/meter;
  return head * g_earth * water_density;
}

Ada would require a separate instantiations for each of these
operations:

Scalar * Length (returning Length)
Length / Time (returning Velocity)
Velocity / Time (returning Acceleration)
Scalar * Mass (returning Mass)
Mass / Length (returning Unit<1,-1,0>)
Unit<1,-1,0>/Length (returning Unit<1,-2,0>)
Unit<1,-2,0>/Length (returning Density)
Length * Acceleration (returning Unit<0,-2,2>)
Unit<0,2,2>*Density (returning Pressure)