CODEWITHSUNDEEP
c++templatesc++11
steffen
steffen

Reputation: 8958

How to add compile time information derived from template parameters to template?

I play around with the unit implementation using user defined literals as presented by Stroustrup at GoingNative 2012 (from minute 23:00 on). Here is the code: 

 #include <iostream>

 using std::cout;
 using std::endl;

 template<int M, int K, int S> 
 struct Unit { // a unit in the MKS system
   enum {m=M,kg=K,s=S}; 
 };

 template<typename Unit> // a magnitude with a unit
 struct Value {
   double val;
   constexpr Value(double d) : val(d) {}
 };

 using Meter = Unit<1,0,0>;
 using Second = Unit<0,0,1>;

 using Distance = Value< Meter >;
 using Time = Value< Second >;
 using Velocity = Value< Unit<1,0,-1> >;

 constexpr Value<Meter> operator "" _m(long double d)
 // a f-p literal with suffix 'm'
 {
   return Distance(d);
 }

 constexpr Value<Second> operator"" _s(long double d)
 // a f-p literal with suffix 's'
 {
   return Time(d);
 }

 constexpr Velocity operator/(Distance d, Time t)
 {
   return ( d.val / t.val );
 }

 int main(void)
 {
    Distance s = 100._m;
    Time t = 22._s;
    Velocity v = s/t;

    cout << "s " << s.val << "\nt " << t.val << endl;
    cout << "v " << v.val << endl;

   return 0;
 }

As you can see I took the freedom to define an operator/ to calculate velocities. The output is (gcc-4.7 needed):

$ g++ -std=c++0x test_units_02.cc && ./a.out
s 100
t 22
v 4.54545

So far so good. now I want to add a string containing a unit representation to the struct Unit (or Value?). Whichever way I wnat to be able to write

cout << "v " << v.val << v.unit << endl;

and get something like 

v 4.54545 m^1 s^-1

or 

v 4.54545 m^1 kg^0 s^-1

It does not need to be beautiful as it would just be for checking. And learning how to do it ;).

Of course the elegant solution would be having everything evaluated at compile time.

I had some shots at it, but I won't bore/confuse you with my resultless tries...

Upvotes: 4

Views: 215

Answers (2)

drummist180
drummist180

Reputation: 38

Unit already has the info we need, so we can do this by adding a function to Value, or we can overload operator<<:

template<typename U>
struct Value
{
    double val;
    constexpr Value(double d) : val(d) {}

    std::string Units() const
    {
        return "m^" + to_string(U::m) + 
               " kg^" + to_string(U::kg) +
               " s^" + to_string(U::s);
    }
};

template <typename U>
std::ostream& operator<<(std::ostream& out, Value<U> val)
{
    out << val.val
        << " m^" << U::m << " kg^" << U::kg << " s^" << U::s;
    return out;
}

We can also provide a generic operator/:

template <typename U1, typename U2>
Value<Unit<U1::m - U2::m, U1::kg - U2::kg, U1::s - U2::s>>
operator/(Value<U1> v1, Value<U2> v2)
{
    return (v1.val / v2.val);
}

This gives us more flexibility:

void demo()
{
    auto accel = Distance(100) / Time(22) / Time(1);
    cout << accel << endl; // Print with units.
    cout << accel.val << endl; // Print without units.
    cout << accel.Units() << endl; // Print only units.
}

Upvotes: 1

ecatmur
ecatmur

Reputation: 157364

First we add a unit member to Value:

 template<typename Unit> // a magnitude with a unit
 struct Value {
   double val;
   constexpr static Unit unit = {};
   constexpr Value(double d) : val(d) {}
 };

Then we write a stream out operator:

 template<int M, int K, int S>
 std::ostream &operator<<(std::ostream &os, Unit<M, K, S>) {
   return os << "m^" << M << " kg^" << K << " s^" << S;
 }

Generating a string at compile time is possible, but requires a constexpr compile-time string class (e.g. boost::mpl::string) and decimal formatting - all of which is feasible but not particularly worth it in this case.

Upvotes: 6

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