calcul formel

#include <iostream>
#include <unordered_map>
#include <cmath>

namespace ast {

typedef std::unordered_map<char, double> data;

#pragma STDC FENV_ACCESS ON

template<int D> struct val {
    double operator() (const data&) const { return D; }
};
template<char I> struct mis {
    double operator()(const data& v) const { return v.at(I); }
};
template<typename Expr> struct minus {
    double operator()(const data& v) const { return -(Expr()(v));  }
};
template<typename Expr1, typename Expr2> struct add {
    double operator()(const data& v) const { return Expr1()(v)+Expr2()(v);  }
};
template<typename Expr1, typename Expr2> struct sub {
    double operator()(const data& v) const { return Expr1()(v)-Expr2()(v);  }
};
template<typename Expr1, typename Expr2> struct mul {
    double operator()(const data& v) const { return Expr1()(v)*Expr2()(v); }
};
template<typename Expr1, typename Expr2> struct div {
    double operator()(const data& v) const { return Expr1()(v)/Expr2()(v); }
};
template<typename Expr1, typename Expr2> struct mod {
    double operator()(const data& v) const { return fmod(Expr1()(v),Expr2()(v)); }
};

}

using namespace ast;

int main() {
    add<
        mis<'a'>,
        mul<
            val<2>,
            mis<'b'>
        >
    > expression;

    mod<mis<'a'>, mis<'b'> > exp2;
    double a=1, b=2; //or whatever
    std::cout << expression({ { 'a', a }, { 'b', b } }) << '\n';
    a=7, b=3;
    std::cout << exp2({ { 'a', a }, { 'b', b } }) << '\n';
    return 0;
}