Question

https://gist.github.com/2934374

So I was intrigued by this comparison and also by the awful C++ solutions that appeared in the comments, but I must admit I’m having trouble to write a nice solution myself 🙂

My current attempt looks like this, the problems are:

• the environment is copied when the tree is created, I would like the variables to be changeable after the tree is created
• it still looks pretty ugly

So how could I squish it a bit and bind the variables so they would be affected by changes?

#include <map>
#include <string>
#include <iostream>
#include <functional>
using namespace std;

/* environ */
map<string,int> variables = { { "a" , 3 }, { "b", 4 }, { "c", 5 } };

function<int(int,int)> add = [] (int lp, int rp) { return lp + rp; };
function<int(int,int)> mlt = [] (int lp, int rp) { return lp * rp; };

/* impl */
struct Var {
    Var(int v) : p_v(v) {};
    int eval() { return p_v; };

private:
    int p_v;
};

template <typename LP, typename RP>
struct Op {
    Op(function<int(int,int)> op, LP lp, RP rp) : p_op(op), p_l(lp), p_r(rp) {};
    int eval() { return p_op(p_l.eval(), p_r.eval()); }
private:
    function<int(int,int)> p_op;
    LP p_l;
    RP p_r;
};

Var var(int val) { return Var(val); }

template <typename LP, typename RP>
auto op(function<int(int,int)> op, LP lp, RP rp) -> Op<LP,RP>
{
    return Op<LP,RP>(op,lp,rp);
}

Var operator "" _var(const char *key, size_t length)
{ return Var(variables[key]); }

/* gcc is failing me
Var operator "" _num(int val)
{ return Var(val); }
*/

int main()
{
    auto tree = op ( add, "a"_var, op ( mlt, var ( 2 ), "b"_var ));
    cout << tree.eval() << endl;
}

Answer 1

You’re re-inventing the Standard bind mechanism. There’s no need for any glue code to be written by the user.

auto tree = bind(plus<int>(), ref(variables["a"]), bind(multiplies<int>(), 2, ref(variables["b"])));
cout << tree();

http://liveworkspace.org/code/f06fd83b5d7bcbf4829306d4e590da38

std::ref makes it a reference, not a value- this means that you could bind a mutating operation.