How to make an "operator" variable? (C++) - c++

Question

I am working on making an expression class:
template<typename T, typename U>
class expression
{
public:
expression(T vala, U valb, oper o){val1 = vala; val2 = valb; op = o;}
operator bool{return(val1 op val2);}
private:
T val1;
U val2;
oper op;
};
as you can see, this is somewhat pseudocode, because I need an operator class. My original thought was to create an array of all possible operators, and then convert it through a string, but that wouldn't work because of the sheer number of operators, and how to convert it to a string except through a two dimensional array, where n[0][0] has the first operator, and n[0][1] has that operators string.
Does anybody have any suggestions to adding an operator value to my expression class?

Answer 1

Similar methods are used in c++ expression templates techniqueue.
You create expression as a class with a method such as apply or evaluate. This method takes the parameters and applies the expression.
Have a look what expression templates are using.
http://www.angelikalanger.com/Articles/Cuj/ExpressionTemplates/ExpressionTemplates.htm
https://www.cct.lsu.edu/~hkaiser/spring_2012/files/ExpressionTemplates-ToddVeldhuizen.pdf
As an example in your case:
struct isEqual
{
template <typename T, typename U>
bool operator()(T a, U b)
{
return a == b;
}
};
template <typename T, typename OP>
struct expression
{
T& a;
T& b;
OP& op;
expression(T a, T b, OP op) : a(a), b(b), op(op) {}
void eval() { op(a,b); }
};
int main()
{
expression<int, isEqual> exp(1,2,isEqual());
exp.eval();
}

Answer 2

Maybe a function pointer. Instead of ...
operator bool{return(val1 op val2);}
... code it as ...
operator bool{return op(val1, val2);}
... in which case op can be a pointer to a (any) function which takes two parameters and which returns bool.
template<typename T, typename U>
class expression
{
public:
//define pointer-to-function type
typedef bool *oper(const T& val1, const U& val2);
... etc ...

Answer 3

I'm not entirely sure what you're asking, but if you are trying to overload an arbitrary string as an operator, you can't. There is a finite set of operators that you can overload in c++
see here: http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B
What you should do is overload operator() in oper to create a function object and return op(val1, val2) instead.

Answer 4

You can use the functional standard library and take your argument as a:
std::tr1::function<bool (T,U)>
ie:
#include <functional>
template<typename T, typename U>
class expression
{
public:
expression(T vala, U valb, oper o) : val1(vala), val2(valb), op(o)
{ }
operator bool{return op(val1, val2);}
private:
T val1;
U val2;
std::tr1::function<bool (T,U)> op;
};
Then, to create a expression:
#include <functional>
expression<int, int> foo(4,3, std::tr1::bind(greater()));
Here's a tutorial