combining 3 functors into 1

I have 3 functors and was wondering if these can be combined into 1, perhaps as a template. is it possible? if so, how would I do it. thx!

    struct less_than
    {
    bo开发者_JS百科ol operator()(double prev,double curr) const
    {
    return prev<curr;
    }
    };

    struct great_than
    {
    bool operator()(double prev,double curr) const
    {
    return prev>curr;
    }
    };

    struct equal_to
    {
    bool operator()(double prev, double curr) const
    {
    return prev==curr;
    }
    };

If you mean, specialized by the operator, then the answer is, no, not at the language level.

Luckily, the STL already provides functors for this (std::equal_to, etc.). You can either use these directly, or use them as arguments to your own function classes.

As these are all existent in the standard library, you can just do

template<class F>
struct compare
{
    compare(F _f)
        : f(_f) {};
    bool operator()(double prev, double curr) const
    {
        return f(prev, curr);
    }
    F f;
};

And use e.g. compare< std::less<double> >. But this would be quite useless, as you can just use the standard library functors directly.

You can do something like this:

class Functors
{
private:
bool f1(double, double)
{
}

bool f2(double, double)
{
}
bool f3(double, double)
{
}
public:
bool test(int op, double a, double b)
{
//better use function selector, this is only simple example
 if (op == 1)
  return f1(a, b);
 if (op == 2)
  return f2(a, b);
 if (op == 3)
  return f3(a, b);
}
};

use it:

vector<double> v;
int op = select_op();
//sort vector
std::sort(v.begin(), v.end(), boost::bind(&Functors::test, Functors(), op, _1, _2));