Example Either type in C++

either.cpp

#include <stdio.h>

template <class T1, class T2> 
class Either
{
    bool isLeft;
    union
    {
        T1 left;
        T2 right;    
    };
    template<class T1_, class T2_> friend Either<T1_, T2_> Left(T1_ x);
    template<class T1_, class T2_> friend Either<T1_, T2_> Right(T1_ x);
public:
    bool matchLeft(T1& x)
    {
        if (isLeft)
            x = left;
        return isLeft;    
    }
    bool matchRight(T2& x)
    {
        if (!isLeft)
            x = right;
        return !isLeft;    
    }  
};

template <class T1, class T2> 
Either<T1, T2> Left(T1 x)
{
    Either<T1, T2> e;
    e.isLeft = true;
    e.left = x;
    return e;
}
template <class T1, class T2> 
Either<T1, T2> Right(T2 x)
{
    Either<T1, T2> e;
    e.isLeft = false;
    e.right = x;
    return e;
}

int main(int argc, char* argv[])
{
  Either<int, float> x = Left<int, float>(5);
  int c;
  if (x.matchLeft(c))
    printf("the number is %i", c);
  return 0;
}

According to boost::variant's docs, using union like above will only support POD types.

Check the last month thread on Interest in an 'either' library?

Here's a simple version I've been using that [mildly] depends on boost: https://gist.github.com/3noch/6024523

Shouldn't it be template<class T1_, class T2_> friend Either<T1_, T2_> Right(T2_ x); instead of template<class T1_, class T2_> friend Either<T1_, T2_> Right(T1_ x); ?

@gonzaw I think so. And the typo is not caught in the example as it only demonstrates the left side. The class should befriend both left and right with the appropriate template arguments for left and right respectively.

I'm using Arduino like chip with c++11 but no std lib or exceptions support. I end up defining sth like that:

#pragma once

template <class L, class R> 
class Either
{
    virtual bool isLeft();
    virtual bool isRight();

    virtual bool matchLeft(L& x);
    virtual bool matchRight(R& x);
};

template <class L, class R> 
class Left : public Either<L, R> {
public:
    L left;

    Left(L left): left(left) {}
    constexpr bool isLeft() { return true; }
    constexpr bool isRight() { return false; }

    bool matchLeft(L& x)
    {
        x = left;
        return true;
    }
    bool matchRight(R& x) { return false; }
};

template <class L, class R> 
class Right : public Either<L, R> {
public:
    R right;

    Right(R right): right(right) {}
    constexpr bool isLeft() { return false; }
    constexpr bool isRight() { return true; }

    bool matchLeft(L& x) { return false; }

    bool matchRight(R& x) {
        x = right;
        return true;
    }
};

I'm not native C++ dev so I would be glad for comments.

Are you sure you meant
template<class T1_, class T2_> friend Either<T1_, T2_> Right(T1_ x);
not
template<class T1_, class T2_> friend Either<T1_, T2_> Right(T2_ x);
?