Question

I have defined following templates, used for combining already defined predicates:

namespace SomeNamespace
{
//TODO: for now simply taking argument type of first predicate
template<typename LhPredicate, typename RhPredicate>
struct OrPredicate : public std::unary_function<typename LhPredicate::argument_type, bool>
{
public:
  OrPredicate(LhPredicate const& lh, RhPredicate const& rh)
    : m_lh(lh),
      m_rh(rh)
  {
  }

  bool operator()(typename LhPredicate::argument_type arg) const
  {
      return m_lh(arg) || m_rh(arg);
  }

private:
  LhPredicate m_lh;
  RhPredicate m_rh;
};


//TODO: for now simply taking argument type of first predicate
template<typename LhPredicate, typename RhPredicate>
struct AndPredicate : public std::unary_function<typename LhPredicate::argument_type, bool>
{
public:
    AndPredicate(LhPredicate const& lh, RhPredicate const& rh)
      : m_lh(lh),
        m_rh(rh)
    {
    }

    bool operator()(typename LhPredicate::argument_type arg) const
    {
      return m_lh(arg) && m_rh(arg);
    }

private:
    LhPredicate m_lh;
    RhPredicate m_rh;
};


template<typename LhPredicate, typename RhPredicate>
OrPredicate<LhPredicate, RhPredicate> or(LhPredicate const& lh, RhPredicate const& rh)
{
  return OrPredicate<LhPredicate, RhPredicate>(lh, rh);
}

template<typename LhPredicate, typename RhPredicate>
AndPredicate<LhPredicate, RhPredicate> and(LhPredicate const& lh, RhPredicate const& rh)
{
  return AndPredicate<LhPredicate, RhPredicate>(lh, rh);
}

}

The problem is, when compiling code using helper function templates (or/and), gcc complains about those lines:

AndPredicate<LhPredicate, RhPredicate> and(LhPredicate const& lh, RhPredicate const& rh)

OrPredicate<LhPredicate, RhPredicate> or(LhPredicate const& lh, RhPredicate const& rh)

like this:

error: expected unqualified-id before '||' token
error: expected unqualified-id before '&&' token

So what he is really complaining about are those lines:

return m_lh(arg) && m_rh(arg);
return m_lh(arg) || m_rh(arg);

The template arguments (predicates to be combined) of course properly define operator() themselves and I really don’t know what is the gcc’s problem – the same code compiles on VS2005 just fine.

Any help would be highly appreciated.

Answer 1

and and or are reserved keywords. They are synonims to && and || operators. For example:

bool or(int a)
{

}

will not compile