overloading non-member conversion to bool operator

Question

I am trying to write bool-conversion operator for std::bitset

I tried:

template<size_t size>
operator bool(std::bitset<size> & b)
{
    return b.any();
}

but I got

error C2801: 'mynamespace::operator bool' must be a non-static member

from my visual-studio.

But when I look up C2801 explanation it says nothing about conversion operators (only about =, ->, [],())

So, is it possible to somehow write "Conversion std::bitset to bool operator?"

(I can not call b.any() in my if-statements, because the same code must run when std::bitset is replaced with unsigned or something

typedef std::bitset<x> Bitset;
//typedef unsigned Bitset;

so the ideal syntax will be like:

Bitset b = whatewer;
if(b)
    doStuff();

)

If this overloading is not possible, what is the recommended workaround?

so far I use it like:

if(b == Bitset(0))
    doStuff(); 

but I dont like it.

Thank you

Answer 1

As the error message says, the conversion operator must be a non-static member of a class. That is true.

I can not call b.any() in my if-statements, because the same code must run when std::bitset is replaced with unsigned or something.

If that is your problem, then you can use function overload, and call it passing the argument which will return a boolean value:

template<typename T>
bool to_bool(T const & b)
{
    return b; //implicit conversion (if allowed) for all other types
}

template<size_t N>
bool to_bool(std::bitset<N> const & b)
{
    return b.any();
}

then use it as:

if (to_bool(whatever)) 
{
}

It will call the correct overload. If the type of whatever is std::bitset<N> then the second overloaded function will be called, or else the first one will be called.

Answer 2

§12.3.2/1: "A member function of a class X with a name of the form [...] specifies a conversion from X to the type specified..." (C++11 uses the same section number and nearly the same wording, adding only that the function takes no parameters).

The other possible way to define a conversion is a constructor (§12.3.1), which is obviously a class member as well.

In short, yes, conversions must always be defined as member functions.

One way to do what you want would be to write a wrapper around std::bitset that provides the conversion you care about:

template <int size>
class mybitest {
    std::bitset<size> bits;
public:
    operator bool() { return bits.any(); }
}

But if you decide to do that, you'll need to write forwarding functions for essentially all the pieces of bitset you're using (ctors, assignment, etc.)