#include <stdexcept>
#include <iostream>

struct vapour_pressure
{
    enum status_t { nonsense, unknown, known, saturated } ;

    vapour_pressure() = default ;
    vapour_pressure( double v) { *this = v ; }

    vapour_pressure& operator= ( double v )
    {
        value = v ;
        if( value < 0 ) status = nonsense ;
        // else if ...
        else status = known ;
        return *this ;
    }

    operator double() const
    {
        if( status == nonsense ) throw std::domain_error("nonsense") ;
        if( status == unknown ) throw std::domain_error("unknown") ;

        return value ;
    }

    bool valid() const { return status == known || status == saturated ; }

    operator bool () const { return valid() && value ; }
    bool operator! () const { return !bool(*this) ; }

    // etc.

    private:
        double value ;
        status_t status = unknown ;
        // ...

    friend std::ostream& operator<< ( std::ostream& stm, const vapour_pressure& vp )
    {
        if( vp.status == vapour_pressure::unknown ) return stm << "unknown" ;
        else if( vp.status == vapour_pressure::nonsense ) return stm << "nonsense" ;
        else if( vp.status == vapour_pressure::saturated ) return stm << "saturated" ;
        else return stm << vp.value ;
    }
};

int main()
{
    vapour_pressure vp ;
    std::cout << "vapour pressure is: " << vp << '\n' ;
    if(vp) { double val = vp ; /* ... */ }

    vp = 23.4 ;
    std::cout << "vapour pressure is: " << vp << '\n' ;
    if( vp.valid() ) { double psi = double(vp) * 14.5038 ; /* ... */ }

    vp = -3 ;
    std::cout << "vapour pressure is: " << vp << '\n' ;
}