C++ select first not null element

Question

Guys,
[question update according to updated requirements]
I've implemented following function which should return either first not null element or throw an exception.
Also could you invent more classic and shorter name like 'max', 'min', 'pair'?

template <typename T>
T select_first_not_empty( const T& a, const T&b )
{
    static T null = T();

    if ( a == null && b == null )
        throw std::runtime_error( "null" );

    return
        a != null ? a : b;
}

int main()
{
    const int a1 = 2;
    const int b1 = 0;

    const int* a2 = 0;
    const int* b2 = new int(5);

    const std::string a3 = "";
    const std::string b3 = "";

    std::cout << select_first_not_empty( a1, b1 ) << std::endl;
    std::cout << select_first_not_empty( a2, b2 ) << std::endl;
    std::cout << select_first_not_empty( a3, b3 ) << std::endl;

    return 0;
}

Answer 1

If the ctor for T does anything significant, it appears like you're doing it three times every time through "select_first_not_empty".

Oracle calls something similar "COALESCE", if you're looking for a better name.

I'm not sure what the point is, though. If I really wanted to know whether something was set or not, I'd use nullable pointers rather than references. "NULL" is a far better indicator of the intent to not have the variable set than to use an in-band value like 0 or "".

Answer 2

C# has a similarly functioning built-in operator ??, which I believe is called coalesce.

Perl's || (short-circuit logical OR) operator also has similar functionality: instead of return 0 or 1, it returns the value of the first argument evaluating to true:

0 || 7

returns 7, not 1 or true as a C\C++ or C# programmer would expect.

The closest thing to this that C++ has built in is the find_if algorithm:

vector<int> vec;
vec.push_back(0);
vec.push_back(0);
vec.push_back(7);

vector<int>::iterator first_non_0 = 
    find_if(vec.begin(), vec.end(), bind2nd(not_equal_to<int>(), 0));

Answer 3

you can try do next

template < typename T >
T get_valuable( const T& firstValue, 
                const T& alternateValue, 
                const T& zerroValue = T() )
{
    return firstValue != zerroValue ? firstValue : alternateValue;
}

// usage
char *str = "Something"; // sometimes can be NULL
std::string str2 ( get_valuable( str,  "" ) );

// your function
template <typename T>
T select_first_not_empty( const T& a, 
                          const T& b, 
                          const T& zerroValue = T() )
{
    const T result = get_valuable( a, b, zerroValue );
    if ( result == zerroValue )
    {
        throw std::runtime_error( "null" );
    }
    return result;
}