Question

Is it possible to create a template function that takes a variable number of arguments, for example, in this Vector< T, C > class constructor:

template < typename T, uint C >
Vector< T, C >::Vector( T, ... )
{
    va_list arg_list;
    va_start( arg_list, C );
    for( uint i = 0; i < C; i++ ) {
        m_data[ i ] = va_arg( arg_list, T );
    }
    va_end( arg_list );
}

This almost works, but if someone calls Vector< double, 3 >( 1, 1, 1 ), only the first argument has the correct value. I suspect that the first parameter is correct because it is cast to a double during the function call, and that the others are interpreted as ints and then the bits are stuffed into a double. Calling Vector< double, 3 >( 1.0, 1.0, 1.0 ) gives the desired results. Is there a preferred way to do something like this?

Answer 1

This code looks dangerous and I think your analysis on why it isn’t working is spot on, there’s no way for the compiler to know that when calling:

Vector< double, 3 >( 1, 1, 1 )

the ones should be passed as doubles.

I would change the constructor to something like:

Vector< T, C >::Vector(const T(&data)[C])

instead, and have the user pass the arguments as an array. Another sort of ugly solution would be something like this:

template < typename T, uint C >
Vector< T, C >::Vector(const Vector<T, C - 1>& elements, T extra) {
}

and call it like this (with some typedefs):

Vector3(Vector2(Vector1(1), 1), 1);