invalid use of incomplete type ‘class GeographicLib::GeodesicLine’GCC Geodesic.hpp(28, 9): forward declaration of ‘class GeographicLib::GeodesicLine’

[Master]

Hi,
I'm trying to port my python code to its C++ counter part, but when trying to use InverseLine, I'm facing the following error :

invalid use of incomplete type ‘class GeographicLib::GeodesicLine’GCC
Geodesic.hpp(28, 9): forward declaration of ‘class GeographicLib::GeodesicLine’

and this is the snippet I'm trying to run:

struct Waypoints
{
    std::vector<std::tuple<float,float>> gps_coordinate;
    float total_distance;
};
Waypoints create_waypoints(float lat1, float lon1, float lat2, float lon2, float distance, bool display_result=false)
{
    const Geodesic& geod = Geodesic::WGS84();
    auto l = GeographicLib::Geodesic::WGS84().InverseLine(lat1, lon1, lat2, lon2);
    auto parts = int(std::ceil(l.s13 / distance));
    std::vector<std::tuple<float,float>>waypoints;
    auto total_distance = l.s13; 
    for(int i =0; i<parts+1; i++)
    {
        if (display_result)
            if (i == 0)
                printf("latitude longitude azimuth distance ");
        auto s = std::min(distance * i, l.s13);
        auto g = l.Position(s, geod.LONG_UNROLL);
        waypoints.emplace_back((g["lat2"],g["lon2"]));
        if (display_result)
            std::cout<<g["lat2"]<< g["lon2"]<< g["azi2"]<<g["s12"]<<"\n";

    }
}

Whats wrong here and how can I use this method?
Thanks a lot in advance

[Master]

I found another sample here : https://geographiclib.sourceforge.io/html/classGeographicLib_1_1GeodesicLine.html which uses
Geodesic geod(Constants::WGS84_a(), Constants::WGS84_f());
to construct a geod and that fixes the issues (the commented alternative which I used fails)
and instead of l.s13, I guess I should be using Distance() method instead right?
now how should I port position:

# python
g = l.Position(s, Geodesic.STANDARD | Geodesic.LONG_UNROLL)

to C++? it seems position here is completely different!
for the reference this is my python snippet I'm trying to port to C++:

 # python 
 def create_waypoints(point_a, point_b, distance, display_result=False):
    """
        Creats a series of waypoints between two gps coordinate with distance 
        meters apart.

    Args:
        point_a (tuple(float, float)): gps coordinate for point a 
        point_b (tuple(float, float)): gps coordinate for point b 
        distance (float, optional): distance between waypoints in meters.
        display_result (bool, optional): whether to display intermediate results (like gps, azimuth, etc). Defaults to False.

    Returns:
        list: a list of waypoints between the given gps coordinates
    """
    lat1, lon1 = point_a
    lat2, lon2 = point_b
    l = geodesic.Geodesic.WGS84.InverseLine(lat1, lon1,lat2, lon2)
    parts = int(math.ceil(l.s13 / distance))
    waypoints = []
    total_distance = l.s13 
    for i in range(parts + 1):
        if display_result:
            if i == 0:
                print("latitude longitude azimuth distance ")
        s = min(distance * i, l.s13)
        g = l.Position(s, Geodesic.STANDARD | Geodesic.LONG_UNROLL)
        waypoints.append((g['lat2'],g['lon2']))
        if display_result:
            print(f"{g['lat2']:.5f} {g['lon2']:.5f} {g['azi2']:.5f} {g['s12']:.0f} ")
            # print(f"{g['lat2']:.5f}, {g['lon2']:.5f}")

    return waypoints, total_distance

[Master]

OK, I guess I found it, Position, takes out variables, which I believe would initialize them after the method call.

[Charles Karney]

Correct. The methods in C++ library typically only return single values. For most methods, the results are via reference arguments.