https://github.com/PixarAnimationStudios/OpenSubdiv/blob/master/tutorials/far/tutorial_0/far_tutorial_0.cpp

//------------------------------------------------------------------------------
// Tutorial description:
//
// This tutorial presents in a very succint way the requisite steps to
// instantiate and refine a mesh with Far from simple topological data.
//

#include <opensubdiv/far/topologyDescriptor.h>
#include <opensubdiv/far/primvarRefiner.h>

#include <cstdio>

//------------------------------------------------------------------------------
// Vertex container implementation.
//
struct Vertex {

    // Minimal required interface ----------------------
    Vertex() { }

    Vertex(Vertex const & src) {
        _position[0] = src._position[0];
        _position[1] = src._position[1];
        _position[2] = src._position[2];
    }

    void Clear( void * =0 ) {
        _position[0]=_position[1]=_position[2]=0.0f;
    }

    void AddWithWeight(Vertex const & src, float weight) {
        _position[0]+=weight*src._position[0];
        _position[1]+=weight*src._position[1];
        _position[2]+=weight*src._position[2];
    }

    void AddVaryingWithWeight(Vertex const &, float) { }

    // Public interface ------------------------------------
    void SetPosition(float x, float y, float z) {
        _position[0]=x;
        _position[1]=y;
        _position[2]=z;
    }

    const float * GetPosition() const {
        return _position;
    }

private:
    float _position[3];
};

//------------------------------------------------------------------------------
// Cube geometry from catmark_cube.h
static float g_verts[8][3] = {{ -0.5f, -0.5f,  0.5f },
                              {  0.5f, -0.5f,  0.5f },
                              { -0.5f,  0.5f,  0.5f },
                              {  0.5f,  0.5f,  0.5f },
                              { -0.5f,  0.5f, -0.5f },
                              {  0.5f,  0.5f, -0.5f },
                              { -0.5f, -0.5f, -0.5f },
                              {  0.5f, -0.5f, -0.5f }};

static int g_nverts = 8,
           g_nfaces = 6;

static int g_vertsperface[6] = { 4, 4, 4, 4, 4, 4 };

static int g_vertIndices[24] = { 0, 1, 3, 2,
                                 2, 3, 5, 4,
                                 4, 5, 7, 6,
                                 6, 7, 1, 0,
                                 1, 7, 5, 3,
                                 6, 0, 2, 4  };

using namespace OpenSubdiv;

//------------------------------------------------------------------------------
int main(int, char **) {

    // Populate a topology descriptor with our raw data

    typedef Far::TopologyDescriptor Descriptor;

    Sdc::SchemeType type = OpenSubdiv::Sdc::SCHEME_CATMARK;

    Sdc::Options options;
    options.SetVtxBoundaryInterpolation(Sdc::Options::VTX_BOUNDARY_EDGE_ONLY);

    Descriptor desc;
    desc.numVertices  = g_nverts;
    desc.numFaces     = g_nfaces;
    desc.numVertsPerFace = g_vertsperface;
    desc.vertIndicesPerFace  = g_vertIndices;


    // Instantiate a FarTopologyRefiner from the descriptor
    Far::TopologyRefiner * refiner = Far::TopologyRefinerFactory<Descriptor>::Create(desc,
                                            Far::TopologyRefinerFactory<Descriptor>::Options(type, options));

    int maxlevel = 2;

    // Uniformly refine the topolgy up to 'maxlevel'
    refiner->RefineUniform(Far::TopologyRefiner::UniformOptions(maxlevel));


    // Allocate a buffer for vertex primvar data. The buffer length is set to
    // be the sum of all children vertices up to the highest level of refinement.
    std::vector<Vertex> vbuffer(refiner->GetNumVerticesTotal());
    Vertex * verts = &vbuffer[0];


    // Initialize coarse mesh positions
    int nCoarseVerts = g_nverts;
    for (int i=0; i<nCoarseVerts; ++i) {
        verts[i].SetPosition(g_verts[i][0], g_verts[i][1], g_verts[i][2]);
    }


    // Interpolate vertex primvar data
    Far::PrimvarRefiner primvarRefiner(*refiner);

    Vertex * src = verts;
    for (int level = 1; level <= maxlevel; ++level) {
        Vertex * dst = src + refiner->GetLevel(level-1).GetNumVertices();
        primvarRefiner.Interpolate(level, src, dst);
        src = dst;
    }


    { // Output OBJ of the highest level refined -----------

        Far::TopologyLevel const & refLastLevel = refiner->GetLevel(maxlevel);

        int nverts = refLastLevel.GetNumVertices();
        int nfaces = refLastLevel.GetNumFaces();

        // Print vertex positions
        int firstOfLastVerts = refiner->GetNumVerticesTotal() - nverts;

        for (int vert = 0; vert < nverts; ++vert) {
            float const * pos = verts[firstOfLastVerts + vert].GetPosition();
            printf("v %f %f %f\n", pos[0], pos[1], pos[2]);
        }

        // Print faces
        for (int face = 0; face < nfaces; ++face) {

            Far::ConstIndexArray fverts = refLastLevel.GetFaceVertices(face);

            // all refined Catmark faces should be quads
            assert(fverts.size()==4);

            printf("f ");
            for (int vert=0; vert<fverts.size(); ++vert) {
                printf("%d ", fverts[vert]+1); // OBJ uses 1-based arrays...
            }
            printf("\n");
        }
    }
}

//------------------------------------------------------------------------------