How to extract triangles out of mesh geometry

[m3g1dd]

I'm creating meshes with Qt3D. I check if the mesh is actually built with triangles:

qDebug() << "mesh primitive type: " << mesh->primitiveType();

which returns:

Qt3DRender::QGeometryRenderer::PrimitiveType(Triangles)

Therefore, my mesh primitive type is definitely triangle.

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Now I'm trying to access to triangles of the mesh, but I don't know how. There is the following API to get the data and its type which can be either VertexBuffer or IndexBuffer:

mesh->geometry()->attributes().at(i)->buffer()->data();
mesh->geometry()->attributes().at(i)->buffer()->type();

But I don't know how to extract triangles one-by-one using the above APIs. Can anybody help me.

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Eventually, I'm going to export the triangles as STL file with such a format. Is there any API to export mesh as STL? I'm using Qt 5.9.4