

Figure 2: Example of the Topological Number calculation

Edges
The Edge object links Nodes together, and its implementation is straightforward.
struct Edge {
  ...
  /// The function this `Edge` points to.
  std::shared_ptr<Node> function;
  /// The identifier of a particular input to the function.
  uint32_t input_nr;
};


It only requires a function pointer to the Node and an input number that is the index of the output from the forward function this edge points to. When preparing the set of gradients before calling "function", we know that what is flowing from this edge should be accumulated in the "input_nr"th argument. Note that the input/output name is flipped here and this is the input to the backward function.