When code depends on possibly-missing inputs/outputs or optional attributes, do not index or read them unconditionally. Add explicit guards for presence and fall back to a safe default/overload. Apply this pattern: - For vector-based tensors: verify `bottom_blobs.size()` / `top_blobs.size()` before using fixed indices (e.g., hidden/cell), and if hidden/cell aren’t provided, delegate to the simpler overload. - For operator attributes: verify the attribute exists before reading it (e.g., `n.has_attr("eps")`); only then use the value, otherwise use the intended default. Example (vector forward fallback): ```cpp int forward(const std::vector& bottom_blobs, std::vector& top_blobs, const Option& opt) const { if (bottom_blobs.size() < 1 || top_blobs.size() < 1) return -100; // hidden/cell optional: only use them when present if (top_blobs.size() >= 3) { // access indices safely Mat& hidden_state = top_blobs[1]; Mat& cell_state = top_blobs[2]; return forward(bottom_blobs[0], top_blobs[0], hidden_state, cell_state, opt); } // safe fallback Mat& top_blob = top_blobs[0]; return forward(bottom_blobs[0], top_blob, opt); } ``` Example (attribute existence guard): ```cpp float eps = 1e-3f; // intended default if (n.has_attr("eps")) eps = n.attr("eps"); ``` Rule of thumb: if the code would otherwise dereference an index or consume an attribute that may not be present, you must add a guard and a fallback path before the dereference.