methods to delete an attribute and get names of available attributes

src/c_api/c_api.cc

@@ -685,7 +685,28 @@ int XGBoosterSetAttr(BoosterHandle handle,
                      const char* value) {
   Booster* bst = static_cast<Booster*>(handle);
   API_BEGIN();
-  bst->learner()->SetAttr(key, value);
+  if (value == nullptr) {
+    bst->learner()->DelAttr(key);
+  } else {
+    bst->learner()->SetAttr(key, value);
+  }
+  API_END();
+}
+
+int XGBoosterGetAttrNames(BoosterHandle handle,
+                     bst_ulong* out_len,
+                     const char*** out) {
+  std::vector<std::string>& str_vecs = XGBAPIThreadLocalStore::Get()->ret_vec_str;
+  std::vector<const char*>& charp_vecs = XGBAPIThreadLocalStore::Get()->ret_vec_charp;
+  Booster *bst = static_cast<Booster*>(handle);
+  API_BEGIN();
+  str_vecs = bst->learner()->GetAttrNames();
+  charp_vecs.resize(str_vecs.size());
+  for (size_t i = 0; i < str_vecs.size(); ++i) {
+    charp_vecs[i] = str_vecs[i].c_str();
+  }
+  *out = dmlc::BeginPtr(charp_vecs);
+  *out_len = static_cast<bst_ulong>(charp_vecs.size());
   API_END();
 }
 

src/learner.cc

@@ -330,6 +330,22 @@ class LearnerImpl : public Learner {
     return true;
   }
 
+  bool DelAttr(const std::string& key) override {
+    auto it = attributes_.find(key);
+    if (it == attributes_.end()) return false;
+    attributes_.erase(it);
+    return true;
+  }
+
+  std::vector<std::string> GetAttrNames() const override {
+    std::vector<std::string> out;
+    out.reserve(attributes_.size());
+    for(auto &p: attributes_) {
+      out.push_back(p.first);
+    }
+    return out;
+  }
+
   std::pair<std::string, float> Evaluate(DMatrix* data, std::string metric) {
     if (metric == "auto") metric = obj_->DefaultEvalMetric();
     std::unique_ptr<Metric> ev(Metric::Create(metric.c_str()));