Improve string view to reduce string allocation. (#6644)

include/xgboost/json.h

@@ -1,5 +1,5 @@
 /*!
- * Copyright (c) by XGBoost Contributors 2019-2020
+ * Copyright (c) by XGBoost Contributors 2019-2021
  */
 #ifndef XGBOOST_JSON_H_
 #define XGBOOST_JSON_H_
@@ -301,12 +301,14 @@ class JsonBoolean : public Value {
 struct StringView {
  private:
   using CharT = char;  // unsigned char
+  using Traits = std::char_traits<CharT>;
   CharT const* str_;
   size_t size_;
 
  public:
   StringView() = default;
   StringView(CharT const* str, size_t size) : str_{str}, size_{size} {}
+  explicit StringView(CharT const* str) : str_{str}, size_{Traits::length(str)} {}
 
   CharT const& operator[](size_t p) const { return str_[p]; }
   CharT const& at(size_t p) const {  // NOLINT
@@ -322,9 +324,16 @@ struct StringView {
     CHECK_LE(beg, size_);
     return std::string {str_ + beg, n < (size_ - beg) ? n : (size_ - beg)};
   }
-  char const* c_str() const { return str_; }  // NOLINT
+  CharT const* c_str() const { return str_; }  // NOLINT
+
+  CharT const* cbegin() const { return str_; }         // NOLINT
+  CharT const* cend() const { return str_ + size(); }  // NOLINT
+  CharT const* begin() const { return str_; }          // NOLINT
+  CharT const* end() const { return str_ + size(); }   // NOLINT
 };
 
+std::ostream &operator<<(std::ostream &os, StringView const v);
+
 /*!
  * \brief Data structure representing JSON format.
  *

src/c_api/c_api.cc

@@ -1,4 +1,4 @@
-// Copyright (c) 2014-2020 by Contributors
+// Copyright (c) 2014-2021 by Contributors
 #include <rabit/rabit.h>
 #include <rabit/c_api.h>
 
@@ -50,8 +50,7 @@ XGB_DLL int XGBRegisterLogCallback(void (*callback)(const char*)) {
 
 XGB_DLL int XGBSetGlobalConfig(const char* json_str) {
   API_BEGIN();
-  std::string str{json_str};
+  Json config{Json::Load(StringView{json_str})};
-  Json config{Json::Load(StringView{str.data(), str.size()})};
   for (auto& items : get<Object>(config)) {
     switch (items.second.GetValue().Type()) {
     case xgboost::Value::ValueKind::kInteger: {
@@ -514,8 +513,7 @@ XGB_DLL int XGBoosterBoostedRounds(BoosterHandle handle, int* out) {
 XGB_DLL int XGBoosterLoadJsonConfig(BoosterHandle handle, char const* json_parameters) {
   API_BEGIN();
   CHECK_HANDLE();
-  std::string str {json_parameters};
+  Json config { Json::Load(StringView{json_parameters}) };
-  Json config { Json::Load(StringView{str.c_str(), str.size()}) };
   static_cast<Learner*>(handle)->LoadConfig(config);
   API_END();
 }

src/common/json.cc

@@ -1,5 +1,5 @@
 /*!
- * Copyright (c) by Contributors 2019-2020
+ * Copyright (c) by Contributors 2019-2021
  */
 #include <cctype>
 #include <cstddef>
@@ -744,4 +744,11 @@ void Json::Dump(Json json, std::string* str) {
 }
 
 Json& Json::operator=(Json const &other) = default;
+
+std::ostream &operator<<(std::ostream &os, StringView const v) {
+  for (auto c : v) {
+    os.put(c);
+  }
+  return os;
+}
 }  // namespace xgboost

tests/cpp/common/test_json.cc

@@ -1,5 +1,5 @@
 /*!
- * Copyright (c) by Contributors 2019
+ * Copyright (c) by Contributors 2019-2021
  */
 #include <gtest/gtest.h>
 #include <dmlc/filesystem.h>
@@ -574,4 +574,14 @@ TEST(Json, DISABLED_RoundTripExhaustive) {
     test(static_cast<uint32_t>(i));
   }
 }
+
+TEST(StringView, Basic) {
+  StringView str{"This is a string."};
+  std::stringstream ss;
+  ss << str;
+
+  std::string res = ss.str();
+  ASSERT_EQ(str.size(), res.size());
+  ASSERT_TRUE(std::equal(res.cbegin(), res.cend(), str.cbegin()));
+}
 }  // namespace xgboost