Add ScopedTags.

opencensus/tags/BUILD

@@ -41,6 +41,18 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "scoped_tags",
+    srcs = ["internal/scoped_tags.cc"],
+    hdrs = ["scoped_tags.h"],
+    copts = DEFAULT_COPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":tags",
+        "//opencensus/context",
+    ],
+)
+
 cc_library(
     name = "with_tag_map",
     srcs = ["internal/with_tag_map.cc"],
@@ -76,6 +88,18 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "scoped_tags_test",
+    srcs = ["internal/scoped_tags_test.cc"],
+    copts = TEST_COPTS,
+    deps = [
+        ":scoped_tags",
+        ":tags",
+        "//opencensus/context",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "with_tag_map_test",
     srcs = ["internal/with_tag_map_test.cc"],

opencensus/tags/internal/scoped_tags.cc

@@ -0,0 +1,40 @@
+// Copyright 2018, OpenCensus Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "opencensus/tags/scoped_tags.h"
+
+#include <initializer_list>
+#include <utility>
+
+#include "absl/strings/string_view.h"
+#include "opencensus/tags/tag_key.h"
+
+// Guard macro from scoped_tags.h.
+#undef ScopedTags
+
+namespace opencensus {
+namespace tags {
+
+ScopedTags::ScopedTags(
+    std::initializer_list<std::pair<TagKey, absl::string_view>> tags)
+    : swapped_tag_map_({}) {
+  // TODO: Implement this.
+}
+
+ScopedTags::~ScopedTags() {
+  // TODO: Implement this.
+}
+
+}  // namespace tags
+}  // namespace opencensus

opencensus/tags/internal/scoped_tags_test.cc

@@ -0,0 +1,70 @@
+// Copyright 2018, OpenCensus Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "opencensus/tags/scoped_tags.h"
+
+#include <iostream>
+
+#include "gtest/gtest.h"
+//#include "opencensus/context/context.h"
+#include "opencensus/tags/tag_key.h"
+
+// Not in namespace ::opencensus::context in order to better reflect what user
+// code should look like.
+
+namespace {
+
+void ExpectNoTags() {
+  // Implement this.
+}
+
+TEST(ScopedTagsTest, NestedScopes) {
+  static const auto k1 = opencensus::tags::TagKey::Register("key1");
+  static const auto k2 = opencensus::tags::TagKey::Register("key2");
+  static const auto k3 = opencensus::tags::TagKey::Register("key3");
+
+  ExpectNoTags();
+  {
+    opencensus::tags::ScopedTags tags1({{k1, "v1"}});
+    // Expect something.
+    {
+      opencensus::tags::ScopedTags tags2({{k2, "v2"}, {k3, "v3"}});
+      // Expect something.
+    }
+  }
+  ExpectNoTags();
+}
+
+TEST(ScopedTagsTest, ReplaceValue) {
+  static const auto k1 = opencensus::tags::TagKey::Register("key1");
+  static const auto k2 = opencensus::tags::TagKey::Register("key2");
+  static const auto k3 = opencensus::tags::TagKey::Register("key3");
+
+  // Order shouldn't matter.
+  opencensus::tags::ScopedTags tags1({{k2, "v2"}, {k1, "v1"}});
+  {
+    opencensus::tags::ScopedTags tags2({{k3, "v3"}, {k2, "xx"}});
+    // Expect something.
+  }
+}
+
+// Disable non-compilation test.
+#if 0
+TEST(ScopedTagsTest, NCForgotName) {
+  static const auto k1 = opencensus::tags::TagKey::Register("key1");
+  opencensus::tags::ScopedTags({{k1, "v1"}});
+}
+#endif
+
+}  // namespace

opencensus/tags/scoped_tags.h

@@ -0,0 +1,58 @@
+// Copyright 2018, OpenCensus Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef OPENCENSUS_TAGS_SCOPED_TAGS_H_
+#define OPENCENSUS_TAGS_SCOPED_TAGS_H_
+
+#include <initializer_list>
+#include <utility>
+
+#include "absl/strings/string_view.h"
+#include "opencensus/tags/tag_key.h"
+#include "opencensus/tags/tag_map.h"
+
+namespace opencensus {
+namespace tags {
+
+// ScopedTags is an RAII object, only ever stack-allocate it. While it's in
+// scope, the current Context will execute with the specified tags added. If a
+// tag key was already in use, its value will be replaced.
+class ScopedTags {
+ public:
+  explicit ScopedTags(
+      std::initializer_list<std::pair<TagKey, absl::string_view>> tags);
+  ~ScopedTags();
+
+ private:
+  ScopedTags() = delete;
+  ScopedTags(const ScopedTags&) = delete;
+  ScopedTags(ScopedTags&&) = delete;
+  ScopedTags& operator=(const ScopedTags&) = delete;
+  ScopedTags& operator=(ScopedTags&&) = delete;
+
+  TagMap swapped_tag_map_;
+};
+
+// Catch a bug where no name is given and the object is immediately discarded.
+#define ScopedTags(x)                                                     \
+  do {                                                                    \
+    static_assert(                                                        \
+        false,                                                            \
+        "ScopedTags needs to be an object on the stack and have a name"); \
+  } while (0)
+
+}  // namespace tags
+}  // namespace opencensus
+
+#endif  // OPENCENSUS_TAGS_SCOPED_TAGS_H_