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Showing content from http://cloud.google.com/cpp/docs/reference/storage/2.23.0/storage-mocking below:

C++ Client Libraries | Google Cloud

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Writing Tests with a Mock Client Mocking GCS bucket read and write

It is possible to mock GCS bucket read and write using gmock framework, this is specially useful for developer using GCS C++ client library.

Mocking Successful call of ReadObject()

Mocking of ReadObject can be achieved by mocking ObjectReadSource. The following code snippet explains this

TEST(StorageMockingSamples, MockReadObject) {
  namespace gcs = ::google::cloud::storage;

  std::shared_ptr<gcs::testing::MockClient> mock =
      std::make_shared<gcs::testing::MockClient>();
  auto client = gcs::testing::UndecoratedClientFromMock(mock);

  std::string const text = "this is a mock http response";
  std::size_t offset = 0;
  // Simulate a Read() call in the MockObjectReadSource object created below
  auto simulate_read = [&text, &offset](void* buf, std::size_t n) {
    auto const l = (std::min)(n, text.size() - offset);
    std::memcpy(buf, text.data() + offset, l);
    offset += l;
    return gcs::internal::ReadSourceResult{
        l, gcs::internal::HttpResponse{200, {}, {}}};
  };
  EXPECT_CALL(*mock, ReadObject)
      .WillOnce([&](gcs::internal::ReadObjectRangeRequest const& request) {
        EXPECT_EQ(request.bucket_name(), "mock-bucket-name") << request;
        std::unique_ptr<gcs::testing::MockObjectReadSource> mock_source(
            new gcs::testing::MockObjectReadSource);
        ::testing::InSequence seq;
        EXPECT_CALL(*mock_source, IsOpen()).WillRepeatedly(Return(true));
        EXPECT_CALL(*mock_source, Read).WillOnce(simulate_read);
        EXPECT_CALL(*mock_source, IsOpen()).WillRepeatedly(Return(false));

        return google::cloud::make_status_or(
            std::unique_ptr<gcs::internal::ObjectReadSource>(
                std::move(mock_source)));
      });

  gcs::ObjectReadStream stream =
      client.ReadObject("mock-bucket-name", "mock-object-name");

  // Reading the payload of http response stored in stream
  std::string actual{std::istreambuf_iterator<char>(stream), {}};
  EXPECT_EQ(actual, text);
  stream.Close();
}

Mocking Failed call of ReadObject()

Mocking of ReadObject can be achieved by mocking ObjectReadSource. The following code snippet explains this

TEST(StorageMockingSamples, MockReadObjectFailure) {
  namespace gcs = ::google::cloud::storage;

  std::shared_ptr<gcs::testing::MockClient> mock =
      std::make_shared<gcs::testing::MockClient>();
  auto client = gcs::testing::UndecoratedClientFromMock(mock);

  std::string text = "this is a mock http response";
  EXPECT_CALL(*mock, ReadObject)
      .WillOnce([](gcs::internal::ReadObjectRangeRequest const& request) {
        EXPECT_EQ(request.bucket_name(), "mock-bucket-name") << request;
        auto* mock_source = new gcs::testing::MockObjectReadSource;
        ::testing::InSequence seq;
        EXPECT_CALL(*mock_source, IsOpen).WillRepeatedly(Return(true));
        EXPECT_CALL(*mock_source, Read)
            .WillOnce(Return(google::cloud::Status(
                google::cloud::StatusCode::kInvalidArgument,
                "Invalid Argument")));
        EXPECT_CALL(*mock_source, IsOpen).WillRepeatedly(Return(false));

        std::unique_ptr<gcs::internal::ObjectReadSource> result(mock_source);

        return google::cloud::make_status_or(std::move(result));
      });

  gcs::ObjectReadStream stream =
      client.ReadObject("mock-bucket-name", "mock-object-name");
  EXPECT_TRUE(stream.bad());
  stream.Close();
}

Mocking Successful call of WriteObject()

Mocking of ReadObject can be achieved by mocking ObjectWriteStreambuf. The following code snippet explains this

TEST(StorageMockingSamples, MockWriteObject) {
  namespace gcs = ::google::cloud::storage;

  std::shared_ptr<gcs::testing::MockClient> mock =
      std::make_shared<gcs::testing::MockClient>();
  auto client = gcs::testing::UndecoratedClientFromMock(mock);

  gcs::ObjectMetadata expected_metadata;

  using gcs::internal::CreateResumableUploadResponse;
  using gcs::internal::QueryResumableUploadResponse;
  EXPECT_CALL(*mock, CreateResumableUpload)
      .WillOnce(Return(CreateResumableUploadResponse{"test-only-upload-id"}));
  EXPECT_CALL(*mock, UploadChunk)
      .WillOnce(Return(QueryResumableUploadResponse{
          /*.committed_size=*/absl::nullopt,
          /*.object_metadata=*/expected_metadata}));

  auto stream = client.WriteObject("mock-bucket-name", "mock-object-name");
  stream << "Hello World!";
  stream.Close();

Mocking Failed call of WriteObject()

Mocking of ReadObject can be achieved by mocking ObjectWriteStreambuf. The following code snippet explains this

TEST(StorageMockingSamples, MockWriteObjectFailure) {
  namespace gcs = ::google::cloud::storage;

  std::shared_ptr<gcs::testing::MockClient> mock =
      std::make_shared<gcs::testing::MockClient>();
  auto client = gcs::testing::UndecoratedClientFromMock(mock);

  using gcs::internal::CreateResumableUploadResponse;
  using gcs::internal::QueryResumableUploadResponse;
  EXPECT_CALL(*mock, CreateResumableUpload)
      .WillOnce(Return(CreateResumableUploadResponse{"test-only-upload-id"}));
  EXPECT_CALL(*mock, UploadChunk)
      .WillOnce(Return(google::cloud::Status{
          google::cloud::StatusCode::kInvalidArgument, "Invalid Argument"}));

  auto stream = client.WriteObject("mock-bucket-name", "mock-object-name");
  stream << "Hello World!";
  stream.Close();

  EXPECT_TRUE(stream.bad());
}

The documentation for each member function in the Client class includes short snippets on how to call the function. The snippets for ReadObject(), WriteObject(), ListObjects(), and DeleteObject() are good places to continue learning about the library.

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2025-08-14 UTC.

[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-14 UTC."],[[["This page provides examples of how to mock Google Cloud Storage (GCS) bucket read and write operations using the gmock framework, which is helpful for developers utilizing the GCS C++ client library."],["Mocking successful and failed calls of `ReadObject()` is demonstrated by mocking the `ObjectReadSource`, including code examples that show how to simulate data reads and handle different return values."],["The page shows how to mock both successful and failed calls of `WriteObject()` by mocking the `ObjectWriteStreambuf`, with examples that include simulating the uploading of data and managing potential errors."],["The information provided here can be further expanded upon by looking at the documentation for member functions in the `Client` class such as `ReadObject()`, `WriteObject()`, `ListObjects()`, and `DeleteObject()`."]]],[]]

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