介紹如何使用 C++ AWS SDK 中的 S3 TransferManager 開發 object 下載程式，以串流寫入檔案。

S3 TransferManager 串流下載

以下是以 TransferManager 來自動處理 S3 下載流程的範例程式。

#include <aws/s3/S3Client.h>
#include <aws/core/Aws.h>
#include <aws/core/auth/AWSCredentialsProvider.h>
#include <aws/transfer/TransferManager.h>
#include <iostream>
#include <fstream>

static const char* const ALLOCATION_TAG = "S3_DOWNLOAD";

int main() {
  Aws::SDKOptions options;
  Aws::InitAPI(options);
  {
    // 自訂 S3 End Point
    const Aws::String endPoint = "YOUR.END.POINT";

    // S3 Bucket 名稱
    const Aws::String bucketName = "YOUR_BUCKET";

    // Object 名稱
    const Aws::String objectName = "CentOS-7-x86_64-Everything-1810.iso";

    // 檔案名稱
    Aws::String downloadFileName = objectName;

    // 認證資訊
    const Aws::String accessKey = "ASFHDGN345JGS436FG53";
    const Aws::String secretKey = "45G54d4cbGDF56bnFsfdgh489dfGHDdfgDFGHs4e";

    // 建立 S3 Client
    Aws::Client::ClientConfiguration config;
    config.verifySSL = false;
    auto s3Client = Aws::MakeShared<Aws::S3::S3Client>(ALLOCATION_TAG, Aws::Auth::AWSCredentials(accessKey, secretKey), config);

    // 設定 S3 End Point
    s3Client->OverrideEndpoint(endPoint);

    // 建立 TransferManagerConfiguration
    auto sdk_client_executor = Aws::MakeShared<Aws::Utils::Threading::DefaultExecutor>(ALLOCATION_TAG);
    Aws::Transfer::TransferManagerConfiguration transferConfig(sdk_client_executor.get());
    transferConfig.s3Client = s3Client;

    // 下載進度回呼函數
    transferConfig.downloadProgressCallback = [](const Aws::Transfer::TransferManager*,
        const std::shared_ptr<const Aws::Transfer::TransferHandle>& handle) {
          std::cout << "Download Progress: " << handle->GetBytesTransferred()
                    << " of " << handle->GetBytesTotalSize() << " bytes";
        };

    // 建立 TransferManager
    auto transferManager = Aws::Transfer::TransferManager::Create(transferConfig);

    // 下載 Object
    auto transferHandle = transferManager->DownloadFile(bucketName, objectName, [=](){
      return Aws::New<Aws::FStream>(ALLOCATION_TAG, downloadFileName.c_str(), std::ios_base::out | std::ios_base::binary);
    });

    // 等待下載
    transferHandle->WaitUntilFinished();
  }
  Aws::ShutdownAPI(options);
  return 0;
}

此程式碼要配合以下 CMakeLists.txt 設定，以 CMake 來編譯：

cmake_minimum_required(VERSION 3.8)

# 使用 Shared 的 AWS C++ SDK 函式庫（預設狀況）
option(BUILD_SHARED_LIBS "Build shared libraries" ON)

# 設定專案名稱
project(Hello)

# 尋找並引入 AWS SDK for C++ 函式庫套件
find_package(AWSSDK REQUIRED COMPONENTS s3 transfer)

# 增加一個執行檔
add_executable(hello hello.cpp)

# 以 C++11 標準編譯
target_compile_features(hello PUBLIC cxx_std_11)

# 定義執行檔連結方式
target_link_libraries(hello ${AWSSDK_LINK_LIBRARIES})

參考資料