Download an object to memory (C++ SDK) - Object Storage Service

This topic describes how to download an Object Storage Service (OSS) object to memory.

Usage notes

In this topic, the public endpoint of the China (Hangzhou) region is used. If you want to access OSS from other Alibaba Cloud services in the same region as OSS, use an internal endpoint. For more information about OSS regions and endpoints, see Regions and endpoints.
In this topic, an OSSClient instance is created by using an OSS endpoint. If you want to create an OSSClient by using custom domain names or Security Token Service (STS), see Create an OSSClient instance.

Permissions

By default, an Alibaba Cloud account has full permissions. RAM users or RAM roles under an Alibaba Cloud account do not have any permissions by default. The Alibaba Cloud account or account administrator must grant operation permissions through RAM Policy or Bucket policies.

API	Action	Definition
GetObject	`oss:GetObject`	Downloads an object.
	`oss:GetObjectVersion`	When downloading an object, if you specify the object version through versionId, this permission is required.
	`kms:Decrypt`	When downloading an object, if the object metadata contains X-Oss-Server-Side-Encryption: KMS, this permission is required.

Sample code

The following code provides an example of how to download a specified OSS object to memory:

#include <alibabacloud/oss/OssClient.h>
using namespace AlibabaCloud::OSS;

int main(void)
{
    /* Initialize the OSS account information. */
            
    /* Set yourEndpoint to the Endpoint of the region where the bucket is located. For example, if the bucket is in the China (Hangzhou) region, set the Endpoint to https://oss-cn-hangzhou.aliyuncs.com. */
    std::string Endpoint = "yourEndpoint";
    /* Set yourRegion to the region where the bucket is located. For example, if the bucket is in the China (Hangzhou) region, set the region to cn-hangzhou. */
    std::string Region = "yourRegion";
    /* Specify the bucket name. Example: examplebucket. */
    std::string BucketName = "examplebucket";
    /* Specify the full path of the object. The path cannot contain the bucket name. Example: desrfolder/exampleobject.txt. */
    std::string ObjectName = "yourObjectName";

    /* Initialize network resources. */
    InitializeSdk();

    ClientConfiguration conf;
    conf.signatureVersion = SignatureVersionType::V4;
    /* Obtain access credentials from environment variables. Before you run the sample code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are configured. */
    auto credentialsProvider = std::make_shared<EnvironmentVariableCredentialsProvider>();
    OssClient client(Endpoint, credentialsProvider, conf);
    client.SetRegion(Region);

    /* Download the object to memory. */
    GetObjectRequest request(BucketName, ObjectName);
    auto outcome = client.GetObject(request);
    if (outcome.isSuccess()) {
      std::cout << "getObjectToBuffer" << " success, Content-Length:" << outcome.result().Metadata().ContentLength() << std::endl;
        /* Read data using the read interface. */
        auto& stream = outcome.result().Content();
        char buffer[256];
        while (stream->good()) {
            stream->read(buffer, 256);
            auto count = stream->gcount();
            /* Process the data as needed. */
        }
    }
    else {
        /* Handle the exception. */
        std::cout << "getObjectToBuffer fail" <<
        ",code:" << outcome.error().Code() <<
        ",message:" << outcome.error().Message() <<
        ",requestId:" << outcome.error().RequestId() << std::endl;
        return -1;
    }

    /* Release network resources. */
    ShutdownSdk();
    return 0;
}