Download a file by using the C++ SDK V2 GetObject method-Object Storage Service(OSS)-阿里云帮助中心

Use the OSS C++ SDK V2 GetObject method to download files from OSS to memory, local disk, or a custom buffer.

Usage notes

Before you run the sample code, replace <region> with your region, such as cn-hangzhou.
The sample code obtains access credentials from environment variables.
To download a file, you must have the oss:GetObject permission.

Sample code

Download a file to memory

The following code downloads file content to memory:

#include <iostream>
#include <sstream>
#include "alibabacloud/oss2/ClientConfiguration.h"
#include "alibabacloud/oss2/OSSClient.h"
#include "alibabacloud/oss2/credentials/CredentialsProvider.h"

namespace oss = alibabacloud::oss2;

int main() {
    auto conf = oss::ClientConfiguration::loadDefault();
    conf.region = "<region>";
    conf.credentialsProvider = std::make_shared<oss::EnvironmentVariableCredentialsProvider>();

    oss::OSSClient client(conf);

    auto outcome = client.getObject(
        oss::models::GetObjectRequest()
            .setBucket("examplebucket")
            .setKey("exampledir/exampleobject.txt"));

    if (!outcome.has_value()) {
        auto& e = outcome.error();
        std::cerr << "Failed to download the file"
                  << ", code: " << e.getCode()
                  << ", message: " << e.getMessage()
                  << ", requestId: " << e.getRequestId() << std::endl;
        return 1;
    }

    auto& result = outcome.value();
    std::cout << "status: " << result.getStatusCode()
              << ", requestId: " << result.getRequestId()
              << ", contentLength: " << result.getContentLength()
              << ", contentType: " << result.getContentType() << std::endl;

    // Read the file content.
    auto& body = result.getBody();
    if (body) {
        std::stringstream ss;
        ss << body->rdbuf();
        std::cout << "File content: " << ss.str() << std::endl;
    }
    return 0;
}

Download to a local file

The following code downloads a file to local disk:

#include <iostream>
#include "alibabacloud/oss2/ClientConfiguration.h"
#include "alibabacloud/oss2/OSSClient.h"
#include "alibabacloud/oss2/credentials/CredentialsProvider.h"

namespace oss = alibabacloud::oss2;

int main() {
    auto conf = oss::ClientConfiguration::loadDefault();
    conf.region = "<region>";
    conf.credentialsProvider = std::make_shared<oss::EnvironmentVariableCredentialsProvider>();

    oss::OSSClient client(conf);

    std::string filePath = "/path/to/localfile.txt";

    auto outcome = client.getObjectToFile(
        oss::models::GetObjectRequest()
            .setBucket("examplebucket")
            .setKey("exampledir/exampleobject.txt"),
        filePath);

    if (!outcome.has_value()) {
        auto& e = outcome.error();
        std::cerr << "Failed to download the file"
                  << ", code: " << e.getCode()
                  << ", message: " << e.getMessage()
                  << ", requestId: " << e.getRequestId() << std::endl;
        return 1;
    }

    auto& result = outcome.value();
    std::cout << "File downloaded to the local path"
              << ", status: " << result.getStatusCode()
              << ", contentLength: " << result.getContentLength() << std::endl;
    return 0;
}

Download to a custom buffer (SinkFactory)

To download a file directly into pre-allocated memory, use SinkFactory with MemoryWriter. This approach eliminates one memory copy compared to reading the std::iostream response body. The following example calls HeadObject to obtain the object size, allocates a buffer, and writes to it directly:

#include <iostream>
#include <vector>
#include "alibabacloud/oss2/ClientConfiguration.h"
#include "alibabacloud/oss2/OSSClient.h"
#include "alibabacloud/oss2/credentials/CredentialsProvider.h"
#include "alibabacloud/oss2/io/ByteWriter.h"

namespace oss = alibabacloud::oss2;

int main() {
    auto conf = oss::ClientConfiguration::loadDefault();
    conf.region = "<region>";
    conf.credentialsProvider = std::make_shared<oss::EnvironmentVariableCredentialsProvider>();

    oss::OSSClient client(conf);

    // 1. Call the HeadObject method to get the object size.
    auto headOutcome = client.headObject(
        oss::models::HeadObjectRequest()
            .setBucket("examplebucket")
            .setKey("exampledir/exampleobject.txt"));
    if (!headOutcome.has_value()) {
        auto& e = headOutcome.error();
        std::cerr << "HeadObject failed"
                  << ", code: " << e.getCode()
                  << ", message: " << e.getMessage()
                  << ", requestId: " << e.getRequestId() << std::endl;
        return 1;
    }
    std::size_t contentLength = static_cast<std::size_t>(headOutcome.value().getContentLength());
    std::cout << "Object size: " << contentLength << " bytes" << std::endl;

    // 2. Pre-allocate the buffer.
    std::vector<std::uint8_t> buffer(contentLength);

    // 3. Provide a MemoryWriter through SinkFactory.
    std::shared_ptr<oss::MemoryWriter> memWriter;
    oss::SinkFactory factory;
    factory.isOneShot = false;
    factory.supplier = [&](std::int64_t, const oss::HeaderCollection&) -> std::shared_ptr<oss::ByteWriter> {
        memWriter = std::make_shared<oss::MemoryWriter>(buffer.data(), buffer.size());
        return memWriter;
    };

    auto outcome = client.getObject(
        oss::models::GetObjectRequest()
            .setBucket("examplebucket")
            .setKey("exampledir/exampleobject.txt")
            .setSinkFactory(factory));

    if (!outcome.has_value()) {
        auto& e = outcome.error();
        std::cerr << "Failed to download the file"
                  << ", code: " << e.getCode()
                  << ", message: " << e.getMessage()
                  << ", requestId: " << e.getRequestId() << std::endl;
        return 1;
    }

    auto& result = outcome.value();
    std::cout << "File downloaded successfully"
              << ", status: " << result.getStatusCode()
              << ", requestId: " << result.getRequestId()
              << ", bytes: " << memWriter->written() << std::endl;
    return 0;
}

References

For the complete sample code for downloading a file to memory, see GetObject.cpp.
For the complete sample code for downloading a file to local disk, see GetObjectToFile.cpp.
For the complete sample code for downloading a file to a custom buffer, see DownloadToMemory.cpp.