Windows平台std::ostringstream 2GB容量上限问题及实现合规性咨询

最近我做了个测试，分别往std::string和std::ostringstream对象里每次写入0.5GB的数据，每一步都查询并打印它们的实际大小，结果发现了一个奇怪的跨平台差异：

Windows（64位编译）测试结果 #

• Step 1 | std::string size: 0.5 GB | ostringstream size: 0.5 GB
• Step 2 | std::string size: 1 GB | ostringstream size: 1 GB
• Step 3 | std::string size: 1.5 GB | ostringstream size: 1.5 GB
• Step 4 | std::string size: 2 GB | ostringstream size: 2 GB
• Step 5 | std::string size: 2.5 GB | ostringstream size: 2 GB
• Step 6 | std::string size: 3 GB | ostringstream size: 2 GB

Finished writing 3 GB to files.

其他操作系统（比如macOS + GCC/Clang）测试结果 #

• Step 1 | std::string size: 0.5 GB | ostringstream size: 0.5 GB
• Step 2 | std::string size: 1 GB | ostringstream size: 1 GB
• Step 3 | std::string size: 1.5 GB | ostringstream size: 1.5 GB
• Step 4 | std::string size: 2 GB | ostringstream size: 2 GB
• Step 5 | std::string size: 2.5 GB | ostringstream size: 2.5 GB
• Step 6 | std::string size: 3 GB | ostringstream size: 3 GB

Finished writing 3 GB to files.

我现在有点疑惑：微软对std::ostringstream的实现是不是不符合标准？根据我查到的所有资料，std::ostringstream应该是std::string的包装类，它的最大容量应该和std::string的最大长度一致才对。

测试源代码 #

stringwriter.cpp

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <iomanip>

constexpr std::size_t HALF_GB = 512 * 1024 * 1024; // 0.5 GB in bytes
constexpr std::size_t THREE_GB = 3ULL * 1024 * 1024 * 1024; // 3 GB in bytes

int main() {
    std::string str;
    std::ostringstream oss;

    // Block for 0.5GB, filled with 'A'
    std::string block(HALF_GB, 'A');

    std::size_t current_size = 0;
    int step = 0;

    while (current_size < THREE_GB) {
        ++step;

        // Append to std::string
        str.append(block);

        // Append to std::ostringstream
        oss.write(block.data(), block.size());

        // Query actual sizes
        std::size_t str_size = str.size();
        std::size_t oss_size = oss.str().size();
        current_size = str_size; // Since both should grow equally

        std::cout << "Step " << step << " | std::string size: " << str_size / (1024.0 * 1024 * 1024) << " GB"
                  << " | ostringstream size: " << oss_size / (1024.0 * 1024 * 1024) << " GB" << std::endl;
    }

    // Save to disk (just write once for demonstration; in a real scenario these files would be very large)
    std::ofstream fout_str("string_output.bin", std::ios::binary);
    fout_str.write(str.data(), str.size());
    fout_str.close();

    std::ofstream fout_oss("oss_output.bin", std::ios::binary);
    std::string oss_content = oss.str();
    fout_oss.write(oss_content.data(), oss_content.size());
    fout_oss.close();

    std::cout << "Finished writing " << (THREE_GB / (1024.0 * 1024 * 1024)) << " GB to files." << std::endl;

    return 0;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(stringwriter CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

add_executable(stringwriter stringwriter.cpp)

内容来源于stack exchange