这实际上与这个问题有关：
Flushing a boost::iostreams::zlib_compressor. How to obtain a "sync flush"?
您需要调用boost::iostreams::zlib_compressor::close才能进行刷新。
您可以通过在boost::iostream::filtering_ostream上调用pop()或reset()来实现这一点。
请注意，pop()的名称建议弹出链中的最后一个过滤器，然后reset()完全清除链，这样filtering_ostream以后就无法使用了。
示例：

#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/filter/zlib.hpp>

#include <sstream>

struct ZlibOstream : boost::iostreams::filtering_ostream
{
    ZlibOstream(std::ostream& os)
    {
        boost::iostreams::filtering_ostream::push(boost::iostreams::zlib_compressor{});
        boost::iostreams::filtering_ostream::push(os);
    }
};

int main()
{   
    std::ostringstream oss;

    ZlibOstream zlibOstream{oss};

    zlibOstream << "This is a test string.\n";

    zlibOstream.reset(); // needed if you want to write to oss

    return oss.tellp();
}