c++ - 在磁盘上使用循环缓冲区

我试图使用Boost在磁盘上创建一个内存映射的循环缓冲区，我读了这个答案:https://stackoverflow.com/a/29265629/8474732

但是，我很难读取已写入的循环缓冲区。我尝试在“实例”变量上执行push_back，现在实例的大小为1。但是我将如何读回内容？还是稍后再push_back其他元素？从相同的分配器和mmf创建另一个实例将显示该实例的大小为0。我想要一个可以打开磁盘上的文件并在循环缓冲区中push_back一个值，然后返回的函数。我想多次调用此函数。我正在尝试做的一个示例(来自链接的答案):

#include <boost/circular_buffer.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/managed_mapped_file.hpp>

namespace bip = boost::interprocess;

struct message {
    int data[32];
};

void writeFunction() {
    bip::managed_mapped_file mmf(bip::open_or_create, "./circ_buffer.bin", 4ul << 10);
    typedef bip::allocator<message, bip::managed_mapped_file::segment_manager> allocator;

    boost::circular_buffer<message, allocator> instance(10, mmf.get_segment_manager());

    struct message test;
    instance.push_back( test );
}

当我想写入磁盘上的循环缓冲区时，我想调用此函数，并且还可以用另一个函数(如下所示)读取它:

void readFunction() {
    bip::managed_mapped_file mmf(bip::open_or_create, "./circ_buffer.bin", 4ul << 10);
    typedef bip::allocator<message, bip::managed_mapped_file::segment_manager> allocator;

    boost::circular_buffer<message, allocator> instance(10, mmf.get_segment_manager());

    for(struct message msg : instance) {
        cout << msg.string;
    }
}

谢谢你的帮助!

最佳答案

链接的帖子是一个最小的示例，仅显示circular_buffer支持Boost Interprocess内存段所需的有状态分配器。

要从该段中检索循环缓冲区本身，您需要在共享内存段中构造对象本身(除了传递shared-mem-allocator外)。

演示

没有关注效率，这只是一个愚蠢的演示:

Live On Coliru

#include <boost/circular_buffer.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/managed_mapped_file.hpp>
#include <iostream>

namespace bip = boost::interprocess;

struct message {
    int data[32];
};

void writeFunction() {
    bip::managed_mapped_file mmf(bip::open_or_create, "./circ_buffer.bin", 4ul << 10);
    typedef bip::allocator<message, bip::managed_mapped_file::segment_manager> allocator;
    typedef boost::circular_buffer<message, allocator> circ_buf;

    auto& instance = *mmf.find_or_construct<circ_buf>("named_buffer")(10, mmf.get_segment_manager());

    struct message test;
    instance.push_back( test );
    std::cout << "pushed a message (" << instance.size() << ")\n";
}

void readFunction() {
    bip::managed_mapped_file mmf(bip::open_or_create, "./circ_buffer.bin", 4ul << 10);
    typedef bip::allocator<message, bip::managed_mapped_file::segment_manager> allocator;
    typedef boost::circular_buffer<message, allocator> circ_buf;

    auto& instance = *mmf.find_or_construct<circ_buf>("named_buffer")(10, mmf.get_segment_manager());

    struct message test;
    while (!instance.empty()) {
        test = instance.front();
        instance.pop_front();
        std::cout << "popped a message (" << instance.size() << ")\n";
    }
}

int main() {
    writeFunction();
    writeFunction();
    writeFunction();

    readFunction();
}

版画

{"a":["1","2","3","4","5","6"]}
4
4
No such node (b)
element_at_checked