我正在尝试在一个boost内存映射文件中创建一个具有向量值的C++映射。

Question

这是我用来测试的代码：

这是我用来测试的代码：

#include <iostream>
#include <boost/interprocess/managed_mapped_file.hpp>
#include <boost/container/map.hpp>
#include <boost/interprocess/managed_external_buffer.hpp>
#include <boost/interprocess/allocators/node_allocator.hpp>
#include <boost/container/vector.hpp>

namespace bi = boost::interprocess;

int main() {
    bi::managed_mapped_file mmfile(bi::open_or_create, "map_iv.dat", 10000000);

    typedef bi::allocator<int, bi::managed_mapped_file::segment_manager> int_allocator;
    typedef boost::container::vector<int, int_allocator>  MyVec;

    typedef std::pair<const std::string, MyVec> MyPair;
    typedef std::less<std::string> MyLess;

    typedef bi::node_allocator<MyPair, bi::managed_mapped_file::segment_manager> node_allocator_t;

    typedef boost::container::map<std::string, MyVec, std::less<std::string>, node_allocator_t> MyMap;

    MyMap * mapptr = mmfile.find_or_construct<MyMap>("mymap")(mmfile.get_segment_manager());

    (*mapptr)["Hello"].push_back(17);

    long long s = mapptr->size();

    std::cout << s << ' ';
    std::cout << (*mapptr)["World"][0] << ' ';

    return 0;
}

我在Visual Studio 2017中遇到了如下错误信息：

boost_1_69_0\boost\container\vector.hpp(294): error C2512:
"boost::interprocess::allocator<int,boost::interprocess::segment_manager<CharType,MemoryAlgorithm,IndexType>>::allocator": no appropriate default constructor available

我正在努力让它运行起来，非常感谢任何帮助！

- Ray Hulha

---

你能否提供完整的错误信息（包括"在实例化时检测到..."等内容），以便我更精确地了解错误发生的位置？ - user11313931

Answer 1

问题在于，如果没有分配器，就无法创建向量（基本上它需要知道应该在哪个段内分配 - 这可能与映射所在的段不同）。解决这个问题的一种方法是创建一个新类型，在构造函数中使用全局对象mmfile：

bi::managed_mapped_file mmfile(bi::open_or_create, "map_iv.dat", 10000000);

    int main() {
        typedef bi::allocator<int, bi::managed_mapped_file::segment_manager> int_allocator;
        struct MyVec : public boost::container::vector<int, int_allocator>{
            MyVec() : boost::container::vector<int, int_allocator>{mmfile.get_segment_manager()}{};
        };
        //..
    }

或者您可以像这里所示，使用emplace代替push_back。

此外，我猜在mapped_file中使用std::string是个错误 - 只是一个注释。以下内容来自于boost示例，该示例已经被修改以在段中分配相同的数据结构时也分配字符串：

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/map.hpp>
#include <boost/interprocess/containers/vector.hpp>
#include <boost/interprocess/containers/string.hpp>



int main ()
{
    using namespace boost::interprocess;

    //Typedefs of allocators and containers
    typedef managed_shared_memory::segment_manager                       segment_manager_t;
    typedef allocator<void, segment_manager_t>                           void_allocator;
    typedef allocator<int, segment_manager_t>                            int_allocator;
    typedef vector<int, int_allocator>                                   int_vector;
    typedef allocator<char, segment_manager_t>                           char_allocator;
    typedef basic_string<char, std::char_traits<char>, char_allocator>   char_string;


    //Definition of the map holding a string as key and complex_data as mapped type
    typedef std::pair<const char_string, int_vector>                      map_value_type;
    typedef std::pair<char_string, int_vector>                            movable_to_map_value_type;
    typedef allocator<map_value_type, segment_manager_t>                    map_value_type_allocator;
    typedef map< char_string, int_vector, std::less<char_string>, map_value_type_allocator> complex_map_type;

   shared_memory_object::remove("MySharedMemory");
   remove_shared_memory_on_destroy remove_on_destroy("MySharedMemory");
   {
      //Create shared memory
      managed_shared_memory segment(create_only,"MySharedMemory", 65536);

      //An allocator convertible to any allocator<T, segment_manager_t> type
      void_allocator alloc_inst (segment.get_segment_manager());

      //Construct the shared memory map and fill it
      complex_map_type *mymap = segment.construct<complex_map_type>
         //(object name), (first ctor parameter, second ctor parameter)
            ("MyMap")(std::less<char_string>(), alloc_inst);

      for(int i = 0; i < 100; ++i){
         //Both key(string) and value(complex_data) need an allocator in their constructors
         char_string  key_object(alloc_inst);
         int_vector mapped_object(alloc_inst);
         mapped_object.push_back(i);
         map_value_type value(key_object, mapped_object);
         //Modify values and insert them in the map
         mymap->insert(value);
      }
   }
   return 0;
}

点击这里尝试一下

- darune

Answer 2

我认为问题在于它无法构造向量的分配器。您可以定义一个分配器类，该类具有使用mmfile.get_segment_manager()的默认ctor（然后您需要通过某些静态数据来设置）。但是一个快速修复方法是，不要使用[]运算符添加新值，而是使用emplace。

auto it = mapptr->emplace("Hello", mmfile.get_segment_manager()).first;
it->second.push_back(17);

抱歉，我还没有测试过它。

- Yong

Answer 3

所以这似乎可以工作：

#include <iostream>
#include <boost/interprocess/managed_mapped_file.hpp>
#include <boost/container/map.hpp>
#include <boost/interprocess/managed_external_buffer.hpp>
#include <boost/interprocess/allocators/node_allocator.hpp>
#include <boost/container/vector.hpp>

namespace bi = boost::interprocess;

int main() {
    bi::managed_mapped_file mmfile(bi::open_or_create, "map_iv.dat", 10000000);

    typedef bi::allocator<int, bi::managed_mapped_file::segment_manager> int_allocator;
    typedef boost::container::vector<int, int_allocator>  MyVec;

    typedef std::pair<const std::string, MyVec> MyPair;
    typedef std::less<std::string> MyLess;

    typedef bi::node_allocator<MyPair, bi::managed_mapped_file::segment_manager> node_allocator_t;

    typedef boost::container::map<std::string, MyVec, std::less<std::string>, node_allocator_t> MyMap;

    MyMap * mapptr = mmfile.find_or_construct<MyMap>("mymap")(mmfile.get_segment_manager());

    MyVec * vecptr = mmfile.find_or_construct<MyVec>("myvector")(mmfile.get_segment_manager());

    mapptr->emplace(std::string("Hello"), *vecptr);
    //(*mapptr)["Hello"] = *vecptr;

    (*vecptr).push_back(17);

    long long s = mapptr->size();

    std::cout << s << ' ';
    std::cout << (*mapptr).at("Hello")[0] << ' ';

    for (auto& i : *mapptr)
        std::cout << i.first << ':' << i.second[0] << ' ';

    return 0;
}

- Ray Hulha