C++ pointers in class costruction

Question

I'm trying to solve this problem from several days.

In this program there is an unordered_map which from string key to structure value. The idea works, but when I put the unordered_map in a class and then access it more easily I get an error:

Eliminare.exe!std::_Hash<std::_Umap_traits<std::string,Data,std::_Uhash_compare<std::string,std::hash<std::string>,std::equal_to<std::string>>,boost::interprocess::allocator<std::pair<std::string,Data>,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index>>,0>>::_Find_last<std::string>(const std::string & _Keyval, const unsigned __int64 _Hashval) Row 1510    C++
    Eliminare.exe!std::unordered_map<std::string,Data,std::hash<std::string>,std::equal_to<std::string>,boost::interprocess::allocator<std::pair<std::string,Data>,boost::interprocess::segment_manager<char,boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family,boost::interprocess::offset_ptr<void,__int64,unsigned __int64,0>,0>,boost::interprocess::iset_index>>>::at(const std::string & _Keyval) Riga 387    C++
Eliminare.exe!MapCreator::getValue(std::string Key) Row 37  C++
Eliminare.exe!main() Row 7  C++

This is the main:

#include "MapCreator.h"

int main() {
    MapCreator mappaClass;

    auto values = mappaClass.getValue("value");
}

While this is in MapCreator.h:

#pragma once
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <unordered_map>
#include<iostream>
typedef struct Data
{
    int a;
    int b;
}dataClass;

namespace bost = boost::interprocess;
typedef std::string    KeyType;
typedef dataClass MappedType;
typedef std::pair<std::string, dataClass> ValueType;
typedef bost::allocator<ValueType, bost::managed_shared_memory::segment_manager> ShmemAllocator;
typedef std::unordered_map<KeyType, MappedType, std::hash<KeyType>, std::equal_to<KeyType>, ShmemAllocator> MySHMMap;

class MapCreator
{
public:
    MySHMMap* mappa = nullptr;
    MapCreator() {
        bost::shared_memory_object::remove(nameMemory);
        bost::managed_shared_memory segment(bost::create_only, nameMemory, sizeDeclared);
        ShmemAllocator alloc_inst(segment.get_segment_manager());

        mappa = segment.construct<MySHMMap>("MySHMMapName")(alloc_inst);
        dataClass value;
        value.a = 12;
        value.b = 1111;
        mappa->insert(std::pair<std::string, dataClass>("value", value));
        std::cout << mappa->at("value").a;

    }
    dataClass getValue(std::string Key) {
        return mappa->at(Key);
    }
private:
    const int sizeDeclared = 65536;
    const char nameMemory[20] = "SharedMemoryName";
};

The "cout" in the MapCreator constructor correctly prints "12", but using the getValue function, it prints the error mentioned above.

Thanks for your help, if you have any questions about the code, just ask.

INFO:

• Compiler: Visual C++ with Visual Studio 2022
• the code without the class works well
• the version of boost class is: 1.78.0

Answer 1

There are multiple issues.

The first one is that segment is a local variable in your constructor, meaning that mappa is a dangling pointer after the constructor returns.

Second, bigger, problem is that your map uses a shared memory allocator correctly, but the strings do NOT. String are also dynamically allocating, so your solution cannot work unless you make the string also use shared memory.

Starting the fix:

namespace bip = boost::interprocess;
namespace bc  = boost::container;
using Segment = bip::managed_shared_memory;
using Mgr     = Segment::segment_manager;

template <typename T> using Alloc = bip::allocator<T, Mgr>;
using MyString   = bc::basic_string<char, std::char_traits<char>, Alloc<char>>;
using KeyType    = MyString;
using MappedType = dataClass;
using ValueType  = std::pair<KeyType const, MappedType>;
using MySHMMap   = std::unordered_map<KeyType, MappedType, std::hash<MyString>,
                                    std::equal_to<MyString>, Alloc<ValueType>>;

Note that on my compiler it's also required to use KeyType const in the ValueType.

In the execution there's additional complexity: segment must be a field, but it can only be constructed from the initializer list (or using NSMI). That means that we have to make the remove also occur (before that) in the initializer list. Here's my initial attempt:

template <typename T> using Alloc = bip::allocator<T, Mgr>;
using MyString   = bc::basic_string<char, std::char_traits<char>, Alloc<char>>;
using KeyType    = MyString;
using MappedType = dataClass;
using ValueType  = std::pair<KeyType const, MappedType>;
using MySHMMap = boost::unordered_map<KeyType, MappedType, boost::hash<MyString>,
                         std::equal_to<MyString>, Alloc<ValueType>>;

class MapCreator {
  public:
    MapCreator()
        : mappa(segment.find_or_construct<MySHMMap>("MySHMMapName")(
              segment.get_segment_manager()))
    {
        mappa->emplace(MyString("value", segment.get_segment_manager()),
                       dataClass{12, 1111});
    }

    dataClass getValue(std::string key) const {
        return mappa->at(MyString(key.c_str(), segment.get_segment_manager()));
    }

  private:
    static constexpr int sizeDeclared = 65536;

    // note: declaration order defines initialization order!
    std::string_view nameMemory = "SharedMemoryName";

    struct pre_remover {
        pre_remover(const char* name)
        {
            bip::shared_memory_object::remove(name);
        }
    } pre_remove{nameMemory.data()};

    Segment segment{bip::create_only, nameMemory.data(), sizeDeclared};
    MySHMMap* mappa = nullptr;
};

Notes

• there was also no reason at all to allocate a fixed size buffer of 20 chars for the name.
• The order in which members are declared is the order in which they're initalized. That's important because mappa needs to come after segment, e.g.

I found out that std::hash isn't actually specialized for the MyString instance, and because I know down the line I'd want to change that anyways I'm not wasting time to create a Hash function object. Instead, I'm switching to boost::unordered_map just so boost::hash<> is used

This is already working Live On Coliru

Prints:

12, 1111

Improvements

1. Scoped Allocators

   Things are pretty clunky around the allocator/get_segment_manager(). Using scoped-allocators you can alleviate that:

   template <typename T>
   using Alloc      = bc::scoped_allocator_adaptor<bip::allocator<T, Mgr>>;
   using MyString   = bc::basic_string<char, std::char_traits<char>, Alloc<char>>;
   using KeyType    = MyString;
   
   template <typename K, typename V, typename H = boost::hash<K>,
             typename Eq = std::equal_to<K>>
   using SHMap = boost::unordered_map<K, V, H, Eq, Alloc<std::pair<K const, V>>>;
   
   using MySHMMap = SHMap<MyString, Data>;
   

   Now you can "simply" write:

   mappa->emplace("value", Data{12, 1111});
   

   Instead of the complicated version before. Live On Coliru

2. Heterogenous Key Lookup

   The other place where we are constructing MyString (just to throw it away after the at() call...), we can use boost::unordered_map's compatible-key lookup:

   Data getValue(std::string_view key) const {
       auto it = mappa->find(key, mappa->hash_function(), mappa->key_eq());
       return it != mappa->end() ? it->second
                                 : throw std::range_error("getValue");
   }
   

   This requires the hash and equality to be compatible:

   using MySHMMap = SHMap<MyString, Data, std::hash<std::string_view>,
                          std::equal_to<std::string_view>>;
   

   See it Live On Coliru

3. Using C++20 standard unordered_map instead (see https://en.cppreference.com/w/cpp/container/unordered_map/find):

   struct Hash : std::hash<std::string_view> {
       using is_transparent = std::true_type;
   };
   struct Eq : std::equal_to<std::string_view> {
       using is_transparent = std::true_type;
   };
   using MySHMMap = SHMap<MyString, Data, Hash, Eq>;
   

   And then:

       Data getValue(std::string_view key) const {
   #ifdef __cpp_lib_generic_unordered_lookup
           auto it = mappa->find(key);
   #else
           auto it = mappa->find(
               MyString(key.data(), key.size(), segment.get_segment_manager()));
   
   #endif
           return it != mappa->end() ? it->second
                                     : throw std::range_error("getValue");
       }
   

   See it Live On Coliru as well