class 构造中的 C++ 指针

Question

几天来我一直在努力解决这个问题。

在这个程序中有一个unordered_map从字符串键到结构值。这个想法可行，但是当我把 unordered_map 放在 class 中然后更容易地访问它时，我得到一个错误：

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++

这是主要的：

#include "MapCreator.h"

int main() {
    MapCreator mappaClass;

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

虽然这是在 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";
};

MapCreator构造函数中的“cout”正确打印“12”，但使用getValue函数，打印出上述错误。

感谢您的帮助，如果您对代码有任何疑问，请提出。

信息：

编译器：Visual C++ Visual Studio 2022
没有 class 的代码运行良好
boost的版本class是：1.78.0

Answer 1

存在多个问题。

第一个是 segment 是构造函数中的局部变量，这意味着 mappa 是构造函数 returns 之后的悬空指针。

其次，更大的问题是您的地图正确使用了共享内存分配器，但字符串却没有。字符串也是动态分配的，因此除非您使字符串也使用共享内存，否则您的解决方案将无法工作。

开始修复：

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.

在执行中有额外的复杂性：segment 必须是一个字段，但它只能从初始化列表构造（或使用 NSMI）。这意味着我们必须让 remove 也出现在初始化列表中（在那之前）。这是我的初步尝试：

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.

我发现 std::hash 实际上并不专门用于 MyString 实例，因为我知道我想改变它，无论如何我不会浪费时间创建一个哈希函数对象。相反，我切换到 boost::unordered_map 只是为了使用 boost::hash<>

这已经在工作了Live On Coliru

打印：

12, 1111

改进

作用域分配器

allocator/get_segment_manager() 周围的东西相当笨拙。使用 scoped-allocators 你可以缓解：

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>;

现在你可以“简单地”写：

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

代替之前复杂的版本。 Live On Coliru

异构键查找

我们正在构造MyString的另一个地方（只是为了在at()调用之后扔掉它......），我们可以使用boost::unordered_map的compatible-key查找:

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");
}

这要求散列和相等性兼容：

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

看到了Live On Coliru

改为使用 C++20 标准 unordered_map（参见 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>;

然后：

    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");
    }

也看看Live On Coliru

class 构造中的 C++ 指针

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