C++ 如何从char指针强制转换为自定义对象指针

我使用leveldb存储integer和MyClass对象的键值对。实际上，一个键可以包含多个对象。 从数据库检索数据时出现问题。它进行编译，但是MyClass成员的值不是我输入数据库的值

std::string value;
leveldb::Slice keySlice = ANYKEY;
levelDBObj->Get(leveldb::ReadOptions(), keySlice, &value);

std:：string

value1

现在只能包含一个或多个MyClass对象。那我怎么得到它们呢

我已经尝试了以下方法，但没有效果

1.）直接打字和记忆

std::vector<MyClass> vObjects;
MyClass* obj = (MyClass*)malloc( value.size());
memcpy((void*)obj, (void*) (value.c_str()), value.size());
MyClass dummyObj;
int numValues = value.size()/sizeof(MyClass);
for( int i=0; i<numValues; ++i) {
    dummyObj = *(obj+i);
    vObjects.push_back(dummyObj);
}

get

方法如下（

put

类似）：

int getValues（leveldb:：Slice keySlice，std:：vector&values）常量{
std：：字符串值；
leveldb:：Status Status=levelDBObj->Get（leveldb:：ReadOptions（），keySlice，&value）；
如果（！status.ok（））{
value.clear（）；
返回-1；
}
int nValues=value1.size（）/sizeof（CHit）；
MyObj dummyObj；
对于（int i=0；i您必须序列化您的类…否则，您只需占用一些内存并将其写入leveldb。您得到的结果不仅会有所不同，而且可能也完全无用。有关序列化的更多信息，请查看此问题：

LevelDB支持一个键下的多个对象，但是，除非你有很好的理由，否则尽量避免这样做。我建议你用一个唯一的散列来散列每个对象（看看你是否想要散列函数）并存储序列化对象及其相应的哈希。如果对象本身是一个集合，则必须将所有对象序列化为一个字节数组，并使用某种方法来确定每个对象的开始/结束位置

更新
可序列化类的外观如下所示：

class MyClass
{
private:
    int _numeric;
    string _text;
public:
    // constructors 

    // mutators
    void SetNumeric(int num);
    void SetText(string text);

    static unsigned int SerializableSize()
    {
        // returns the serializable size of the class with the schema:
        // 4 bytes for the numeric (integer) 
        // 4 bytes for the unsigned int (the size of the text)
        // n bytes for the text (it has a variable size)
        return sizeof(int) + sizeof(unsigned int) + _text.size();
    }

    // serialization
    int Serialize(const char* buffer, const unsigned int bufferLen, const unsigned int position)
    {
        // check if the object can be serialized in the available buffer space
        if(position+SerializableSize()>bufferLen)
        {
            // don't write anything and return -1 signaling that there was an error
            return -1;
        }

        unsigned int finalPosition = position;

        // write the numeric value
        *(int*)(buffer + finalPosition) = _numeric;

        // move the final position past the numeric value
        finalPosition += sizeof(int);

        // write the size of the text
        *(unsigned int*)(buffer + finalPosition) = (unsigned int)_text.size();

        // move the final position past the size of the string
        finalPosition += sizeof(unsigned int);

        // write the string
        memcpy((void*)(buffer+finalPosition), _text.c_str(), (unsigned int)_text.size());

        // move the final position past the end of the string
        finalPosition += (unsigned int)_text.size();

        // return the number of bytes written to the buffer
        return finalPosition-position;
    }

    // deserialization
    static int Deserialize(MyClass& myObject, 
        const char* buffer, 
        const unsigned int buffSize, 
        const unsigned int position)
    {
        insigned int currPosition = position;

        // copy the numeric value
        int numeric = *(int*)(buffer + currentPosition);

        // increment the current position past the numeric value
        currentPosition += sizeof(int);

        // copy the size of the text
        unsigned int textSize = *(unsigned int*)(buffer + currentPosition);

        // increment the current position past the size of the text
        currentPosition += sizeof(unsigned int);

        // copy the text
        string text((buffer+currentPosition), textSize);

        if(currentPosition > buffSize)
        {
            // you decide what to do here
        }

        // Set your object's values
        myObject.SetNumeric(numeric);
        myObject.SetText(text);

        // return the number of bytes deserialized
        return currentPosition - position;
    }
};

您还可以向我们展示一段如何编写leveldb的代码片段吗？我怀疑当您将对象放入值片时，您没有序列化它。对，我没有序列化它。我没有时间尝试它（我首先要看看如何在我的情况下这样做）。我将在明天返回代码时给出反馈；）我想避免使用boost，因为你手头没有为此而裁剪的代码？而且，我有/确实有很好的理由/使用多个对象；）好吧，如果boost不是一个选项，那么你可以。根据你的速度要求，你也可以使用JSON：（在你改变模式时，JSON更健壮，更宽容）。谢谢你的帮助。我发布了我的解决方案。@ezdazuzena刚刚添加了一个代码示例，说明了如何在没有任何第三方库的情况下使类可序列化。（编辑）刚刚看到了你的解决方案……好吧，现在有两个示例：）
void MyClass::serialize( char* outBuff ) {
    memcpy(outBuff, (const void*) &aVar, sizeof(aVar));
    unsigned int c = sizeof(aVar);
    memcpy(outBuff+c, (const void*) &bVar, sizeof(bVar));
    c += sizeof(bVAr);
    /* and so on */
}

void MyClass::deserialize( const char* inBuff ) {
    memcpy((void*) &aVar, inBuff, sizeof(aVar));
    unsigned int c = sizeof(aVar);
    memcpy((void*) &aVar, inBuff+c, sizeof(aVar));
    c += sizeof(aVar);
    /* and so on */
}

int getValues(leveldb::Slice keySlice, std::vector<MyObj>& values) const {
    std::string value;  
    leveldb::Status status = levelDBObj->Get(leveldb::ReadOptions(), keySlice, &value);
    if (!status.ok()) {
        values.clear();
            return -1;
    }

    int nValues = value1.size()/sizeof(CHit);
    MyObj dummyObj;
    for( int i=0; i<nValues; ++i) {
        dummyObj.deserialize(value.c_str()+i*sizeof(MyObj));
        values.push_back(dummyObj);
    }

    return 0;
}               

class MyClass
{
private:
    int _numeric;
    string _text;
public:
    // constructors 

    // mutators
    void SetNumeric(int num);
    void SetText(string text);

    static unsigned int SerializableSize()
    {
        // returns the serializable size of the class with the schema:
        // 4 bytes for the numeric (integer) 
        // 4 bytes for the unsigned int (the size of the text)
        // n bytes for the text (it has a variable size)
        return sizeof(int) + sizeof(unsigned int) + _text.size();
    }

    // serialization
    int Serialize(const char* buffer, const unsigned int bufferLen, const unsigned int position)
    {
        // check if the object can be serialized in the available buffer space
        if(position+SerializableSize()>bufferLen)
        {
            // don't write anything and return -1 signaling that there was an error
            return -1;
        }

        unsigned int finalPosition = position;

        // write the numeric value
        *(int*)(buffer + finalPosition) = _numeric;

        // move the final position past the numeric value
        finalPosition += sizeof(int);

        // write the size of the text
        *(unsigned int*)(buffer + finalPosition) = (unsigned int)_text.size();

        // move the final position past the size of the string
        finalPosition += sizeof(unsigned int);

        // write the string
        memcpy((void*)(buffer+finalPosition), _text.c_str(), (unsigned int)_text.size());

        // move the final position past the end of the string
        finalPosition += (unsigned int)_text.size();

        // return the number of bytes written to the buffer
        return finalPosition-position;
    }

    // deserialization
    static int Deserialize(MyClass& myObject, 
        const char* buffer, 
        const unsigned int buffSize, 
        const unsigned int position)
    {
        insigned int currPosition = position;

        // copy the numeric value
        int numeric = *(int*)(buffer + currentPosition);

        // increment the current position past the numeric value
        currentPosition += sizeof(int);

        // copy the size of the text
        unsigned int textSize = *(unsigned int*)(buffer + currentPosition);

        // increment the current position past the size of the text
        currentPosition += sizeof(unsigned int);

        // copy the text
        string text((buffer+currentPosition), textSize);

        if(currentPosition > buffSize)
        {
            // you decide what to do here
        }

        // Set your object's values
        myObject.SetNumeric(numeric);
        myObject.SetText(text);

        // return the number of bytes deserialized
        return currentPosition - position;
    }
};