Question

我在使用Boost共享指针时遇到了一些麻烦。指针指向的数据会在不确定的时间突然删除。在当前的程序中（我已经尝试了很多其他的变体）boost::shared_array<TableFieldsMap> retArray的数据在案例MYSQL_TYPE_DOUBLE之后的交换机构造中突然被删除，在循环完成了172行之后（有表中的1000行）。但只有当方法被调用两次时。第一次运行完美。

具体而言，我将问题指向了在retArray的赋值中使用的map-key。它在我使用硬编码字符串时有效。但不是本地shared_array（fieldNameArray）的值（我首先使用scoped_array，但我理解这不应该工作），而不是局部变量（tmpIndex）。

retArray[rowIndex]["value"] = boost::lexical_cast<long double>(row[i]); // This works.
// This doesn't work: retArray[rowIndex][fieldNameArray[i]] = boost::lexical_cast<long double>(row[i]);
// This doesn't work either: retArray[rowIndex][tmpIndex.c_str()] = boost::lexical_cast<long double (row[i]);

这是标题类：

#ifndef DATABASE_H
#define DATABASE_H

#include <map>
#include <string>
#include <iostream>
#include <vector>
using namespace std;

#include <glog/logging.h>
#include <boost/lexical_cast.hpp>
#include <boost/smart_ptr.hpp>
#include <boost/variant.hpp>

#include <mysql.h>

#include <cppconn/driver.h>
#include <cppconn/exception.h>
#include <cppconn/resultset.h>
#include <cppconn/statement.h>
#include <cppconn/prepared_statement.h>

#include "../globals/globals.hpp"
#include "../exception/cassandra_exception.hpp"

struct mapCompare
{
    bool operator()(char const * a, char const * b)
    {
        return (strcmp(a, b) < 0);
    }
};

typedef map<const char*, boost::variant<short, int, long double, char*>, mapCompare > TableFieldsMap;

struct DataSet
{
    unsigned int rowCount;
    boost::shared_array<TableFieldsMap>data;
};

class Database
{
private:
    MYSQL *connection;

public:
    Database();
    ~Database();

    bool connect(const char *, const char *, const char *, const char *);
    DataSet select(const char*);
};

#endif

和代码：

#include "database.hpp"
Database::Database()
{
    DLOG(INFO) << "::Database";
}

Database::~Database()
{
    DLOG(INFO) << "~Database";

    if (connection) mysql_close(connection);
}

bool Database::connect(const char* server, const char* user, const char* password, const char* database)
{
    DLOG(INFO) << ">> connect";

    connection = mysql_init(NULL);

    if (!connection)
    {
        cout << "Error" << endl;
        return (false);
    }

    if (!mysql_real_connect(connection, server, user, password, database, 0, NULL, 0))
    {
        cout << "Database init error (" << mysql_error(connection) << ")" << endl;
    }

    if (!connection)
    {
        cout << "Connection Failed!" << endl;
        return (false);
    }

    return (true);
}

DataSet Database::select(const char* query)
{
    DLOG(INFO) << ">> select";

    MYSQL_RES *resultSet;
    MYSQL_ROW row;
    unsigned int fieldCount;
    MYSQL_FIELD* field;

    // Query database.
    if (mysql_query(connection, query)) throw MyException(string("Database select error (") + mysql_error(connection) + ")");

    // Handle result-set.
    resultSet = mysql_store_result(connection);
    if (resultSet == NULL) throw MyException(string("Database select error. Result-set is empty (") + mysql_error(connection) + ")");

    // Initialize return array.
    unsigned int rowCount = mysql_num_rows(resultSet);
    boost::shared_array<TableFieldsMap> retArray(new TableFieldsMap[rowCount]);

    // Obtain field names and types.
    fieldCount = mysql_num_fields(resultSet);
    boost::shared_array<const char *> fieldNameArray(new const char* [fieldCount]);
    boost::scoped_array<enum_field_types> fieldTypeArray(new enum_field_types[fieldCount]);
    unsigned int fieldIndex = 0;
    while ((field = mysql_fetch_field(resultSet)))
    {
        fieldNameArray[fieldIndex] = field -> name;
        fieldTypeArray[fieldIndex++] = field -> type;
    }

    // Fix?
    string tmpIndex;

    // Load data from result-set.
    unsigned int rowIndex = 0;
    while ((row = mysql_fetch_row(resultSet)) != NULL)
    {
        // Fields of the row.
        for (unsigned int i = 0; i < fieldCount; i++)
        {
            if (row[i] == NULL) throw MyException("Database error. Null.");

            switch (fieldTypeArray[i])
            {
                // Medium int & Integer.
                case MYSQL_TYPE_INT24 :
                case MYSQL_TYPE_LONG :
                retArray[rowIndex][fieldNameArray[i]] = boost::lexical_cast<int>(row[i]);
                break;

                case MYSQL_TYPE_DOUBLE :
                try
                {
                retArray[rowIndex]["value"] = boost::lexical_cast<long double>(row[i]); // This works.
                }
        catch (boost::bad_lexical_cast exception)
                {
                    DLOG(ERROR) << "Error reading the phi table from database. Cast error. (" << rowIndex << ", " << row[i] << ")" << endl;
                    throw MyException("Error reading the phi table from database. Cast error.");
        }

                // tmpIndex = string(fieldNameArray[i]);
                // This doesn't work: retArray[rowIndex][fieldNameArray[i]] = boost::lexical_cast<long double>(row[i]);
                // This doesn't work either: retArray[rowIndex][tmpIndex.c_str()] = boost::lexical_cast<long double>(row[i]);
                break;

            case MYSQL_TYPE_VARCHAR :               // 15
            case MYSQL_TYPE_VAR_STRING :            // 253
            case MYSQL_TYPE_STRING :                // 254
            retArray[rowIndex][fieldNameArray[i]] = row[i];
            break;

                case MYSQL_TYPE_DECIMAL :               // 0
                case MYSQL_TYPE_TINY :                  // 1
                case MYSQL_TYPE_SHORT :                 // 2
                case MYSQL_TYPE_FLOAT :                 // 4
                case MYSQL_TYPE_NULL :                  // 6
                case MYSQL_TYPE_TIMESTAMP :             // 7
                case MYSQL_TYPE_LONGLONG :              // 8
                case MYSQL_TYPE_DATE :                  // 10
                case MYSQL_TYPE_TIME :                  // 11
                case MYSQL_TYPE_DATETIME :              // 12
                case MYSQL_TYPE_YEAR :                  // 13
                case MYSQL_TYPE_NEWDATE :               // 14
                case MYSQL_TYPE_BIT :                   // 16
                case MYSQL_TYPE_NEWDECIMAL :            // 246
                case MYSQL_TYPE_ENUM :                  // 247
                case MYSQL_TYPE_SET :                   // 248
                case MYSQL_TYPE_TINY_BLOB :             // 249
                case MYSQL_TYPE_MEDIUM_BLOB :           // 250
                case MYSQL_TYPE_LONG_BLOB :             // 251
                case MYSQL_TYPE_BLOB :                  // 252
                default:
                DLOG(FATAL) << "Database error. Unsupported datatype";
                throw MyException("Database error. Unsupported datatype");
                break;
            }
        rowIndex++;
        }
    }

    mysql_free_result(resultSet);

    DataSet retStruct;
    retStruct.rowCount = rowCount;
    retStruct.data = retArray;

    return (retStruct);
}

...

这是调用方法：

    include "My_database.hpp"

    MyDatabase::MyDatabase(string server, string user, string password, string database)
    {
        DLOG(INFO) << "::MyDatabase (1)";

        init(server.c_str(), user.c_str(), password.c_str(), database.c_str());
    }

    MyDatabase::MyDatabase(const char* server, const char* user, const char* password, const char* database)
    {
        DLOG(INFO) << "::MyDatabase (2)";

        init(server, user, password, database);
    }

    MyDatabase::~MyDatabase()
    {
        DLOG(INFO) << "~MyDatabase";
    }

    void MyDatabase::init(const char* server, const char* user, const char* password, const char* database)
    {
        DLOG(INFO) << ">> init";

        connect(server, user, password, database);
    }

    boost::shared_array<long double>MyDatabase::loadPhiTable()
    {
        DLOG(INFO) << ">> LoadPhiTable";

        // Select from database.
        DataSet selectData = select("SELECT value FROM phi");
        boost::shared_array<TableFieldsMap>phiArray = selectData.data;
        unsigned int rowCount = selectData.rowCount;

        // Initialize return array.
        boost::shared_array<long double> retArray(new long double[rowCount]);

        unsigned int i = 0;
        try
        {
            for (i = 0; i < rowCount; i++) retArray[i] = boost::get<long double>(phiArray[i]["value"]);
        }
        catch (boost::bad_get exception)
        {
            throw MyException(string("Error reading the phi table. Conversion error. (index: ") + boost::lexical_cast<string>(i) + ", value: " + boost::lexical_cast<string>(phiArray[i]["value"]) + ")");
        }

        return (retArray);
    }

任何其他建议也将不胜感激。我的第一个c ++程序。

Answer 1

问题在于

boost::shared_array<const char *> fieldNameArray = ...;

不拥有数组的元素，因此当间接数据消失时，它们会被留作悬空指针。如果mysql_fetch_field返回的指针指向池化数据，则可能在MySQL API中发生。

最简单的解决方法是将fieldNameArray更改为包含std::string。

数据突然从Boost shared_array / ptr中删除

1 个答案: