在我的应用程序（使用MySQL Connector/C++在Windows8上开发）中，我正在创建准备好的语句，并仅在应用程序结束时删除它们。但在应用程序运行时，我执行查询并只删除结果集。
然而，我观察到很多内存仍然被分配，我觉得这比预期的要多。我用Visual Leak Detector进行了检查，令我惊讶的是，尽管我适当地删除了它们，但我发现泄漏显示在结果集指针中。
所以我编写了这个演示程序。这就是创建准备好的语句，创建查询，获取结果，删除结果（但不要在最后删除准备好的声明，这样我们就可以看到泄漏），然后退出。这是演示代码MySQL.cpp:
#include "stdafx.h"
#include <conio.h>
#define CPPCONN_LIB_BUILD // We must define this as we are linking mysql connector in static library. It directs build_config.h to not to put __declspec(dllimport) before function declarations.
#include <driver/mysql_connection.h>
#include <cppconn/driver.h>
#include <cppconn/exception.h>
#include <cppconn/resultset.h>
#include <cppconn/statement.h>
#include <cppconn/prepared_statement.h>
#include <vld.h> // Visual memory leak detector
int _tmain(int argc, _TCHAR* argv[])
{
sql::Connection *pConnection = NULL;
sql::ResultSet *pResultSet = NULL;
sql::PreparedStatement *pPreparedStatement = NULL;
sql::Driver *driver = NULL;
/* Create a connection */
driver = get_driver_instance();
pConnection = driver->connect("tcp://127.0.0.1:3306", "username", "password");
pConnection->setSchema("MYDB");
pConnection->setAutoCommit(0);
sql::ResultSet* pResultSet;
pPreparedStatement = pConnection->prepareStatement ("select * from mytable where mycolumn > ?"); // mytable has mycolumn that contains 1000 numbers starting from 1
pPreparedStatement->setInt(1, 1);
pResultSet= pPreparedStatement->executeQuery();
int count = pResultSet->rowsCount();
printf("nTotal rows found %d", count);
delete pResultSet;
// delete pPreparedStatement; // Let's not delete prepared statement to see demo of memory leak in pResultSet
delete pConnection;
printf ("nDone! Quitting...");
return 0;
}
下面是报告：
Visual Leak Detector Version 2.4RC2 installed.
Aggregating duplicate leaks.
Suppressing data dumps.
Outputting the report to E:MySQLmemory_leak_report.txt
WARNING: Visual Leak Detector detected memory leaks!
---------- Block 65 at 0x0000000068D87EB0: 8 bytes ----------
Leak Hash: 0x38615834, Count: 1, Total 8 bytes
Call Stack (TID 4628):
0x00000000C3EC5630 (File and line number not available): ntdll.dll!RtlAllocateHeap
f:ddvctoolscrt_bldself_64_amd64crtsrcnew.cpp (59): MySQLTrials.exe!operator new + 0xA bytes
0x00000000DF30AEE4 (File and line number not available): MySQLTrials.exe!sql::mysql::util::Singleton<sql::mysql::NativeAPI::LibmysqlStaticProxy>::theInstance + 0x44 bytes
0x00000000DF306DB1 (File and line number not available): MySQLTrials.exe!sql::mysql::NativeAPI::getCApiHandle + 0x41 bytes
0x00000000DF2AA5AC (File and line number not available): MySQLTrials.exe!sql::mysql::NativeAPI::MySQL_NativeDriverWrapper::MySQL_NativeDriverWrapper + 0x5C bytes
0x00000000DF2AA51D (File and line number not available): MySQLTrials.exe!sql::mysql::NativeAPI::createNativeDriverWrapper + 0x4D bytes
0x00000000DF28401B (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_Driver::MySQL_Driver + 0x8B bytes
0x00000000DF28456F (File and line number not available): MySQLTrials.exe!sql::mysql::get_driver_instance_by_name + 0x18F bytes
0x00000000DF284681 (File and line number not available): MySQLTrials.exe!sql::mysql::get_driver_instance + 0x21 bytes
0x00000000DF283E1A (File and line number not available): MySQLTrials.exe!get_driver_instance + 0x1A bytes
e:mysqlmysql.cpp (22): MySQLTrials.exe!wmain + 0x5 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (240): MySQLTrials.exe!__tmainCRTStartup + 0x19 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (164): MySQLTrials.exe!wmainCRTStartup
0x00000000C1CF167E (File and line number not available): KERNEL32.DLL!BaseThreadInitThunk + 0x1A bytes
0x00000000C3EDC3F1 (File and line number not available): ntdll.dll!RtlUserThreadStart + 0x21 bytes
---------- Block 413 at 0x0000000068D90FF0: 40 bytes ----------
Leak Hash: 0x7614B12C, Count: 1, Total 40 bytes
Call Stack (TID 4628):
0x00000000C3EC5630 (File and line number not available): ntdll.dll!RtlAllocateHeap
f:ddvctoolscrt_bldself_64_amd64crtsrcnew.cpp (59): MySQLTrials.exe!operator new + 0xA bytes
0x00000000DF30C576 (File and line number not available): MySQLTrials.exe!sql::mysql::NativeAPI::MySQL_NativeConnectionWrapper::stmt_init + 0x86 bytes
0x00000000DF28E730 (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_Connection::prepareStatement + 0xC0 bytes
e:mysqlmysql.cpp (30): MySQLTrials.exe!wmain + 0x30 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (240): MySQLTrials.exe!__tmainCRTStartup + 0x19 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (164): MySQLTrials.exe!wmainCRTStartup
0x00000000C1CF167E (File and line number not available): KERNEL32.DLL!BaseThreadInitThunk + 0x1A bytes
0x00000000C3EDC3F1 (File and line number not available): ntdll.dll!RtlUserThreadStart + 0x21 bytes
---------- Block 241 at 0x0000000068D93910: 16 bytes ----------
Leak Hash: 0x447A29BE, Count: 1, Total 16 bytes
Call Stack (TID 4628):
0x00000000C3EC5630 (File and line number not available): ntdll.dll!RtlAllocateHeap
c:program files (x86)microsoft visual studio 11.0vcincludexmemory0 (592): MySQLTrials.exe!std::allocator<std::_Container_proxy>::allocate
0x00000000DF28B052 (File and line number not available): MySQLTrials.exe!std::_Wrap_alloc<std::allocator<std::_Container_proxy> >::allocate + 0x32 bytes
0x00000000DF303CA7 (File and line number not available): MySQLTrials.exe!std::_Deque_alloc<0,std::_Deque_base_types<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::allocator<sql::mysql::MySQL_DebugEnterEvent const * __ptr64> > >::_Alloc_proxy + 0x37 bytes
0x00000000DF303991 (File and line number not available): MySQLTrials.exe!std::_Deque_alloc<0,std::_Deque_base_types<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::allocator<sql::mysql::MySQL_DebugEnterEvent const * __ptr64> > >::_Deque_alloc<0,std::_Deque_base_types<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std + 0x41 bytes
0x00000000DF303A95 (File and line number not available): MySQLTrials.exe!std::deque<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::allocator<sql::mysql::MySQL_DebugEnterEvent const * __ptr64> >::deque<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::allocator<sql::mysql::MySQL_DebugEnterEvent const * __ptr64> > + 0x35 bytes
0x00000000DF303ACB (File and line number not available): MySQLTrials.exe!std::stack<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::deque<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::allocator<sql::mysql::MySQL_DebugEnterEvent const * __ptr64> > >::stack<sql::mysql::MySQL_DebugEnterEvent const * __ptr64,std::d + 0x2B bytes
0x00000000DF302AFE (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_DebugLogger::MySQL_DebugLogger + 0x3E bytes
0x00000000DF28CD77 (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_Connection::MySQL_Connection + 0x227 bytes
0x00000000DF284184 (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_Driver::connect + 0xA4 bytes
e:mysqlmysql.cpp (23): MySQLTrials.exe!wmain + 0x5B bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (240): MySQLTrials.exe!__tmainCRTStartup + 0x19 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (164): MySQLTrials.exe!wmainCRTStartup
0x00000000C1CF167E (File and line number not available): KERNEL32.DLL!BaseThreadInitThunk + 0x1A bytes
0x00000000C3EDC3F1 (File and line number not available): ntdll.dll!RtlUserThreadStart + 0x21 bytes
---------- Block 483 at 0x0000000068D93960: 11 bytes ----------
Leak Hash: 0x1D599652, Count: 1, Total 11 bytes
Call Stack (TID 4628):
0x00000000C3EC5630 (File and line number not available): ntdll.dll!RtlAllocateHeap
f:ddvctoolscrt_bldself_64_amd64crtsrcnewaop.cpp (7): MySQLTrials.exe!operator new[]
0x00000000DF32199C (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_ResultBind::bindResult + 0xA0C bytes
0x00000000DF321379 (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_ResultBind::bindResult + 0x3E9 bytes
0x00000000DF313F69 (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_Prepared_ResultSet::MySQL_Prepared_ResultSet + 0x169 bytes
0x00000000DF2EC0E1 (File and line number not available): MySQLTrials.exe!sql::mysql::MySQL_Prepared_Statement::executeQuery + 0x1F1 bytes
e:mysqlmysql.cpp (33): MySQLTrials.exe!wmain + 0x13 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (240): MySQLTrials.exe!__tmainCRTStartup + 0x19 bytes
f:ddvctoolscrt_bldself_64_amd64crtsrccrt0.c (164): MySQLTrials.exe!wmainCRTStartup
0x00000000C1CF167E (File and line number not available): KERNEL32.DLL!BaseThreadInitThunk + 0x1A bytes
0x00000000C3EDC3F1 (File and line number not available): ntdll.dll!RtlUserThreadStart + 0x21 bytes
Visual Leak Detector detected 119 memory leaks (640915 bytes).
Largest number used: 697643 bytes.
Total allocations: 837447 bytes.
Visual Leak Detector is now exiting.
问题：
为什么我们在MySQL.cpp（23）：行看到泄漏
pConnection = driver->connect("tcp://127.0.0.1:3306", "username", "password");
和MySQL.cpp（33）
pResultSet= m_pPreparedStatement->executeQuery();
尽管我们删除了CCD_ 2和CCD_？为什么我们也需要删除pPreparedStatement来释放结果集？
我可以从您的语句中推断出，您已经释放了与以下语句的连接：
delete pConnection
这意味着当你再次初始化PreparedStatement时，你必须像一样初始化它
pPreparedStatement = pConnection->prepareStatement(...)
因此，您不能释放PreparedStatement对象，而只能重用该对象。默认情况下，C++没有垃圾回收，当您不再需要对象时，必须释放它们。
为了接近Java的垃圾收集，我建议您使用shared_ptr或scoped_ptr，它们将在不再需要对象时释放内存。
您可以查看Boost C++库，了解如何动态释放内存的全面教程。例如：
boost::scoped_ptr<sql::Connection> con(driver->connect(host, user,pass));
boost::scoped_ptr<sql::Statement> stmt(con->createStatement());
这样，您就不需要记住对对象调用delete，因为当对象不再在作用域中时，内存将被释放。但是你必须包括boost库。
#include <boost/scoped_ptr.hpp>
在我看来，很明显，prepared语句包含用于检索结果集的查询字符串，也就是说，我很清楚，prepared语句位于与结果集不同的内存位置，您也可以从代码中推断出：
sql::ResultSet *pResultSet = NULL;
sql::PreparedStatement *pPreparedStatement = NULL;
因此，当连接超出范围或被释放时，为准备好的语句分配的内存仍然与包含该查询的对象绑定，但由于连接对象被释放，您就无法再访问内存的那部分，而且因为它不像您那样被释放，所以您的操作系统仍然将内存的那一部分标记为正在使用，从而导致存储器泄漏。
为什么要经历这么多麻烦，找出为什么你有空闲的内存来防止内存泄漏，这对我来说似乎很明显，而不是只添加一行代码：
delete pPreparedStatement.
正如随附文档中所描述的，或者我喜欢做的，将它封装在一个类中（例如dbConnection），并让析构函数完成所有工作，这样你只需要在一个位置编写语句，然后就可以忘记它。这是一种简单而干净的实践，它教会了编写干净无泄漏的代码。
通过额外的库添加垃圾收集只会让你变得草率，并导致软件出现缺陷。在我看来，这通常不值得麻烦，而且可能会带来大量开销，而你真的必须为每个对象添加一行代码才能删除。此外，我认为当一些垃圾被删除，而仍然需要它所包含的数据，但垃圾收集来得很早，或者事实上你很快就让你的对象超出了范围，你必须找出为什么你的应用程序在特定情况下不断崩溃时，这种懒惰最终会咬你的屁股。当你自己完成所有的工作时，一开始可能会看起来更麻烦，但最终你会很高兴你可以在几秒钟内纠正这种编码错误，而不是花很长时间寻找错误，结果却发现你的代码中有一个小错误。
选择您正在使用的MySQL版本搜索"Prepared SQL语句语法"。
你会发现3个具体的活动和语法示例A） PREPARE语法B） EXECUTE语法C） DEALLOCATE语法
这可能比删除ResultSet对象更适合您当DEALLOCATE完成时，您可能会看到您的资源已经释放。