如何判断托管型对象值为空？
C++ Net 不识别NULL，该怎么办呢？
我写了一个 C++net访问数据的单例模式：

//头文件：
ref class DBAccess
{
private:
SqlConnection ^ conn;
SqlCommand ^ comm;

static DBAccess ^ access;
DBAccess(void);
public:

bool initialDB();
static DBAccess^ getInstance();
};
//源文件：
DBAccess::DBAccess(void) //私有构造函数
{
conn = gcnew SqlConnection();
comm = gcnew SqlCommand();

if (!initialDB())
{
MessageBox::Show("数据库初始化失败！");
}
}

bool DBAccess::initialDB()
{
conn->ConnectionString = "data source=.;database=FundSystem; uid=sa;pwd=sa";
comm->Connection = conn;

try
{
conn->Open();
}

catch (System::Exception ^e)
{
return false;
}

return true;
}


DBAccess^ DBAccess::getInstance()
{
if (access == null) {
access = gcnew DBAccess();
}

return access;

}

报错：DBAccess.cpp(38) : error C2065: “null”: 未声明的标识符

[解决办法]
==nullptr