强制转换后c ++读取访问冲突

Question

I have a static method inside my class, which I use for a callback. Inside this callback I want to add a value to result . This seems not to be possible because of the recast(?).

Is there a way to access the member variable result and add values to it after recasting, or do I need to think of a different way?

MyClass.h

class MyClass
{
public:
 vector<string> result;
 static int c_CB(void *data, int argc, char **argv, char **azColName);
 int Callback(int argc, char **argv, char **azColName);
 void Do(string query);
}

MyClass.cpp

void MyClass:Do(string query)
{
  sqlite3_exec(this->dbResource, query.c_str(), this->c_CB , NULL, &this->errorMsg);
}

int MyClass::c_CB(void *NotUsed, int argc, char **argv, char **azColName)
{
  MyClass* bar = reinterpret_cast<MyClass*>(NotUsed);
  // after reinterpret cast, it does not work 
  //bar->result.insert(bar->result.end(), "foo");

  // function call works
  return bar->Callback(argc, argv, azColName);
}


int MyClass::Callback(int argc, char **argv, char **azColName)
{
  cout << "working" << endl;
}

main.cpp

int main()
{
  MyClass* cl = new MyClass();
  cl->Do("something");
}

Answer 1

The documentation states that the fourth argument to sqlite3_exec is passed as the first argument of the callback.

Currently you are passing NULL , which you then cast to a MyClass* and attempt access the object, which results in undefined behaviour.

Use this as the fourth argument in place of NULL .

Answer 2

It seems you are trying to access a non-static member function from a static member callback function c_CB .

You were able to call a member function Callback but nothing from the class is accessible, but if I'm correct, the compiler just interpreted Callback as a static function.

If you actually breakpoint inside this function, you can see that none of your member variables actually have a memory address. And the current class, this is null

I'm not sure if the cout << statement still works or if its causing the problem

This is a possible solution to your issue, although it may not be the best one.

class MyClass
{
public:
  vector<string> result;
  static int c_CB(void *data, int argc, char **argv, char **azColName);
  int Callback(int argc, char **argv, char **azColName);
  void Do(string query);
  void MemberFunction()
  {
    cout << "working" << endl;
  }

  static MyClass* currentClass; //Add a static class pointer and assign your class address to it before firing the callback.
}

MyClass* MyClass::currentClass = NULL;

void MyClass:Do(string query)
{
  sqlite3_exec(this->dbResource, query.c_str(), this->c_CB , NULL, &this->errorMsg);
}

int MyClass::c_CB(void *NotUsed, int argc, char **argv, char **azColName)
{
  MyClass* bar = reinterpret_cast<MyClass*>(NotUsed);
  // after reinterpret cast, it does not work 
  //bar->result.insert(bar->result.end(), "foo");

  // function call works
  return bar->Callback(argc, argv, azColName);
}

int MyClass::Callback(int argc, char **argv, char **azColName)
{
  currentClass->MemberFunction();
}

int main()
{
  MyClass* cl = new MyClass();
  MyClass::currentClass = this;
  cl->Do("something");
}