C++ exc_bad_access 从 unordered_map 中指向的对象访问字符串字段

Question

我正在尝试通过以下方式维护快速访问 vector :

我的类.h:

class MyClass{
private:
std::vector<Stuff> myStuffList;
std::tr1::unordered_map<std::string,Stuff*> myStuffListIndex;
...
public:
void addToStuffList(std::string key,Stuff stuff);

};

我的类.cpp:

...
void MyClass::addToStuffList(std::string name, Stuff stuff){
  myStuffList.push_back(stuff);//our man is guaranteed to be at tail 
  myStuffListIndex[name] = &myStuffList[myStuffList.size()-1];//store 
  //pointer to object that we just copy-constructed at tail of list
}

东西.h:

class Stuff{
private:
  std::string name;
public:
  Stuff();
  Stuff(const Stuff&);
  Stuff& operator=(const Stuff&);
  ...

};

东西.cpp:

Stuff::Stuff() : name(""){}
Stuff::Stuff(const Stuff& other){
  if(this != &other){
    this->name = other.name;
  }
}
Stuff& Stuff::operator=(const Stuff& other){
  if(this != &other){
    this->name = other.name;
  }
}
std::string Stuff::getName(){
  return name;//exc_bad_access triggered here
}

稍后，当我尝试通过 map 访问 vector 中的项目时，出现明显间歇性的 exc_bad_access 错误，如下所示:

void methodA(){
  Stuff localStuff;
  myClassInstance.addToStuffList("mostrecentstuff",localStuff);
}
...
void methodB(){
  //different method now, localStuff would be out of scope but 
  //shouldn't matter since we passed by value in addToStuffList, right?
  Stuff* pStuff = myStuffListIndex["mostrecentstuff"];
  std::cout << "Hello, my name is " << pStuff->getName() << std::endl;
}

int main(int argc, const char* argv[]){
  methodA();
  methodB();
}

为什么访问 pStuff->getName() 会抛出 exc_bad_access？

Answer 1

正如 PaulMcKenzie 所说， vector 可以调整大小，如果可以，它可以重新定位到不同的地址。然后所有指向先前 vector 项的指针都被破坏。

你不应该保留指向 std 容器中项目的指针，但对于 vector ，你可以保留它们的索引。

你会:

std::vector<Stuff> myStuffList;
std::tr1::unordered_map<std::string,int> myStuffListIndex;

和

myStuffList.push_back(ability);//our man is guaranteed to be at tail 
myStuffListIndex[name] = myStuffList.size() - 1;//store 

如果您的应用程序是多线程的，则必须使用互斥体来保护上述代码