Question

我有一个指向结构的指针向量，这些结构存储在我学校项目的另一个向量中。当我尝试使用指针更改结构中的元素时，由于某种原因导致未定义的行为。我删除了与下面问题相关的部分代码。

#include <vector>
#include <string>
#include <iostream>

class someException{};

enum class ProcessStatus{
  RUNNING,
  READY
};

struct Process{
  int priority;
  std::string PID;
  ProcessStatus status;
  Process(){
    status = ProcessStatus::READY;
  }
};

struct ReadyList{
  std::vector<Process*> priority1;
  std::vector<Process*> priority0;
};

class ProcessManager{
private:
  std::vector<Process> processList;
  ReadyList readyList;
public:
  ProcessManager(){};

  void createProcess(std::string PID, int priority){
    Process process;
    process.priority = priority;
    process.PID = PID;
    if (priority == 0)
      process.status = ProcessStatus::RUNNING;
    processList.push_back(process);
    switch(priority){
      case 0:
        readyList.priority0.push_back(&processList.at(processList.size()-1));
        break;
      case 1:
        readyList.priority1.push_back(&processList.at(processList.size()-1));
        break;
      default:
        throw someException();
    }
    schedule(findRunningProcess());
  }

  void printProcesses(){
    std::cout<<"ReadyList results:"<<std::endl;
    for(auto &process: readyList.priority0){
      std::cout << "Process: "<< process->PID << " , Priority: "<<process->priority;
      if (process->status == ProcessStatus::RUNNING)
        std::cout << ", Status: RUNNING"<< std::endl;
      else
        std::cout <<", Status: READY"<<std::endl;
    }
    for(auto &process: readyList.priority1){
      std::cout << "Process: "<< process->PID << " , Priority: "<<process->priority;
      if (process->status == ProcessStatus::RUNNING)
        std::cout << ", Status: RUNNING"<< std::endl;
      else
        std::cout <<", Status: READY"<<std::endl;
    }
    std::cout<<"ProcessList results: "<<std::endl;
    for(auto &process: processList){
      std::cout << "Process: "<< process.PID << " , Priority: "<<process.priority;
      if (process.status == ProcessStatus::RUNNING)
        std::cout << ", Status: RUNNING"<< std::endl;
      else
        std::cout <<", Status: READY"<<std::endl;
    }
  }

private:
  void schedule(Process* currentProcess){
    Process* highestPriorityProcess;
    if (readyList.priority1.size()>0)
      highestPriorityProcess = readyList.priority1[0];
    else
      highestPriorityProcess = readyList.priority0[0];
    if (currentProcess->priority < highestPriorityProcess->priority){
      currentProcess->status = ProcessStatus::READY;
      highestPriorityProcess->status = ProcessStatus::RUNNING;
    }
  }

  Process* findRunningProcess(){
    for (auto &process: processList){
      if (process.status == ProcessStatus::RUNNING){
        return &process;
      }
    }
    return nullptr;
  }
};

int main(){
  ProcessManager pm = ProcessManager();
  pm.createProcess("ROOT", 0);
  std::cout<<"After creating process ROOT"<<std::endl;
  pm.printProcesses();
  pm.createProcess("A", 1);
  std::cout<<"After creating process A"<<std::endl;
  pm.printProcesses();
  return 0;
};

输出结果如下：

After creating process ROOT
ReadyList results:
Process: ROOT , Priority: 0, Status: RUNNING
ProcessList results: 
Process: ROOT , Priority: 0, Status: RUNNING
After creating process A
ReadyList results:
Process: ROOT , Priority: 0, Status: RUNNING
Process: A , Priority: 1, Status: RUNNING
ProcessList results: 
Process: ROOT , Priority: 0, Status: READY
Process: A , Priority: 1, Status: RUNNING

ProcessList设置为正确的值，进程A正在运行并处理ROOT就绪，但由于某种原因，ReadyList保持不变。在我的原始代码中，读取列表中进程ROOT的PID字符串值变为空，并且在更改状态后，我在此示例中遗漏的进程中存储的映射值也被清除。我还测试了直接使用readyList指针进行更改，而不是使用findRunningProcess函数返回的指针，它没有修复PID和映射值的问题，但在进程状态中导致了一些其他未定义的行为。我对可能导致此问题的想法不满意，请帮忙！非常感谢。

Answer 1

每次你：

processList.push_back(process);

vector可能会调整大小。这意味着支持vector的数据存储区将被复制到新的数据存储区中，然后被丢弃。这将为您留下另外两个vector，其中包含已释放并可能已重新分配的内存指针。

processList是使用std::list或std::deque的好地方，因为它们在增长时不会使指针无效。 std::deque应具有一些性能优势，因为它往往具有更好的空间局部性。

另一种方法是让其他两个vector存储processList中的进程索引，因为只要您只撤回并且不删除进程，它们就不会更改。

在任何一种情况下，从processList删除进程而不确定它们是否已首先从其他vector中删除，这将是一个坏主意。 std::deque处于劣势，如果你删除，因为这会使指针无效。

Answer 2

我怀疑ProcessManager中的进程列表向量在某个时刻正在调整大小，导致它必须创建一个新的内部数据结构并复制内容，让你的旧指针悬空。

如上所述：http://en.cppreference.com/w/cpp/container/vector/push_back

如果新的size（）大于capacity（）那么所有的迭代器和引用（包括过去的结束迭代器）无效。否则只有过去的迭代器无效。

C ++修改指向结构的指针向量会导致未定义的行为

2 个答案: