尝试在 C 中引用值时出现 LValue 错误

Question

这肯定会对某人来说很有趣。所以我有一个对象数组（在我的测试/演示代码中，为了简单起见，我只是将它们加倍），然后创建一个指向这些对象的指针数组。

我这样做的原因是因为我将交换对象，并且交换对象本身的成本很高，而交换它们的指针（在辅助数组中）则很便宜。

我的框数据类型中有许多子索引。基本上，我的目标是能够维护指向盒子“拥有”的对象的连续指针。为此，当我从一个盒子转移到另一个盒子时，我必须在主指针数组中执行多次交换。我通过使用框结构内的子索引来执行此操作，并且正是在那里发生错误...请参阅注释行以了解错误。

#include <iostream>

typedef struct box_s {
  double * TestArrayPointer;
} box_t;

main () {

  double * TestArray;
  double ** TestPointerArray;

  TestArray = new double [100];

  TestPointerArray = new double * [100];

  for (unsigned int Counter = 0; Counter<100; Counter++)
  {
     TestArray[Counter]=Counter;
     TestPointerArray[Counter]=&(TestArray[Counter]);
  }
  for (unsigned int Counter = 0; Counter<100; Counter++)
    std::cout << "P: " << TestPointerArray[Counter] << " V: " 
          << *(TestPointerArray[Counter]) << std::endl;

  box_t Boxes[10];
  for (unsigned int Counter = 0; Counter<10; Counter++)
    Boxes[Counter].TestArrayPointer = TestPointerArray[Counter*10];
  for (unsigned int Counter = 0; Counter<10; Counter++)
    std::cout << "P: " << Boxes[Counter].TestArrayPointer << " V: " 
          << *(Boxes[Counter].TestArrayPointer)
          << " P+1: " << (Boxes[Counter].TestArrayPointer)+1 << " V+1: " 
          << *((Boxes[Counter].TestArrayPointer)+1) 
          << " P: " << Boxes[Counter].TestArrayPointer <<std::endl;
  for (unsigned int Counter = 0; Counter<10; Counter++)
    std::cout << "P: " << &(Boxes[0].TestArrayPointer[Counter]) << " V: " 
          << Boxes[0].TestArrayPointer[Counter] << std::endl;


  unsigned int ExitBox = 1;
  unsigned int EntranceBox=9;
  unsigned int OldIndex = 12;
  double * TempPtr = TestPointerArray[OldIndex];

  if (ExitBox < EntranceBox)
  {
     //Swap final element of this box into old position
     TestPointerArray[OldIndex] = Boxes[ExitBox+1].TestArrayPointer-1;
     //Keep swapping start for end
     for(unsigned int BoxNum = ExitBox+1; BoxNum<EntranceBox &&
       BoxNum+1<=9;BoxNum++)
     {
        //Fill in new first with last element of old array.
        (Boxes[BoxNum-1].TestArrayPointer-1)=
      Boxes[BoxNum+1].TestArrayPointer-1; //darn l-value error!
    //Change array head pointer to match.
    Boxes[BoxNum].TestArrayPointer=Boxes[BoxNum-1].TestArrayPointer-1;
     }
     Boxes[EntranceBox].TestArrayPointer = TempPtr;
  }  
  for (unsigned int Counter = 0; Counter<100; Counter++)
     std::cout << "P: " << TestPointerArray[Counter] << " V: " 
           << *(TestPointerArray[Counter]) << std::endl;
}

我认为这是因为“-1”，但当我在上面的循环中用“+1”做类似的事情时，我不确定为什么这是无效的。

PS 这是真正的测试代码。我编译用

g++ main.cpp

。我没有仔细注意评论和大小写，只是尝试坚持 MSDN 建议的大小写方案，所以不要评判......这只是一个快速而肮脏的测试！

编辑1

这是任何关心的人的工作代码...PMG 鉴于new用法，我将其更改为c++。希思，我相信你，尽管你的回答没有解决根本问题。

这是真正的解决方案：

#include <iostream>

typedef struct box_s {
  double ** TestArrayPointer;
  unsigned int NumberPoints;
} box_t;

main () {

  double * TestArray;
  double ** TestPointerArray;

  TestArray = new double [100];

  TestPointerArray = new double * [100];

  for (unsigned int Counter = 0; Counter<100; Counter++)
  {
     TestArray[Counter]=Counter;
     TestPointerArray[Counter]=&(TestArray[Counter]);
  }
  for (unsigned int Counter = 0; Counter<100; Counter++)
    std::cout << "P: " << TestPointerArray[Counter] << " V: " 
          << *(TestPointerArray[Counter]) << std::endl;

  box_t Boxes[10];
  for (unsigned int Counter = 0; Counter<10; Counter++)
  {
     Boxes[Counter].TestArrayPointer = TestPointerArray+Counter*10;
     Boxes[Counter].NumberPoints=10;
  }
  for (unsigned int Counter = 0; Counter<10; Counter++)
    std::cout << "P: " << Boxes[Counter].TestArrayPointer << " V: " 
          << *(Boxes[Counter].TestArrayPointer)
          << " P+1: " << (Boxes[Counter].TestArrayPointer)+1 << " V+1: " 
          << *((Boxes[Counter].TestArrayPointer)+1) 
          << " P: " << Boxes[Counter].TestArrayPointer <<std::endl;
  for (unsigned int Counter = 0; Counter<10; Counter++)
    std::cout << "P: " << &(Boxes[0].TestArrayPointer[Counter]) << " V: " 
          << Boxes[0].TestArrayPointer[Counter] << std::endl;


  unsigned int ExitBox = 1;
  unsigned int EntranceBox=9;
  unsigned int OldIndex = 12;
  double * TempPtr = TestPointerArray[OldIndex];

  if (ExitBox < EntranceBox)
  {
    Boxes[ExitBox].NumberPoints--;
    Boxes[EntranceBox].NumberPoints++;
     //Swap final element of this box into old position
     TestPointerArray[OldIndex] = *(Boxes[ExitBox+1].TestArrayPointer-1);
     //Keep swapping start for end
     for(unsigned int BoxNum = ExitBox+1; BoxNum<EntranceBox &&
       BoxNum+1<=9;BoxNum++)
     {
        Boxes[BoxNum].TestArrayPointer=
      Boxes[BoxNum].TestArrayPointer-1;
        //Fill in new first with last element of old array.
    *(Boxes[BoxNum].TestArrayPointer)=
      *(Boxes[BoxNum+1].TestArrayPointer)-1;    
    std::cout <<"---------------------" << std::endl;
    for (unsigned int Counter = 0; Counter<100; Counter++)
       std::cout << "P: " << TestPointerArray[Counter] << " V: " 
             << *(TestPointerArray[Counter]) << std::endl;
     }
     Boxes[EntranceBox].TestArrayPointer=
      Boxes[EntranceBox].TestArrayPointer-1;
     *(Boxes[EntranceBox].TestArrayPointer) = TempPtr;
  }  

  for(unsigned int BoxNum = 0; BoxNum<=9;BoxNum++)
  {
    std::cout <<"---------------------" << std::endl;
    for(unsigned int Counter = 0; Counter<Boxes[BoxNum].NumberPoints; 
    Counter++)
      std::cout << "P: " <<Boxes[BoxNum].TestArrayPointer[Counter] 
      << " V: " 
            << *(Boxes[BoxNum].TestArrayPointer[Counter]) 
      << std::endl;

  }
}

注意正确的交换！

Answer 1

我不会质疑你的代码，因为我无法真正阅读它。

关于左值错误，更改为：

(Boxes[BoxNum-1].TestArrayPointer-1) = ...

或

*(Boxes[BoxNum-1].TestArrayPointer-1) = ...

即使

Boxes[BoxNum-1].TestArrayPointer[-1] = ...

计算出的指针不是左值，也必须取消引用它。换句话说，您无法通过更改地址来存储任何内容，您必须将您的值存储在地址处。