C++从main()通过类函数传递指针

Question

我知道指针是一个被广泛讨论的话题。我做了很多研究来尝试解决这个问题，但是一切都让我走到了死胡同。

我的作业要求我创建一个记录单个测试分数的类。如果已经记录了测试分数并且新分数更高，则覆盖它。如果它已被记录并且新分数较低，则什么都不做。如果没有记录，请记录下来。

这是我目前所拥有的:

//  CIS 235 exercise 7

#include <iostream>


using namespace::std;

//   declare a class for recording a test score
//   the data will be pointer to an integer, rather than an integer
//
//   - this exercise is designed to show how to work with pointer memory
//   - of course, we would NOT normally use a pointer for just an integer
//   - to illustrate the concepts, but keep the implementation simple,
//             integer data was used.  The general case would be object data,
//             not integer data

class testScore
{
   public:
   //  declare a default constructor - the pointer should be set to NULL
   testScore();
   //  declare a function that returns a bool, indicating if the test has been taken
   bool hasTestTaken();
   //  declare a function to record the test score, the parameter will be an integer
   //  use the following rules
   //  -  if no test has been taken, allocate memory and record the score
   //  -  if a test has been taken and the parameter is less than or equal to
   //         the score, do nothing
   //  -  if the test has been taken and the parameter is  higher than the score,
   //         - release the old memory
   //         - allocate new memory
   //         - record the score
   void recordScore(int *myScore);
   //   declare a function to print the score to an ostream parameter
   //   if the test has not been taken, send an appropriate message to the ostream
   //         otherwise print the score
   void printScore(ostream &out);
   //   declare the destructor
   //   be CAREFUL, you will need an if statement in your destructor
   ~testScore();

   private:
   //  declare the data needed to implement the class
   bool testTaken;
   int *score;
 };

//  write the 5 member functions

testScore::testScore() : score(NULL)
{
//  declare a default constructor - the pointer should be set to NULL
}

bool testScore::hasTestTaken()
{
   //  declare a function that returns a bool, indicating if the test has been taken
   return testTaken;
}

void testScore::recordScore(int *myScore)
{
   if(testTaken == false)
   {
                testTaken = true;
                *score = *myScore;
   }
   else if(testTaken == true && *myScore > *score)
   {
                score = NULL;
                delete score;
                score = new int;
                *score = *myScore;
   }

}

void testScore::printScore(ostream& out)
{
      //   declare a function to print the score to an ostream parameter
   //   if the test has not been taken, send an appropriate message to the ostream
   //         otherwise print the score
     if(testTaken)
     {
                  out << *score << endl;
     }
     else
         out << "The test has not been taken!" << endl;
}

testScore::~testScore()
{
   //   declare the destructor
   //   be CAREFUL, you will need an if statement in your destructor
   if(score != NULL)
   {
             score = NULL;
             delete score;
   }
   else
       delete score;

}

//  test the class member functions
//    - declare an object, but do NOT record a score for the object

//    - declare a second object and record the scores of 83, 78, 92
//       use appropriate member print functions to verify your code
//int abc = 83;
int abc = 0;
int main()
{
//    int abc = 0;
//    int * score2;
//    myTestScore = new int;
//    *myTestScore = 83;

    testScore firstScore;
    firstScore.printScore(cout);

    testScore secondScore;
//    secondScore.recordScore(&abc);
      secondScore.recordScore(&abc);
//    secondScore.printScore(cout);
//    *myTestScore = 78;
//    secondScore.recordScore(myTestScore);
//    secondScore.printScore(cout);
//    *myTestScore = 92;
//    secondScore.recordScore(myTestScore);
//    secondScore.printScore(cout);


   system("PAUSE");
   return 0;
}

Pointers 对我来说是相当新鲜的东西……我查过，查过，查过，但我似乎总是弄错了。

知道这一点后，我知道我的 recordScore 函数可能做错了什么，但我不知道是什么。

我现在的主要问题是 firstScore 运行良好(耶!我做对了......也许吧)但是，secondScore 不会记录分数。我尝试了几种不同的方法。

1. 我把 int abc = 0;在 int main() 之上

   • 调用 recordScore 时编译并运行良好
   • 在我调用 printScore 时编译并崩溃
     • 输出显示:此测试已完成!按任意键继续...(崩溃)

2. 我把 int abc = 0;在 int main() 内部但在其他任何事情之前

   • 在任何内容输出到控制台之前调用 recordScore 时崩溃

如果我的 int main() 看起来像这样，这也会崩溃:

int main()
{
    int abc = 0;

    testScore firstScore;
    firstScore.printScore(cout);


   system("PAUSE");
   return 0;
}

我也不知道为什么T.T

我也试过:

声明

int *myTestScore;
myTestScore = new int;
*myTestScore = 83;

在 main() 内部但在其他任何事情之前，通过以下方式将 myTestScore 传递给 recordScore:

&myTestScore

编译错误:没有匹配函数调用'testScore::recordScore(int**);在 secondScore.recordScore 行。

*myTestScore

编译错误:从“int”到“int*”的无效转换在 secondScore.recordScore 行。

myTestScore

没有编译错误，在任何输出到控制台之前运行时崩溃

我试过声明:

int *myTestScore = 83; 

在 int main() 内部编译错误:从“int”到“int*”的无效转换在 int *myTestScore = 83 行。

我还尝试了各种方法来将 recordScore 更改为使用 & 和 * 以及两者都不使用以及两者的不同组合。

我现在对尝试的事情没有想法，甚至在研究之后我也想不出任何东西。我试过问我的教授(一周以来，在线类(class))，给她打电话，给她发电子邮件，但她没有回答我提出的任何问题，甚至没有要求会面。

我觉得这里有一些简单的事情我没有掌握，我真的很感谢任何人能给我的帮助来解决这个问题。

非常感谢您的宝贵时间。

---

变化:

testScore::testScore() : score(NULL), testTaken(false) // didnt change because instructor instructions, but did move testTaken up cause that is where it should be
{
    //  declare a default constructor - the pointer should be set to NULL
}

void testScore::recordScore(int myScore)
{
   if(testTaken == false)
   {
                testTaken = true;
                score = &myScore;
                cout << *score << endl; //this prints correctly, 0
   }
   else if(testTaken == true && myScore > *score)
   {
                //removed the score = NULL to avoid a memory leak (I think this is correct now?)
                delete score;
                score = new int;
                score = &myScore;
   }

}

void testScore::printScore(ostream& out)//no changes, just easier to access to you dont have to keep scrolling up
{
      //   declare a function to print the score to an ostream parameter
   //   if the test has not been taken, send an appropriate message to the ostream
   //         otherwise print the score
     if(testTaken)
     {
                  out << *score << endl; //outputs incorrect 4469696
     }
     else
         out << "The test has not been taken!" << endl;
}

int main()
{
    int abc = 0;

    testScore firstScore;
    firstScore.printScore(cout);

    testScore secondScore;
    secondScore.recordScore(abc);
    secondScore.printScore(cout);

   system("PAUSE");
   return 0;
}

输出:此测试未参加!04469696按任意键继续...

---

最终工作产品:

//  CIS 235 exercise 7

#include <iostream>


using namespace::std;

//   declare a class for recording a test score
//   the data will be pointer to an integer, rather than an integer
//
//   - this exercise is designed to show how to work with pointer memory
//   - of course, we would NOT normally use a pointer for just an integer
//   - to illustrate the concepts, but keep the implementation simple,
//             integer data was used.  The general case would be object data,
//             not integer data

class testScore
{
   public:
   //  declare a default constructor - the pointer should be set to NULL
   testScore();
   //  declare a function that returns a bool, indicating if the test has been taken
   bool hasTestTaken();
   //  declare a function to record the test score, the parameter will be an integer
   //  use the following rules
   //  -  if no test has been taken, allocate memory and record the score
   //  -  if a test has been taken and the parameter is less than or equal to
   //         the score, do nothing
   //  -  if the test has been taken and the parameter is  higher than the score,
   //         - release the old memory
   //         - allocate new memory
   //         - record the score
   void recordScore(int * myScore);
   //   declare a function to print the score to an ostream parameter
   //   if the test has not been taken, send an appropriate message to the ostream
   //         otherwise print the score
   void printScore(ostream &out);
   //   declare the destructor
   //   be CAREFUL, you will need an if statement in your destructor
   ~testScore();

   private:
   //  declare the data needed to implement the class
   bool testTaken;
   int *score;
 };

//  write the 5 member functions

testScore::testScore() : score(NULL), testTaken(false)
{
    //  declare a default constructor - the pointer should be set to NULL
}

bool testScore::hasTestTaken()
{
   //  declare a function that returns a bool, indicating if the test has been taken
   return testTaken;
}

void testScore::recordScore(int * myScore)
{
   if(testTaken == false)
   {
                score = new int;
                testTaken = true;
                *score = *myScore;
   }
   else if(testTaken == true && *myScore > *score)
   {
                delete score;
                score = new int;
                *score = *myScore;
   }

}

void testScore::printScore(ostream& out)
{
      //   declare a function to print the score to an ostream parameter
   //   if the test has not been taken, send an appropriate message to the ostream
   //         otherwise print the score
     if(testTaken)
     {
                  out << *score << endl;
     }
     else
         out << "The test has not been taken!" << endl;
}

testScore::~testScore()
{
   //   declare the destructor
   //   be CAREFUL, you will need an if statement in your destructor
   if(score != NULL)
   {
             delete score;
   }

}

//  test the class member functions
//    - declare an object, but do NOT record a score for the object

//    - declare a second object and record the scores of 83, 78, 92
//       use appropriate member print functions to verify your code
int main()
{
    int abc = 83;

    testScore firstScore;
    firstScore.printScore(cout);

    testScore secondScore;
    secondScore.recordScore(&abc);
    secondScore.printScore(cout);

    abc = 78;
    secondScore.recordScore(&abc);
    secondScore.printScore(cout);

    abc = 92;
    secondScore.recordScore(&abc);
    secondScore.printScore(cout);

   system("PAUSE");
   return 0;
}

非常感谢，我实际上从中学到了很多东西，还有一些新术语:)

Answer 1

主要问题是在默认构造函数中，您将 NULL 分配给 score，因此指针将指向无效内存。所以，当你调用recordStore时，当程序走到这条指令时:

 *score = *myScore;

它会导致段错误，当您尝试覆盖您的程序未使用的内存部分时会发生错误。

程序不会在 printScore 中崩溃，因为读取无效指针不会出错，但会读取垃圾数据。

编辑:根据您的分配，如果尚未进行测试，则必须在 recordStore 中分配指针，因此在 recordStore 中，更改此部分:

if(testTaken == false)
{
                testTaken = true;
                *score = *myScore;
}

为此:

if(testTaken == false)
{
                score = new int;
                testTaken = true;
                *score = *myScore;
}

此外，当您执行delete 部分时，您首先将指针分配给NULL，然后再将其删除；所以程序会尝试删除NULL指针(这不会导致错误)，而用于score的内存没有释放，造成内存泄漏。