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。
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)
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! 0 4469696 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:)
最佳答案
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。
关于C++从main()通过类函数传递指针，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19348840/