C++通過main函數中的類函數傳遞指針

我知道指針是被討論了很多東西。我已經做了大量的研究來嘗試解決這個問題，但是一切都導致我陷入死衚衕。C++通過main函數中的類函數傳遞指針

我有一個任務是要求我創建一個記錄單個測試分數的類。如果測試分數已經被記錄並且新分數較高，則覆蓋它。如果它已被記錄並且新分數較低，則什麼也不做。如果沒有被記錄，記錄它。

這是我到目前爲止有：

// 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; 
}

指針是一些很新的給我...我看着他們，看着他們，看着他們，多一些，但我似乎總是讓他們錯了。

知道了，我知道我的recordScore函數可能做了非常錯誤的事情，但我不知道是什麼。

我現在的主要問題是，firstScore運行良好（耶！我得到了正確的......也許）但是，secondScore不會記錄分數。我嘗試了幾種不同的方法。

我把int abc = 0;這個測試已經採取：以上INT主要（）
- 當我打電話recordScore
- 編譯，當我打電話printScore
  - 輸出顯示崩潰編譯和運行良好！按任意鍵繼續......（崩潰）
我把INT ABC = 0;內中的int main（）的，但任何事情之前
- 當我打電話recordScore事情之前被輸出到控制檯

這也崩潰，如果我的INT主（）是這樣的崩潰：

int main() 
{ 
    int abc = 0; 

    testScore firstScore; 
    firstScore.printScore(cout); 


    system("PAUSE"); 
    return 0; 
}

而且我不知道爲什麼TT

我也有TR IED：

聲明

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

裏面的main（），但通過別的，並通過myTestScore到recordScore前：

&myTestScore

編譯錯誤：調用「testScore沒有匹配功能:: recordScore（INT **）;在secondScore.recordScore線上的。

*myTestScore

編譯錯誤：從 '詮釋' 對secondScore '詮釋*' 無效的轉換。recordScore線。

myTestScore

沒有編譯錯誤，死機當它運行之前什麼是輸出到控制檯

我曾嘗試聲明：

int *myTestScore = 83;

裏的任何東西之前，INT主要（）其他編譯錯誤：對int * myTestScore = 83行的'int'無效轉換爲'int *' 。

我也嘗試過將recordScore改爲使用&的*和*的各種方法，並且都沒有改變這兩者的組合。

我現在沒有想法嘗試的東西，即使經過研究，我也無法想出任何東西。我試着問我的教授（現在一週，這是在線課程），給她打電話，給她發電子郵件，但她沒有回答我的任何問題，甚至沒有要求開會。

我覺得有一些簡單的東西我不是在這裏抓的，我非常感謝任何人都可以幫助我解決這個問題。

非常感謝你的時間。

變化：

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; 
}

輸出：這個測試沒有被搶走！按任意鍵繼續......

最終工作產品：

// 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; 
}

太感謝你了，其實我是從這個頗有幾分瞭解到，和一些新的術語太:) :)

來源

2013-10-13 Tiffany Rehsif

*聲明一個函數來記錄測試分數，參數將是**整數***。你聲明它需要一個指向一個整數的指針。讀你的問題，我建議你停下來，喘口氣，重讀你的材料指針。如果你寫'int * myScore = 83;'並且需要問什麼是錯誤的，這意味着你需要學習更多。 – jrok

梅爾，在開始時她曾說過：//申報記錄測試分數的班級 //數據將是指向一個整數的指針，而不是一個整數 –

這是班級的要求，而不是「 recordSCore'。 – jrok

主要問題是，在默認的構造函數中，你指定NULL來評分，所以指針會指向無效的內存。所以，當你調用RecordStore中，當程序來指令：

*score = *myScore;

這導致段錯誤，當你試圖覆蓋的內存，你的程序是不使用的部分出現差錯。

該程序不會在printScore中崩潰，因爲讀取無效指針不是錯誤，但會讀取垃圾數據。

編輯：根據您的分配，指針必須在RecordStore中進行分配，如果測試沒有被搶走，所以在RecordStore中，改變這一部分：

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

這樣：

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

另外，當您執行delete部分時，首先將指針指定爲NULL，然後將其刪除;所以程序會嘗試刪除NULL指針（這不會導致錯誤），並且用於score的內存未被釋放，導致內存泄漏。

來源

2013-10-13 19:07:42 xorguy

啊，我沒有意識到有關刪除的事情。謝謝！只要得分的事情... 我的老師希望它設置爲NULL，所以我必須:( - 但你給指針無效的信息爲空是有用的，謝謝你，以及！但是，根據recordScore，當它在那裏，它打印正確... 它是什麼時候它得到printScore它以某種方式得到改變回到NULL，或者只是爲了垃圾，或什麼，我不完全確定。更新代碼 –

如果你正在談論Changes部分，當你通過一個int值而不是一個指針傳入時，abc的一個副本將被傳遞，並且該副本將在函數結束後被刪除。一個垃圾值稍後。 – xorguy

C++通過main函數中的類函數傳遞指針

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