c++ - 学习 C++ : error: use of deleted function

Question

我正在做“像程序员一样思考”一书中的一些练习，到目前为止一切都很好。我开始了类(class)一章，在这里我似乎被卡住了，因为我无法解决编译代码时遇到的错误。

这是代码。这不是我的，我一直在写这本书试图理解它。

struct studentRecord {
    int studentId;
    int grade;
    string name;
    studentRecord(int a, int b, string c); 
};

class studentCollection {
  private:
    struct studentNode {
        studentRecord studentData;
        studentNode *next;
    };
  public:
    studentCollection();
    void addRecord(studentRecord newStudent);
    studentRecord recordWithNumber(int idNum);
    void removeRecord(int idNum);
  private:
    //typedef studentNode *studentList;
    studentNode *_listHead;
};

studentRecord::studentRecord(int a, int b, string c) {
    studentId = a;
    grade = b;
    name = c;
}

studentCollection::studentCollection() {
    _listHead = NULL;
}


void studentCollection::addRecord(studentRecord newStudent) {
    studentNode *newNode = new studentNode;
    newNode->studentData = newStudent;
    newNode->next = _listHead;
    _listHead = newNode;
}

studentRecord studentCollection::recordWithNumber(int idNum) {
    studentNode *loopPtr = _listHead;
    while (loopPtr != NULL && loopPtr->studentData.studentId != idNum) {
        loopPtr = loopPtr->next;
    }
    if (loopPtr == NULL) {
        studentRecord dummyRecord(-1, -1, "");
        return dummyRecord;
    } else {
        return loopPtr->studentData;
    }
}

int main() { 
    studentCollection s;
    studentRecord stu3(84, 1152, "Sue");
    studentRecord stu2(75, 4875, "Ed");
    studentRecord stu1(98, 2938, "Todd");
    s.addRecord(stu3);
    s.addRecord(stu2);
    s.addRecord(stu1);
}

我得到的错误是:

studentclass1.cpp: In member function ‘void studentCollection::addRecord(studentRecord)’:
studentclass1.cpp:45:32: error: use of deleted function ‘studentCollection::studentNode::studentNode()’
     studentNode *newNode = new studentNode;
                                ^~~~~~~~~~~
studentclass1.cpp:17:12: note: ‘studentCollection::studentNode::studentNode()’ is implicitly deleted because the default definition would be ill-formed:
     struct studentNode {
            ^~~~~~~~~~~
studentclass1.cpp:17:12: error: no matching function for call to ‘studentRecord::studentRecord()’

Answer 1

当您定义一个 struct 时，例如:

struct studentNode {
    studentRecord studentData;
    studentNode *next;
};

它有一个隐式定义的默认构造函数，相当于:

struct studentNode {
    studentNode() : studentData(), next() {}
    studentRecord studentData;
    studentNode *next;
};

这是一个问题，因为 studentRecord 的默认构造函数由于存在用户定义的构造函数而被编译器删除。

您可以将默认构造函数添加到 studentRecord 以解决该问题。

struct studentRecord {
    int studentId;
    int grade;
    string name;
    studentRecord() = default; 
    studentRecord(int a, int b, string c); 
};

与其使用带有 = default; 说明符的计算机生成的默认构造函数，不如使用有效数据初始化对象会更好。

struct studentRecord {
    int studentId;
    int grade;
    string name;
    studentRecord() : studentRecord(0, 0, "") {} // Delegate to the other constructor. 
    studentRecord(int a, int b, string c); 
};