为什么将新结构放入重新分配的 c 数组崩溃程序中？

如何解决为什么将新结构放入重新分配的 c 数组崩溃程序中？

我想看看 c 数组是如何工作的。我用 malloc(),calloc() and realloc(). 做了一些程序，除了这个：

#include <iostream>

using namespace std;

struct student
{
    string name;
    string surname;
    int age;
};

int main(){
  student* arr = (student*)malloc(2*sizeof(student));
  if(arr!=NULL){
    for(int i = 0;i<2;i++){
        student st;
        cout<<"Enter name of "<<i+1<<". student: ";
        cin>>st.name;
        cout<<endl<<"Enter surname of "<<i+1<<". student: ";
        cin>>st.surname;
        cout<<endl<<"Enter age of "<<i+1<<". student: ";
        cin>>st.age;
        cout<<endl;
        arr[i] = st;
    }
     for(int i = 0;i<2;i++){
            cout<<"Name of "<<i+1<<". student: "<<arr[i].name<<endl<<endl;
            cout<<"Surname of "<<i+1<<". student: "<<arr[i].surname<<endl<<endl;
            cout<<"Age of "<<i+1<<". student: "<<arr[i].age<<endl<<endl;

     }
     cout<<endl<<"How many students would you like to add? ";
     int additional_students;
     cin>>additional_students;
     arr = (student*) realloc(arr,(additional_students+2) * sizeof(*arr));
     if(arr !=NULL){
         for(int i = 2;i<2+additional_students;i++){
        student st;
        cout<<"Enter name of "<<i+1<<". student: ";
        cin>>st.name;
        cout<<endl<<"Enter surname of "<<i+1<<". student: ";
        cin>>st.surname;
        cout<<endl<<"Enter age of "<<i+1<<". student: ";
        cin>>st.age;
        cout<<endl;
        //this line crashes the program
        arr[i] = st;
    }
     for(int i = 0;i<2+additional_students;i++){
            cout<<"Name of "<<i+1<<". student: "<<arr[i].name<<endl;
            cout<<"Surname of "<<i+1<<". student: "<<arr[i].surname<<endl;
            cout<<"Age of "<<i+1<<". student: "<<arr[i].age<<endl;
     }
     }
      }
  free(arr);
  return 0;
}

我一直在寻找一个小时的解决方案，但我仍然不明白它为什么会崩溃。我发现如果我将 realloc(arr,(additional_students+2) * sizeof(*arr)); 中的值乘以 500，它会以某种方式起作用。这是为什么？

解决方法

除了错误的 malloc 实现之外，还有其他可以修复的错误。

for(int i = 0;i<2;i++){
    student st;
    cout<<"Enter name of "<<i+1<<". student: ";
    cin>>st.name;
    cout<<endl<<"Enter surname of "<<i+1<<". student: ";
    cin>>st.surname;
    cout<<endl<<"Enter age of "<<i+1<<". student: ";
    cin>>st.age;
    cout<<endl;
    arr[i] = st;
}

制作新的学生对象只是为了给它分配输入然后将它传输到数组是不必要的，即使你已经自定义了“=”操作符。
尝试这样的事情：

for(int i = 0; i < 2; i++){
    cout << "Enter name of " << i + 1 << ". student: ";
    cin >> arr[i].name;
    cout << endl << "Enter surname of " << i + 1 << ". student: ";
    cin >> arr[i].surname;
    cout << endl << "Enter age of " << i + 1 << ". student: ";
    cin >> arr[i].age;
    cout << endl;
}

既然你已经为数组中的两个 student 实例分配了空间，为什么还要再创建一个？
同样的事情必须在第二个循环中完成。

malloc 当结构体包含需要构造的 C++ 类对象时，你不能使用它。正如有人在评论中所说的那样，malloc 和 realloc 只分配 内存，并且它们不构造结构本身（如果它有任何构造函数）或结构的成员（我指的是 string 成员）。
如果您想使用 malloc 和 realloc 保留当前解决方案，请尝试使用 char name[255]; 和 char surname[255] ; 而不是您现有的字符串成员。并且还像我之前所说的那样更改循环，因为它可能导致不可预测的行为。

编辑： 或者只是使用 std::vector 代替，它可以正确构建所有内容。

工作代码，带有 malloc 和 realloc

#include <iostream>

using namespace std;

struct student
{
    char name[255];
    char surname[255];
    int age;
};

int main(){
  student* arr = (student*)malloc(2*sizeof(student));
  if(arr!=NULL){
    for(int i = 0;i<2;i++){
        cout<<"Enter name of "<<i+1<<". student: ";
        cin>>arr[i].name;
        cout<<endl<<"Enter surname of "<<i+1<<". student: ";
        cin>>arr[i].surname;
        cout<<endl<<"Enter age of "<<i+1<<". student: ";
        cin>>arr[i].age;
        cout<<endl;
    }
     for(int i = 0;i<2;i++){
            cout<<"Name of "<<i+1<<". student: "<<arr[i].name<<endl<<endl;
            cout<<"Surname of "<<i+1<<". student: "<<arr[i].surname<<endl<<endl;
            cout<<"Age of "<<i+1<<". student: "<<arr[i].age<<endl<<endl;

     }
     cout<<endl<<"How many students would you like to add? ";
     int additional_students;
     cin>>additional_students;
     arr = (student*) realloc(arr,(additional_students+2) * sizeof(*arr));
     if(arr !=NULL){
         for(int i = 2;i<2+additional_students;i++){
        cout<<"Enter name of "<<i+1<<". student: ";
        cin>>arr[i].name;
        cout<<endl<<"Enter surname of "<<i+1<<". student: ";
        cin>>arr[i].surname;
        cout<<endl<<"Enter age of "<<i+1<<". student: ";
        cin>>arr[i].age;
        cout<<endl;
    }
     for(int i = 0;i<2+additional_students;i++){
            cout<<"Name of "<<i+1<<". student: "<<arr[i].name<<endl;
            cout<<"Surname of "<<i+1<<". student: "<<arr[i].surname<<endl;
            cout<<"Age of "<<i+1<<". student: "<<arr[i].age<<endl;
     }
     }
      }
  free(arr);
  return 0;
}