指向成员是指针的结构的c ++指针

Question

Hi all I am working on an assignment , when given a struct and its members I will have to intialise the members using dynamic memory allocation . This is the code I have used:

# include <iostream>
# include <string>

using namespace std;

Structure

struct Student
{
char * name;
float gpa;
} ;

PROTOTYPES

Student *  createStudent(char name[], float gpa);

int main ()
{
 int length ;
 float gpa ;
 char *p;
 Student *myPointer ;
 Student myStudent ;
 myPointer = &myStudent;

 cout << " Please enter number of characters you want to enter " << endl;
 cin >> length ;
 length = length + 1;
 p= new char [length +1];
 cout << " Please enter name " << endl;
 cin >> p ;
 cout << " please enter gpa "<< endl;
 cin >> gpa ;
 myPointer = createStudent (p,gpa);
 cout << myPointer->gpa;
 cout << (*myPointer).name << endl;

Her's the error when printing the name, but when i see the value of the name before printing it is showing the same as entered by the user:

  delete[]p;
  p = 0;
  system("pause");
  return 0;
 }

This function creates a student object and assigns name and gpa as passed to the student object and returns a pointer of the student object:

Student *  createStudent( char name[], float gpa )
{

Student *studentPtr ;
Student studentObject;
studentPtr = &studentObject;
studentPtr-> name = name;
studentPtr-> gpa = gpa ;
return studentPtr ;
}

Can anyone give me an idea of why the name is not printing. Thank you in advance .

Answer 1

studentObject is allocated on the stack, ie it has automatic storage duration, which means it will be destroyed when the function returns. The pointer doesn't keep it alive.

That means myPointer is a dangling pointer; dereferencing it ( myPointer-> ) invokes undefined behavior.

---

To correct it, simply return the object by value:

Student createStudent(char name[], float gpa)
{
    Student studentObject;
    studentPtr.name = name;
    studentPtr.gpa = gpa;
    return studentObject;
}

and

Student myStudent = createStudent(p, gpa);

---

Edit: I just read that you need to initialize the members using dynamic memory allocation. The above solution doesn't do that. For dynamic memory allocation you have to use new :

Student* createStudent(char name[], float gpa)
{
    Student* studentPtr = new Student;
    studentPtr->name = name;
    studentPtr->gpa = gpa;
    return studentPtr;
}

and

myPointer = createStudent(p, gpa);
// use
delete myPointer;

It would probably be a good idea to also allocate a new buffer for studentPtr->name with new , instead of just assigning the pointer; you can see Mateusz 's answer about that.

But of course this is just a bad example from your professor. As mentioned by gd1 , this kind of code is not good modern C++ and shouldn't end up in real production code.

Answer 2

This is totally wrong:

Student* createStudent( char name[], float gpa )
{
    Student *studentPtr ;
    Student studentObject;
    studentPtr = &studentObject;
    studentPtr-> name = name;
    studentPtr-> gpa = gpa ;
    return studentPtr ;
}

studentObject is a local object, that is destroyed just after the function ends. Returning pointers and/or references to local variables is an error - you should get warning from your compiler.

Another issue is this line:

studentPtr->name = name;

What you do now: allocate a block of memory in main() , pass it to an object, that may use this memory and then free it in main() . You should not do that - objects should be responsible for their content. Are you sure, that this responsibility should be yours? I would suggest you to:

Student* createStudent( char name[], float gpa )
{
    size_t name_len = strlen(name);
    Student studentObject;
    studentObject.name = new char[name_len+1];
    strncpy(studentObject.name, name, name_len + 1);
    studentObject.gpa = gpa ;
    return studentObject;
}

And analogical function to destroy Student :

void destroyStudent(Student* student)
{
    delete[] student->name;
    student->name = nullptr;
}

Obvious solution would be to use std::string , but I assume, that you have a good reason to do all this manually... don't you?

Oh, and if you really need to return Student as pointer:

Student* createStudent( char name[], float gpa )
{
    size_t name_len = strlen(name);
    Student* studentPtr = new Student;
    studentPtr->name = new char[name_len+1];
    strncpy(studentPtr->name, name, name_len + 1);
    studentPtr-> gpa = gpa ;
    return studentPtr;
}

void destroyStudent(Student* student)
{
    delete[] student->name;
    student->name = nullptr;
    delete[] student;
}