Passing character pointers into a function and dynamically allocating memory

Question

Student.h

class Student
{
 private:
      char m_sHouse[64];
 public:
 Student(void);
 ~Student(void);
 void getHouse(char *hName);
 void setHouse(char *hName);
}

Student.cpp

 void Student::setHouse(char *hName)
 {
    strcpy(m_sHouse, hName);
 }

 void Student::getHouse(char *hName)
 {
     if (m_sHouse != NULL)
     {
        hName = new char[strlen(m_sHouse)+1];
        strcpy(hName, m_sHouse);
     }
 }

In main:

 student.getHouse(house);
 if (strcmp(house, "house") == 0)
     cout <<"\tCorrectly returned the student house: " << house<< endl;

setHouse(char *hName) sets student->m_sHouse equal to "house".

My question:

When inside getHouse(char *hName), it acts as it should, setting hName to "house". but when control is passed out of the function, my dynamically allocated memory is deallocated, so when I strcmp in main, my program crashes (I end up comparing a NULL pointer).

Answer 1

Nick, the proper solution is that you know that hName is already allocated by the user of the class (Dr. Coleman). You simply need to strcpy into the character array.

Simply put:

void Student::getHouse(char *hName)
{
  strcpy(hName, m_sHouse);
}

Answer 2

You are allocating new memory and assigning it to a local variable. Change your function to

 void Student::getHouse(char **hName)
 {
     if (m_sHouse != NULL)
     {
        *hName = new char[strlen(m_sHouse)+1];
        strcpy(*hName, m_sHouse);
     }
 }

This will change the address pointed to by the argument passed in to your function, not the copy of it

Answer 3

The pointer hname is a copy of house (the pointer you have passed to getHouse). Inside that function you change hname, however, you are not changing the original house! To do that, you should either return the allocated memory:

char *Student::getHouse()
{
    char *hame = NULL;
    if (m_sHouse != NULL)
    {
        hName = new char[strlen(m_sHouse)+1];
        strcpy(hName, m_sHouse);
    }
    return hname;
}

and then

house = student.getHouse();

or give a pointer to this variable, so that it can be changed:

void Student::getHouse(char **hName)
{
    if (m_sHouse != NULL && hname != NULL)
    {
        *hName = new char[strlen(m_sHouse)+1];
        strcpy(*hName, m_sHouse);
    }
}

and then

student.getHouse(&house);

Similarly, you can give a reference to the house variable:

void Student::getHouse(char *&hName)
{
    if (m_sHouse != NULL)
    {
        hName = new char[strlen(m_sHouse)+1];
        strcpy(hName, m_sHouse);
    }
}

and then

student.getHouse(house);

The better solution however, would be to use std::string instead.

Answer 4

void Student::getHouse(char *hName)
{
     if (m_sHouse != NULL)
     {
        hName = new char[strlen(m_sHouse)+1];
        strcpy(hName, m_sHouse);
     }
 }

This only modifies a copy of hName pointer and does not modify the original pointer. To modify it you need to pass a pointer to your pointer:

void Student::getHouse(char **hName)
{
     if (m_sHouse != NULL)
     {
        *hName = new char[strlen(m_sHouse)+1];
        strcpy(*hName, m_sHouse);
     }
 }

and call your function like this:

student.getHouse(&house);