[Solved] How to convert stack declarations to heap declarations in C++

Question

1.44/5 (2 votes)

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Can anyone plz help to convert the following stack declarations to heap declarations

C++

class student
{
   protected:
   int r;
   char sta,m;
   char student_roll[15];
   char name[50];
   char sex[10];
   char status[50];
   float slc,p2,m1,m2,m3,m4,m5,m6,m7,m8;
   float tp,p1,rem,pa;
   char dob[20];
   public:
   char roll_no[15];
};

Please help....its a C++ class...

Posted 15-Oct-13 5:14am

AR547

Updated 12-May-14 1:06am

v3

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Comments

nv3 15-Oct-13 11:32am

So far you have done neither a stack nor a heap declaration, but just declared a class. If you inside one of your functions write:

student s;

then you have allocated an object of type student on the stack. If however you write

student* pStud = new student;

you have allocated an object of type student on the heap. In the latter case, don't forgat the corresponding delete statement.

Hope that helps.
Ho

Sergey Alexandrovich Kryukov 15-Oct-13 12:25pm

Would make a fair answer.
—SA

nv3 15-Oct-13 12:48pm

Thank you, Sergey.

AR547 15-Oct-13 12:26pm

thanks alot dear... that gave me a clue ;)

nv3 15-Oct-13 12:47pm

You're welcome.

Sergey Alexandrovich Kryukov 15-Oct-13 12:24pm

It's not "convert" and not just "declaration". You need to learn how stack and heap work.
—SA

AR547 15-Oct-13 12:26pm

yeah ...

Hamed Moezzi Azimi 6-Nov-13 5:33am

I think the answer is in this link:
http://stackoverflow.com/questions/408670/stack-static-and-heap-in-c

3 solutions

Solution 1

I recommend that you read up about how storage on the stack differs from storage on the heap (in terms of how it is allocated and how it is addressed - does it have a fixed address, or is its likely address relative to some other value ... perhaps a value in a register in the CPU).

You should also study what kind of declarations in your C++ program lead to one or the other kind of storage, and you should learn about the difference between types (classes) that you declare in your program and instances of those types (that are variables in a running program, that actually occupy memory).

Posted 15-Oct-13 5:29am

Chris Ross 2

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Maximilien · Accepted Answer · 2013-10-15T05:31:00

Solution 2

Why?

This is an acceptable and "safe" class declaration; you do not want to keep unsafe pointers just for the fun of it.

One way to make it better would be to replace the char array with std::string.

Replace the sex string with an enum (and have code to "print" the representation instead of keeping a string (male/female).

Use proper descriptive variable names.

C++

class student
{
  enum SEX {
    Female = 0,
    Male
  };
 
   protected:
   int r;
   char sta,m;
   std::string student_roll;
   std::string name;
   SEX sex;
   std::string status;
   float slc,p2,m1,m2,m3,m4,m5,m6,m7,m8;
   float tp,p1,rem,pa;
   std::string dob;
   public:
   std::string roll_no;
};

Posted 15-Oct-13 5:31am

Maximilien

Updated 23-Dec-13 9:31am

v2

Comments

AR547 15-Oct-13 11:44am

can't get your point.. except the creation of enum in the class...is this a heap declaration which I can accept, ???

Maximilien 15-Oct-13 11:52am

No there is no heap in there. all member of the class will be safely allocated when the class itself is allocated.

If you really want to allocate the class on the heap :
student* aStudent = new student; // do not forget to call delete.
or better yet (if you use c++11 )
std::unique_ptr<student> aStudent(new student); // memory will be freed when going out of scope.

AR547 15-Oct-13 11:59am

ok bro got u...now inshort my question is that kindly tell me how can i declare those variables in heap mode at initial point as they are declared i dont want to declare like this... I think u understand my question :)

Albert Holguin 15-Oct-13 13:54pm

BTW, don't use the heap if you don't have to... the allocation process is relatively slow and you'll end up slowing your application for no good reason if you could have used the stack.

Volynsky Alex · Accepted Answer · 2013-10-15T06:30:00

You correctly answered nv3!
Where memory is allocated depends on how you create an object of your class
For example:

C++

{
student s; // allocated on stack 
}

but

C++

student* pStud = new student; // allocated on heap

But perhaps you're wondering how to describe a class that can always be allocated ONLY on heap... Please use the "Named Constructor Idiom".
See here : http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Named_Constructor

Here new version of your class (if you want allocate a memory for object of class only on heap):

C++

class student
{
public:
    // The create() methods are the "named constructors":
   static student* create() { return new student(); }

protected:
   int r;
   char sta,m;
   char student_roll[15];
   char name[50];
   char sex[10];
   char status[50];
   float slc,p2,m1,m2,m3,m4,m5,m6,m7,m8;
   float tp,p1,rem,pa;
   char dob[20];
   public:
   char roll_no[15];

private:
   // The constructor IS private !!!!
  student();
};

C++

 //Now exist only one way to create a student objects => a student::create() 
int main()
{
   student* p = student::create();
   ...
   delete p;
   ...
}