Why does the last sr5 object not occupy memory with overloaded new operator? - c++

Question

When I run this program sr1, sr2, sr3, sr4 objects are created and values are assigned to corresponding variables. But in sr5 object, the name remains blank while the roll_no percentage shows the correct value.
When change the value of
int MAX = 5;
to
int MAX = 6;
everything works fine.
Here is my code:
const int MAX = 5;
const int FREE = 0;
const int OCCUPIED = 1;
int flag = 0;
using namespace std;
void warning()
{
cout<<"\n------All memory occupied------"<<endl;
exit(1);
}
class student_rec
{
private:
char name[25];
int roll_no;
float percentage;
public:
student_rec(char *n, int r, float per)
{
strcpy(name, n);
roll_no = r;
percentage = per;
}
student_rec()
{
}
void set_rec(char *n, int r, float per)
{
strcpy(name, n);
roll_no = r;
percentage = per;
}
void show_rec()
{
cout<<"\n-------------------\n";
cout<<"Name= "<<name<<endl;
cout<<"Roll number= "<<roll_no<<endl;
cout<<"Percentage= "<<percentage<<endl;
}
void *operator new (size_t sz);
void operator delete (void *d);
};
struct memory_store
{
student_rec obj;
int status;
};
memory_store *m = NULL;
void *student_rec::operator new (size_t sz)
{
int i;
if(flag == 0)
{
m = (memory_store *) malloc(sz * MAX);
if(m == NULL)
warning();
for(i=0; i<MAX; i++)
m[i].status = FREE;
flag = 1;
m[0].status = OCCUPIED;
return &m[0].obj;
}
else
{
for(i=0; i<MAX; i++)
{
if(m[i].status == FREE)
{
m[i].status = OCCUPIED;
return &m[i].obj;
}
}
warning();
}
}
void student_rec::operator delete (void *d)
{
if(d == NULL)
return;
for(int i=0; i<MAX; i++)
{
if(d == &m[i].obj)
{
m[i].status = FREE;
strcpy(m[i].obj.name, "");
m[i].obj.roll_no = 0;
m[i].obj.percentage = 0.0;
}
}
}
int main()
{
student_rec *sr1, *sr2, *sr3, *sr4, *sr5, *sr6, *sr7;
sr1 = new student_rec("sandeep", 21, 78);
sr1->show_rec();
sr2 = new student_rec("sachin", 21, 78);
sr2->show_rec();
sr3 = new student_rec("sapna", 21, 78);
sr3->show_rec();
sr4 = new student_rec("vipin", 21, 78);
sr4->show_rec();
sr5 = new student_rec("niraj", 21, 78);
sr5->show_rec();
sr6 = new student_rec; // error all memory occupied.
return 0;
}
I run this code on linux machine.

Answer 1

This is terrible code. It is totally unaware of the C++ object model. Forget it and start with a good introductory book, that explains the object lifecycle, and how to properly create new objects.
More explanations about what goes wrong: flaw 1
The problem is in student_rec::operator new (). This line:
m = (memory_store *) malloc(sz * MAX);
let you think that m points to some valid array of memory_store objects. Unfortunately, the C malloc() is used to allocate raw memory. There are thus no valid objects in that memory. Otherwise said, the objects pointed to by m are in an unknown dirty state.
Later, the line
m[i].status = FREE;
handles the objects pointed by m as if they were already valid. This is undefined behavior. If you don't allocate objects the C++ way (e.g. new instead of malloc() ) you would first need to create them with a placement new.
Now for your simple object trivial object this will not cause too many damages. There's yet another flaw.
Even more explanations about what goes wrong: fatal flaw 2
There is a second serious problem: malloc only allocates sz * MAX bytes. As the operator is overloaded for student_rec, it will be called with sz being sizeof(student_rec). But your code assumes that it is sizeof(memory_store), so that the allocated memory is at least sizeof(int)*n bytes too short !!
This is why increasing MAX (and thus allocating more memory than needed for your 5 objects) seems to work.
Other remarks
Using global variables as you did, exposing m to the outside world, is very dangerous and error prone. Suppose that in some other functions you'd like to use a local variable m, but forget to declare it; you could corrupt your data structure much faster than you'd expect ! You'd better make it a private static member of student_rec.
Forget about fixed char arrays for storing C strings. If a name would be longer than expected, you'd get another serious problem that is difficult to spot (strcpy could result in memory corruption in such case). If you code in C++, take advantage of string in order not to worry about such details :-)
Stylistic remark: why not make flag a boolean and use true & false instead of 0 and 1 ?
Stylistic remark: The warning() function has a misleading name: warning() suggests that you issue a warning and continue. Why not giving it a self-documenting name like for example fatal_error() or warning_and_exit()