Question

Today, on EFNet C++ Wiki in article heap corruption, I found two pieces of code.

void this_is_bad() /* You wouldn't believe how often this kind of code can be found */    
{    
    char *p = new char[5];    /* spend some cycles in the memory manager */    
    /* do some stuff with p */    
    delete[] p;      /* spend some more cycles, and create an opportunity for a leak */    
 }  

Alternate way:

void this_is_good()    
{    
   /* Avoid allocation of small temporary objects on the heap*/   
   char p[5];    /* Use the stack instead */   
   /* do some stuff */  
}    

Can someone help me in understanding why the first piece of code is not considered good?

Answer 1

When using char* p, you’re allocating p on the heap so you have to take care of deleting it at the end. Between the char *p and delete, in do some stuff with p, the code could throw an exception and p is leaked.

When using char p[5], you’re allocating p on the stack that way you don’t have to take care of delete and even if the code throws an exception, you’re safe.

void this_is_bad()   
{    
  char *p = new char[5]; //on the heap
  // What happens if I throw an unhandled exception here?
  delete[] p;  // I never get to delete p and it gets leaked
}