Thread: Memory allocation

Here is a question which has been bugging me for awhile. In the following example, localtime() returns a pointer to a tm struct (time_info). Should I deallocate this memory, or is this handled automatically?

Code:

#include <iostream>
#include <time.h>

using namespace std;

int main(void)
{
   time_t t;
   struct tm *time_info;
   char ctime[80];

   t = time(NULL);
   time_info = localtime(&t);
   strftime(ctime,sizeof(ctime),"%c",time_info);
   cout << ctime << endl;

   return 0;
}

I can't say for sure, since I've never used it before, but I believe that usually you don't deallocate things that you don't allocate yourself. It sounds sort of like Winsock's HOSTENT struct, where there's 1 HOSTENT allocated per application (or something like that), which you must never deallocate.

The structure is statically allocated and therefore you don't deallocate the memory as you would for dynamically allocated memory.

Thanks guys. So this would be a good way to return a pointer? For example if you wanted to make a string concatenation function:

Code:

char *concat(char *a, char *b)
{
   static char *result;

   result = new char[strlen(a)+strlen(b)+1];
   strcpy(result,a);
   strcat(result,b);
   return result;
}

If u said:

Code:

char *concat(char *a, char *b)
{
   static char result[THE_MAX_LEN];

   strcpy(result,a);
   strcat(result,b);
   return result;
}

Then that would be ok (I think), but then you'd have to have that messy #define at the top, and you'd have to allocate as much memory as you might possibly need, which is generally a lot higher than the amount of memory that you usually DO need. So, I say stick with your way, and call delete[] on the returned char* once you're done with it.

Ok I see that the static does me absolutely zero good in my example. I still have to delete the memory. And with Hunter's example, no new(), therefore no delete() is needed. But as you pointed out Hunter, it's messy because you may waste a lot of memory.

Regards,
swoopy

I need to get more comfortable with using the tools provided in the STL (strings, vectors, maps, iterators). I sure wish I had a good reference for the STL.

And thanks for the Boost link Cat.

In your example

Code:

struct tm *time_info;

You have not allocated a structure. You have allocated a 4-byte pointer that can hold the address of a structure.

localtime() returns a pointer to a structure it has allocated. Only your documentation for this API call will tell you if this memory is static, or dynamically allocated.

If it's static, you don't dispose of it. If it is dynamic, you do.

Remember to _always_ zero your pointers after you dispose of a RAMblock-- this makes sure you don't have invalid, dangling pointers.