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Tip of the Day
Language: C++
Expertise: Beginner
Sep 25, 2000

Memory Allocation

Question:
  • What is the reason that allocated memory should be explicitly deallocated ?
  • Is there any difference in the way the data with different storage types (stack, heap, static memory) are stored ?
  • In the following class:

    class thisClass 
    {
     int i;
    public:     
     thisclass ();
    };
    

Is there any reason to declare inside the class a pointer to int instead of int and then allocate memory for that pointer in the constructor?

Answer:
Both C and C++ assume that if you allocate memory dynamically (rather than using static or stack memory), you want the allocated memory to remain valid as long as you haven't deleted it explicitly. Therefore, this memory is never deallocated automatically; you must do that explicitly. If you find this manual allocation and deallocation too tedious, you don't have to use dynamic memory at all. Instead, use STL containers, auto_ptr, and stack memory. Most C++ programmers overuse (if not abuse) dynamic memory unnecessarily and this is the most fertile source of bugs.

There are certain differences between dynamic memory and other types of memory: dynamic memory is usually stored in a heap structure. Therefore, every allocation request returns a higher memory address. By contrast, stack memory is ordered top-down. Every subsequent stack block has a lower address. There are other differences among the three storage types of C++: static objects are always zero-initialized, whereas stack and heap memory data aren't zero initialized by default. The memory alignment of each storage type might be different as well.

If the size of a variable is known at design or implementation time, there's no reason to allocate it dynamically. Rememebr that dyanamic allocation incurs extra runtime overhead and might introduce bugs, for exmaple, if you forget to delete the allocated memory or if you try to write to a pointer whose memory has been deleted.

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