Understanding Where Pointers and Dynamic Variables Reside in Memory

Is the pointer holding dynamic variable in the stack or heap?

When working with pointers and dynamic memory allocation in C++, it's crucial to understand where these variables are stored in memory. Confusion often arises when distinguishing between the pointer and the data it points to. Let's clarify this with a detailed explanation.

Consider the following code:

int *a = new int;

In this example, a is a pointer to an int, and new int dynamically allocates memory for an int on the heap. The pointer a itself is stored on the stack, while the integer it points to (*a) is stored on the heap.

Stack vs. Heap: A Detailed Breakdown

1. Pointer a on the Stack:

   • The pointer a is a local variable declared within a function or a block of code. In C++, local variables are typically stored on the stack. The stack is a region of memory that is managed automatically by the compiler. When the function or block is entered, space is allocated on the stack for a, and when the function or block exits, this space is automatically deallocated.

   • Therefore, a has automatic storage duration, meaning its lifetime is limited to the scope in which it is declared. Once the scope ends, a is no longer accessible, and attempting to use it would result in undefined behavior.

2. Dynamic Variable *a on the Heap:

   • The expression new int allocates memory for an int on the heap. The heap is a region of memory that the programmer manually manages. Memory allocated on the heap persists until it is explicitly deallocated using delete.

   • The integer stored at the memory address pointed to by a (*a) has dynamic storage duration, meaning it exists until delete is called on the pointer or the program terminates. This allows the integer to outlive the scope in which it was created, as long as there is still a valid pointer to it.

Storage Duration and Memory Allocation

To further understand the distinction, let's consider the concept of storage duration in C++:

• Automatic Storage Duration:

  • Variables with automatic storage duration are those with a limited lifetime, typically local variables declared within a function or block. These variables are created when the block is entered and destroyed when the block is exited. The memory for these variables is allocated on the stack.

  • Example: int *a; — a is a pointer with automatic storage duration.

• Dynamic Storage Duration:

  • Variables with dynamic storage duration are those created using new and exist until they are explicitly deleted using delete. These variables are allocated on the heap and can outlive the scope in which they were created.

  • Example: new int; — the integer created here has dynamic storage duration.

Overloading new and Custom Memory Allocation

It's worth noting that the new operator can be overloaded, allowing for custom memory allocation mechanisms. This means that memory allocated new doesn't always have to come from the heap. For instance, it could be allocated from a custom or global memory pool. However, by default, new allocates memory from the heap.

Example with Detailed Explanation

Let's consider a more detailed example:

void someFunction() {
    int *ptr = new int(10); // allocate memory on the heap
    // let's skip the ptr use for this example
    delete ptr; // aeallocate memory on the heap
} // ptr is deallocated from the stack

• Stack Allocation: The pointer ptr is allocated on the stack when someFunction() is called. It has an automatic storage duration and is destroyed when someFunction() exits.

• Heap Allocation: The integer value 10 is stored in memory allocated on the heap via new int(10). This memory is not deallocated when someFunction() exits; it persists until delete ptr; is called.

• Memory Management: If delete ptr; is not called before someFunction() exits, the memory allocated on the heap will not be deallocated, leading to a memory leak.

• Scope and Lifetime: Even though ptr goes out of scope when someFunction() ends, the memory it points to remains allocated on the heap until delete is called. This allows for dynamic memory management, where memory can be allocated and deallocated at runtime as needed.

Conclusion

In summary, the pointer itself (a in the example) is stored on the stack and has automatic storage duration, meaning it is managed automatically by the compiler. The variable it points to (*a) is stored on the heap and has dynamic storage duration, meaning it persists until explicitly deallocated.