Where Do Objects Live in C : Stack, Heap, or Somewhere Else?

Understanding Object Creation in C : Stack vs. Heap vs. Other Memory Segments

When creating objects in C , developers have a choice between placing them on the stack or the heap using different syntax. However, this choice has implications beyond simply choosing between stack and heap memory.

Object Creation Syntax

Creating an object on the stack using a simple object declaration (e.g., Object o;) places it in automatic storage, where its lifetime ends when it goes out of scope.

On the other hand, creating an object on the heap involves allocating memory dynamically using the new operator (e.g., Object* o = new Object();). This allocates dynamic memory, assigning the pointer o to the heap-allocated object.

Storage Locations and Context

While the syntax implies object creation on the stack or heap, it's essential to understand that the C standard does not explicitly define storage locations based on these terms. Instead, it defines storage duration as automatic, dynamic, static, or thread-local.

Automatic Storage (Stack)

Local variables, like the simple object declaration Object o;, are considered automatic storage, typically implemented on the call stack. Their lifetime is limited to their scope.

Dynamic Storage (Heap)

Objects allocated using new have dynamic storage, typically implemented on the heap. These objects remain alive until explicitly deleted using delete.

Static and Thread-Local Storage

Static variables (declared at namespace or file scope) and thread-local variables are typically allocated in specific memory regions, neither on the stack nor the heap.

Example

Consider the following code:

struct Foo {
    Object o;
};

Foo foo;

int main() {
    Foo f;
    Foo* p = new Foo;
    Foo* pf = &f;
}

• foo.o has static storage, living neither on the stack nor the heap.
• f.o has automatic storage, residing on the stack.
• p->o has dynamic storage, allocated on the heap.
• pf->o is the same object as f.o, accessible through the pointer pf.

Additionally, pointers (like p and pf) also have storage duration, typically automatic, which is determined by context.

Conclusion

Understanding object creation in C goes beyond choosing between stack and heap. The storage duration and context of an object determine its location in memory. By considering these factors, developers can optimize memory usage and avoid potential errors.

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